Concise Intro To
A Concise Introduction to Geographical Information Systems and Science
M Morad and T Connolly
Kingston Centre for GIS Kingston University, UK Contents
Section 1 An Overview of Main Concepts
Section 2 GIS Terminology
Section 3 Major GIS Acronyms
Section 4 Recommended Further Reading
2 Section 1
An Overview of Main Concepts
3 GIS and geospatial data
A Geographical Information System is, technically speaking, a computerised system for capturing, processing, enhancing, querying, analysing and visualising spatially-referenced data. GIS draws on several related disciplines, and its cross- disciplinary nature is illustrated below.
GIS draws upon several related disciplines
What distinguishes GIS from other forms of information systems, such as databases and spreadsheets, is that GIS deals with spatial information. GIS has the capability to relate layers of data for the same points in space, combining, analysing and, finally, mapping out the results. Spatial information uses location, within a coordinate system, as its reference base. The most common representation of spatial information is a map on which the location of any point could be given using latitude and longitude, or local grid references such as the British National Grid (BNG) or the New Zealand Map Grid (NZMG). See the diagram below for an example of a special engineering survey grid.
y Location of North Sea wells 5 Well id x y depth f g a 1 1 2000 4 e b 2 2 3000 c 4 2 2900 3 d d 3 3 5400 e 2 4 1750 2 b c f 1 5 2500 g 3 5 6000 1 a 0 0 12345x
Point data and related attribute information
Some applications of GIS are obvious, for example water supply companies use GIS as a spatial database of pipes and manholes; local governments can use GIS to manage and update property boundaries, emergency operations and environmental resources. GIS may also be used to map out the provision of services, such as health care and primary education, taking into account population distribution and access to facilities. Increasingly, GIS is being used to 4 assist businesses in identifying their potential markets and maintaining a spatial database of their customers.
In broad terms, a Geographic Information System could be defined as a set of principles and techniques employed to achieve one (or both) of the following objectives:
1. Finding suitable locations that have the relevant attributes. For example, finding a suitable location where an airport, a commercial forest or a retail outlet can be established. This is usually achieved through the use of Boolean (logical) operations, as illustrated schematically in the following figure.
OR =
(union)
AND =
(intersection)
XOR =
NOT =
Boolean operations
2. Querying the geographical attributes of a specified location. For example, examining the roads in a particular locality, to check road density or find the shortest path, and so on. This is often achieved by ‘clicking’ onto the location or object of interest, and examining the contents of the database for that location or object. The following figure shows examples of spatial query functions in GIS.
5 Some of the basic spatial query functions
GIS data are usually stored in more than one layer in order to overcome the technical problems caused by handling very large amounts of information at once. It is easier to work with complex spatial problems one layer at a time, to enable the revision of the data without having to overhaul the entire information system. This is a fundamental aspect of GIS, and working with layers of geographical information is known as data integration (see the figure on the next page).
For example, a comprehensive GIS of a UK county might consist of several layers of data:
• Layer 1 property boundaries and land use types (area maps) • Layer 2 road and railway networks (line maps) • Layer 3 terrain characteristics (spot-height or contour maps) • Layer 4 hospital and school locations (point maps)
Area data
Line data
Surface data
Point n data property boundaries
road and rail n
contours
n
n schools
Examples of layers of area, line, surface and point data
6 Spatial data may be represented in GIS in one (or both) of the two following formats:
• Vector model, as geometric objects: points, lines, polygons • Raster model, as image files composed of grid-cells known as pixels
The following figure outlines this idea.
Vector representation
Real world Map
Raster representation
Examples of vector and raster data models for the same object
7 The importance of geographical data
The nature of any natural or economic activity with a spatial dimension cannot be properly understood without reference to its spatial qualities. Spatial data have two essential parts: location and attributes.
A GIS requires locational references. Typical locational references are latitude and longitude and national grid references such as the British National Grid. However, other geospatial codes can also be used to identify location, such as postcodes. Any locality would have a number of characteristics or properties associated with it. These attributes are usually kept in tables, containing such information as vegetation types, population, annual income, and so on.
GIS systems store and process data in two formats, vector and raster (or as the filed and object views of reality, as some experts would prefer to say).
In the vector data model (see the next Figure), the world is represented as a mosaic of points and interconnecting lines representing the location and boundaries of geographical entities. In vector data models, the data are represented as: • lines (arcs) • polygons (traversed areas) • nodes (intersection points) • points (labelled nodes)
The raster (or grid-cell) data model has arisen from aerial and satellite imaging technology, which represents geographical objects as grid-cell structures known as pixels. See the following figure.
10 7,10 5,9 9,8 4,7 polygon 1,6 8,6 6,6 5 2,5 line 5,4
point 2,2 7,2
0 0 510
Geographic entities represented in a vector format Each data model has particular strengths and weaknesses, and the type of model used is determined by the nature of the work being undertaken and the data available.
8 The main advantage of the vector data format is that it permits a neat representation of points, boundaries, and linear features. This makes it particularly useful for analysis tasks that require accurate positioning, for example in engineering or cadastral boundary databases. It is also possible in a vector-based GIS to define the spatial relationship (ie the connectivity and adjacency) between coverage features. This aspect of GIS is known as topology, and is important for such purposes as network analysis (for example to find an optimal path between two nodes in a complex transport network).
By contrast, raster-based GIS defines the position of features in terms of (x, y) co- ordinates where topological associations are more difficult to identify. However, the main disadvantage of vector data is that the boundaries of the resulting map polygons are discrete (enclosed by well-defined boundary lines), whereas in reality, assuming maps scales are not an issue, the map polygons may represent continuous gradation or gradual change, as in soil maps.
Cadastral Vegetation boundaries River Thematic network Utility Soil diagram
Advantages of Vector and raster data
Grid Data Models use a raster matrix (a grid of image cells) to represent information. Grid data are also known as raster data. The resolution (visual definition) of the raster depends on its pixel (cell) size. In other words, pixel resolution represents the size of the ground area covered by each pixel in the image. The smaller the cell size, the higher the resolution. The raster data model is, therefore, good for representing indistinct boundaries, such as thematic information on soil types, soil moisture, vegetation, ground temperatures, and so on. Furthermore, as reconnaissance satellites and aerial surveys use raster-based scanners, the information (ie scanned images) can be directly incorporated into GIS programmes capable of working with raster data. However, the higher the grid resolution, the larger the data file is going to be. This is the main limitation of raster based GIS.
9 50 m pixel 25 m pixel (grid 20 x 16 = 320 pixels) (grid 40 x 32 = 1280 pixels)
Raster resolution
The question of which data model to use in GIS depends on the nature and objective of the GIS project. Primarily the model type will depend on the nature of the data. Issues of concern are the volume of the data generated, ease of analysis and accuracy. Generally, vector data sets are economical in terms of file size (ie do not take up much memory space) and have a high level of positional precision, but are relatively difficult to use in mathematical computations. On the other hand, grid data sets tend to take up more file space and have a coarser resolution, but are easier to work with mathematically (ie easier to process computationally).
10 Geospatial data processing
There are several techniques for capturing, processing and analysing geospatial data in GIS. Some of these are explained below.
Data capture. A GIS cannot analyse the information in a map, if the data are not already in digital form which the computer can recognise. Maps can be digitised (hand-traced with computer mouse) to collect the coordinates of the map features. Electronic scanning devices can also be used to convert map lines and points to digital information.
PC or Workstation
Digitising tablet connected via serial port
A typical digitiser
Information retrieval. With a GIS we can point at a location, object, or area on the screen and retrieve recorded information about it from the Database Management System (DBMS) which holds the information abut the map’s features.
Spatial data Attribute data (DBMS) Record Area Value Owner Number 100 100 109 3900 Scott 101 117 2300 Digby 101 102 101 2750 Taylor 102 105 103 113 3800 Lloyd 104 97 4700 Turner 104 103 105 122 3550 Jones
Spatial and attribute
Spatial calculations. GIS makes spatial measurements easy to perform. Spatial measurements can be the distance between two points, the area of a polygon or the length of a line or boundary. Calculations can be of a simple nature, such as measuring areas on one map, or more complex, such as measuring overlapping areas on two or more maps.
11 Distance Area Perimeter
4 miles 3 hectares 5 kilometres
Measurements
Overlays (data integration). Overlaying is an important procedure in GIS analysis. Overlaying involves superimposing two or more map layers to produce a new map layer. Consider the following example: a new variety of wheat grows well in dry environments, with long growing seasons and alkaline soils. Given the availability of data on the length of the growing season, moisture regime and soil alkalinity, where is the best place to plant the wheat? The answer can be found by overlaying (superimposing) several maps showing (separately) water-budget, growing season length, soil pH, sodium content, and so on. The GIS analysis can establish the locations where all the favourable soil conditions coincide, as the places where the wheat will grow best.
Layer 1Layer 2Layer 3Result
++=
Data integration
Spatial interpolation. A GIS can be used to estimate the characteristics of terrain or ecological conditions from a limited number of field measurements. For example, a rainfall map can be created from the interpolation of rainfall point measurements taken at a number of different locations on a map.
Spatial interpolation
12 Buffering and corridors. Buffers are used when the distance from a point or line is important. For example a classic use of GIS in earth sciences involves estimating the damage caused by an earthquake, in terms of distance belts away from the epicentre. If the influence of the earthquake is grouped into four classes then the buffers around the epicentre might look like a map of semi-concentric zones of earthquake damage. Figure 9 shows an example of buffering, involving a raster model and a vector model.
Raster representation Vector representation
5 mile buffer around an airport 5 mile buffer around a school
5 miles
5 miles
Buffer or proximity measurements
Network analysis. GIS can handle complex network problems, such as road network analysis. A GIS can work out travel times and the shortest path from A to B. This facility can be built into more complicated models that might require estimates of travel time, accessibility or impedance along a route system. An example is how a road network can be used to calculate the risks of accidents. There are, of course, other types of network analysis, involving stream networks.
For example, GIS could be used to model the flow of water through a river system, to plan a flood warning system. Real time data would be transmitted by flood warning monitors, such as rain gauges and river height alarms, which could be received and passed to a GIS system to assess the hazard. If the amount and intensity of rain exceeds a certain limit, determined by the GIS flood model for the area, a flood protection plan could be put into operation with computer-generated maps demarcating the vulnerable areas at any point in time.
River network Rain gauges Flood model
+=
Network analysis 13 Registration and edge-matching. Before they can be used in GIS, maps and images should be geometrically rectified. The process begins by registering the raw maps and images to known (control) co-ordinates. The registration record is then used or georectify the map or image in need of rectification. The latter step is also known as rubber-sheeting. The rectification of raster data may involve "re- sampling" the original data, which means re-interpolating the grid cell values of the raster by filling in new ones in some places and eliminating others.
Corrected image
Simple (linear) and polynomial (non-linear) image correction
Topology Topological relationships define the manner in which geographical objects are related spatially. In particular, topology defines two aspects of spatial relationship: § Adjacency/contiguity (the identification of adjacent polygon, by recording the left-hand and right-hand polygons for each line). § Connectivity (the identification of linkage points for each line).
1
2 5
4 3 6
Topological construction of geographical areas
Map projection. Map projection is a fundamental aspect of mapmaking. A map projection is essentially a geometric model that transforms the locations of 14 features on the Earth's surface to corresponding locations on a two-dimensional map. It is impossible to project every attribute of a spheroid perfectly onto a plane; but some projections can preserve shape, while others can preserve area, distance, or direction.
Different projections are used for different types of maps because each projection is particularly appropriate for certain uses. For example, a projection that preserves the shape of the continents may distort their relative size. The diagram below demonstrates how three map projections can be constructed.
Normal Transverse Oblique
Map projections (Note: transverse and oblique projections are less common!)
Digital terrain analysis. GIS can build three dimensional models, where the topography of a geographical location can be represented with an x, y, z data model known as Digital Terrain (or Elevation) Model (DTM/DEM). The x and y dimensions of a DTM represent the horizontal plane, and z represents spot heights for the respective (x,y) co-ordinates. As illustrated in the following Figure, the data represented by a DEM array (grid cells) or a Triangulated Irregular Network (TIN). The latter consists of a networking of intersecting lines that make up irregular triangles.
15 DTM array TIN triangles 8 9878 5 10759 10 9
9 4 15 10 9 15
7 5 10 10 5
An illustration of TIN and DEM representation
The data sets derived from a Digital Terrain Model can be used to analyse environmental phenomena or engineering projects that are influenced by elevation, aspect or slope, as in forestry and stream sedimentation studies. The visualisation (display) power of the computer allows the terrain data to be visualised in three- dimensional form, often from any angle of view (this is known as point-of-view analysis).
For example, using a DTM, civil engineers can assess how well a new structure (say, an open-cast mine) is screened by the surrounding terrain, and estimate how much additional screening might be needed to conceal the structure or reduce noise levels.
Digital terrain analysis
Global Positioning System (GPS) and GIS The GPS is a system of Earth-orbiting satellites, each providing precise time and position information which enables portable GPS receiving devices to compute positions on the Earth. Signals must be received from at least three satellites in order to establish the receiver’s position in latitude and longitude (or a national coordinate system provided with the equipment). Measurement from a fourth satellite is required to calculate altitude (height) position.
GPS configuration consists of three parts: 16 1. Twenty four US Navy satellites orbiting the earth. 2. Portable mobile receiver (aerial plus portable console). 3. Base station receiver positioned at a known geographic position, in order to perform differential correction of the raw satellite signals.
Global positioning system
Each satellite in orbit carries a computer and an accurate (atomic) clock. The computer onboard calculates the satellite's orbit a week or so into the future, and predicts ionospheric conditions over that period. By consulting its clock and computer-generated data, the satellite can tell where it is in the sky at any given microsecond, and transmits its position and the current time continuously.
Remote Sensing and GIS
Remote sensing is the study of phenomena sensed from air or satellites. The use of remote sensing provides scientists with an opportunity to interpret data as seen from space. This facility is particularly useful in surveying very large areas at relatively little cost. Remote sensing is therefore widely used in such locations as the arctic zones, deserts, forested areas, marshlands and mountainous regions, where small-scale ("magnified") maps of an adequate quality can be reproduced relatively cheaply from satellite images. In general, remote sensing is suitable for studying extensive areas with difficult or inhospitable terrain, or where conventional surveys may be very costly.
17 Sun (source of solar energy) Remote sensing systems
Earth (source of thermal energy)
lake
Absorption and transmission through Earth surface features
Satellite and aerial remote sensing provide an almost infinite pool of information. They provide the experienced interpreter with knowledge of ground conditions, including geomorphology, vegetation, soils and mineral composition. A great deal of time and cost can be saved, through remote sensing, by reducing the necessary amount of field work. Taken at regular intervals and over a long period of time, satellite remote sensing can also help scientists to identify the extent of change in ground conditions over the seasons, such as seasonal soil moisture changes. This is particularly useful in mapping areas that witness frequent changes in land cover, like agricultural lands, tidal areas and marshlands.
Stereoscopy (three-dimensional view), which is important for such purposes as topographic and forest canopy studies, is possible through remotely sensed images. Stereoscopy is made possible by the overlap of adjacent images (neighbouring images which cover common areas). This can be achieved through the overlap of orbit paths, or the tilting of the sensor to get a sideways view of adjacent areas. Images taken by the French satellite SPOT, for example, are especially popular with land resource scientists using remote sensing techniques, because they combine fairly high resolution (maximum 10 metres) with some overlap.
Visible light (the colours sensed by our eyes) constitutes only a narrow band (wavelength range) within the vast electromagnetic spectrum. One distinct advantage of remote sensing is that it offers the opportunity to use many invisible bands outside the spectrum of visible light. There are many natural phenomena which could only be sensed from distance by using invisible bands in the electromagnetic spectrum.
For example, satellites can identify geothermal reserves close to the Earth’s surface by sensing the heat emanating from such spots. The procedure is similar, in principle, to the use of infrared (night-vision) security cameras to detect intruders in the dark. The results of sensing invisible phenomena can be developed 18 for human use by assigning false colours to the picture (or false grey shades in black-and-white prints). The next diagram shows the wavelength spectrum.
In land use applications, several bands of the electromagnetic spectrum are often used to sense the same place at once. This is useful because each band is especially sensitive to a given phenomenon: showing vegetation boundaries in one case, soil-moisture differences in another, and so on. Therefore, by using multiband images, more information can be obtained as a result of having more colour combinations available for analysis. The following diagram demonstrates the spectral reflectance for three different spectral responses.
Wavelength
Spectral reflectance curves for vegetation, soil and water
Sensors can be divided into two groups: passive and active. Passive sensing resembles the way our eyes operate, by detecting the radiation reflected by the sensed objects from the sun (or another source of illumination). Active sensing is achieved in the reverse manner: the energy is transmitted by the sensor itself, bounces back on impact with the object, giving a ‘backscatter’ echo which is registered by the sensor. In remote sensing, microwave radar (LiDAR: Light Detection And Ranging) operates by active sensing.
Because of Rdar's ability to penetrate clouds, water, snow and (thin) vegetation, a radar image offers the possibility of obtaining a view of the examined phenomena through such obstacles. The technique is especially suitable for surveying areas with frequent cloud cover or under grass.
Radar is also very useful because it accentuates surface roughness on remote sensing images. This occurs because relative height of sensed objects increases when the wavelength of the sensing radiation becomes longer. Relative height also increases when the angle of incidence (the degree of camera tilt from vertical position) is increased. To the soil scientist, this facility can be very useful when studying such phenomena as stoniness, where relative height must be accentuated in order to improve detection. An image of a stony soil surface, taken by a side- looking radar sensor, can show a distinctly mottled texture.
19 Before any remote sensing image can be made available for mapping, it must be cleared of errors caused by geometric displacement or atmospheric interference. The common sources of error in satellite imagery are:
• orbit and flight errors, • Earth curvature, • Earth rotation • surface roughness • atmospheric interference (cloud, fog and smog).
Normal Rolling Yawing Pitching Georectified
However, as shown above image correction is not generally recommended in soil and vegetation inventories, because it may give false data (as image corrections usually involve pixel-value manipulations). Images marred by significant interference from atmospheric phenomena (such as clouds) should be avoided. Atmospheric errors are especially difficult to correct, often making guess-work inevitable. Similarly, images marred by 'noise' (communication errors, which occur during the transmission of images from satellites to the receiving stations) should also be avoided.
Digital correction of image errors is made possible by computerised image processing techniques. Images are normally kept in digitised form (recorded in binary code). Correcting the geometric errors of individual images is usually undertaken by giving the computer the correct coordinates of a number of control points (whose position is known exactly) on each image. The image is then automatically stretched (rescaled and rotated) to produce a geographically-corrected image map.
20 Section 2
GIS Terminology Accuracy: is the degree to which information on a map or in a digital database matches control values. Accuracy is not the same as precision. See Precision.
Address geocoding: See Geocoding.
Address Point™: A product created by the Ordnance Survey to locate all known address locations e.g. houses, office blocks, factories.
Address matching: A mechanism for relating two files using (street or postal) address as the common reference.
Adjacency: Where two objects share a common boundary or side. (These properties do not change when geometry is altered through transformation).
Aggregated data: The grouping together of data to form larger units of measurement. Aggregates are often ordered into a base hierarchy eg UK census enumeration districts which can be aggregated to ward boundary areas (census blocks and tracts in the US). See also Disaggregated data
Algorithm: A procedure used to solve a mathematical or computational problem, or to address a data processing issue. Algorithms usually consist of a set of rules written in a computer language.
American National Standards Institute (ANSI): ANSI standards have been established for many elements of GIS and computer systems. See BSI.
American Standard Code for Information Interchange (ASCII): A widely used industry standard code for transferring alphanumeric codes between computers.
Analogue: A continuously varying electronic signal (contrast with binary – see Binary codes and binary files) The term is also used to describe traditional paper mapping products and aerial photographs.
Application programme interface (API): Computer software designed to access services from programmes across a network.
Application: In computing terminology, application is the technical name for a computer programme.
22 Arc: A line connecting a set of points that can form one side of a polygon. In ARC/INFO, the vertices at either endpoint of an arc are called nodes. In a topological GIS system (see Topology), arcs are linked to nodes (arc-node topology) and to polygons (polygon-arc topology).
Area: Any object or field that has length and breadth. A fundamental unit of geographical information, defined by a continuous, closed boundary. Also known as polygon. Examples include fields, counties, lakes, local authority boundaries, school districts and census enumeration districts.
Array: Data values that are held in the form of a grid. (Arrays can also be one-dimensional (row or column). See also Grid.
Aspect: The geographical direction toward which a slope faces, measured in degrees from north, in a clockwise direction.
Attribute data: Descriptive information about features or elements of a database, listed as numbers or characters. For a database feature like census districts, attributes might include demographic facts such as population, average income, gender and age. Each row represents a geographical feature, and each column represents one attribute variable.
Automated cartography: The procedure of drawing maps with the aid of computer-based display devices such as plotters and visual display units (monitors).
Automated Mapping / Facilities Management (AM/FM): The systems used by public and private utilities, local governments, and other organisations that manage very large capital equipment, stock, or facilities. AM/FM most commonly describes databases designed to manage networks of road, pipe, or cable assets.
Azimuth: The horizontal direction of a vector, measured clockwise in degrees of rotation from the positive y-axis. For example, degrees on a compass measured from north.
Band: A class of multispectral data for an interval within the electromagnetic spectrum (such as light or infrared energy). Satellite imagery such as Landsat and SPOT provide multispectral images of the Earth, containing several bands.
23 Band separate: An image format that stores each band of data in a separate file. The original data are usually collected by multispectral scanners, onboard satellites or aeroplanes.
Bandwidth: A measure of the volume of data that can flow through a communications link (cable). Also known as throughput.
Base map: A map containing geographical features used for positional (map grid) reference. Property boundaries are commonly used as base maps because they are accurately referenced. Base Maps provide the background on which other data layers are overlaid and analysed.
Basic Land and Property Unit (BLPU): In the UK, a reference number used in the UK address gazetteer system known as BS7666. See Gazetteer and UPRN.
Baud rate: The speed of data transmission between computer and other devices, measured in bits per second.
Benchmark Tests: Procedures for comparing the performance of competing hardware and software. Specific benchmark tests can be developed by GIS managers to test new equipment or software cope under conditions close to those which will be encountered in day-to- day use. See Sensitivity analysis.
Binary code and binary files: Digital information and commands stored and used by hardware and software, as sets of on-off (1-0) signals. Most systems of binary encoding in GIS are specific to particular hardware and software vendors. Binary data are usually the most compact means of storing information. However, binary files are not easy to transfer between computer systems which use different binary configurations. Binary files are therefore usually translated into ASCII (text) form for transfer between computers. See American Standard Code for Information Interchange (ASCII).
Binary Large Object (BLOB): The data type of a column in an RDBMS (Relational Database Management System) which can store large images or textual data.
Bit: The smallest unit of information that a computer can store and process digitally. A bit has two possible values, 1 or 0, which correspond to yes and no (or on and off). When several bits are arranged according to an agreed number to the base 2, they make up a byte. See byte. 24 Boolean operator: A keyword that specifies how to combine simple logical expressions into complex expressions. Boolean operators negate a predicate (NOT), specify a combination of predicates (AND), or specify a list of alternative predicates (OR). For example: slope > 100 AND vegetation = forest.
Breakline: A linear feature that defines the areal extent and controls the surface interpolation of a digital elevation model. The term is used as a command in ARC/INFO’s TIN (Triangular Irregular Network) module. Terrain features containing shorelines are often clipped as breakline features, otherwise sea-level surfaces could be erroneously interpolated as hills or valleys. See Digital Terrain Model (DTM).
British Standards Institute (BSI): The UK equivalent of the American National Standards Institute (ANSI).
Buffer: Enclosed polygon created around points, lines or areas at an equal distance in all directions. The results represent areas at set distances from the original object. For example, the creation of buffer zones around a polluted industrial site may represent the varying extent of pollution measured at specified distances from the source of the contamination. Buffers are therefore useful for proximity analysis and environmental impact assessment.
Bundled GIS: A reference to the way software and hardware are sold together or separately. Some GIS vendors offer a bundled package of hardware and software, at a discount negotiated with the software developers and hardware manufacturers.
Byte: A computer-memory and data storage unit composed of contiguous bits, usually 8. File sizes are measured in bytes or megabytes (one million bytes). Bytes consist of values of 0 to 255 (ie 0, 1, 2…255) and a collection of bytes (often 4 or 8 bytes) represents real numbers and integers larger than 255. A Megabyte is a unit of computer storage made up of one million bytes.
Cadastre: A cadastral survey involves the mapping, tracing and recording of private and public land resources. The term cadastre is French in origin, meaning a record of the ownership, extent and market value of a property, for tax and legal purposes.
Cartesian coordinate system: A two-dimensional, planar coordinate system in which x and y represent distances from a point of origin, and where each point on the plane is defined by an (x, y) coordinate. Locations in the coordinate system can be established 25 using any unit of measurement such as metres or yards. Relative measures of distance, area, and direction are uniform throughout a Cartesian coordinate plane.
Cartography: The geographical discipline concerned with map preparation and communicating geographical information.
CD-ROM: Common abbreviation for Compact Disk-Read Only Memory. A CD-ROM disk can hold about 650 megabytes of information, as text, maps, tables or images.
Cell: The basic element of spatial information in the raster (grid) description of spatial features. See Pixel.
Census: The official count of the population, which in the UK is carried out on a 10 year basis by the Office for National Statistics.
Centroid: A measure of central tendency for a polygon (i.e. there are many calculation techniques and many centroids). In the case of an irregularly shaped polygon, the centroid is derived mathematically and is roughly the equivalent of its centre of mass. Centroids are often used in GIS to index the polygon within which they are located (ie, the polygon’s attributes are referenced to its centroid).
Computer Graphics Metafile (CGM): A graphic image exchange standard.
Chain: A sequence of coordinates defining a line or boundary.
Character: An alphanumerical (alphabetical or numerical) value that represents a single unit of data
Chorley Report: The committee of inquiry into the handling of geographic information, completed in 1987. Recognised as an important milestone of the development of GIS in the UK.
Choropleth map: A map in which areas of different value are demarcated by clearly defined boundaries. The values of the underlying data (eg, soil moisture) are represented by different colour or shading intensities, and the map legend acts as a look-up key to explain the values shown on the map.
Classification function: The ability to analyse information according to a group of classes, often according to common attribute values. 26 Client/server architecture: A computer system consisting of a file server software that handles requests from multiple end users (clients) on the network. A file server is usually a high performance Personal Computer (PC) with a large hard drive that is not used exclusively by any individual on the network. Instead, it serves all the other "client" PCs on the network by providing a common place to store data.
Clip: A polygon which defines the boundaries of features in a map, by cutting the lines off its edge. Clips are used to restrict the extent of data processing or querying done in GIS. See Breakline.
Column: The vertical dimension of a table, often used to representing the attributes of the geographical features listed in the rows. Also know as field.
Command line interface: The software interface which allows the GIS operator to type in commands at a prompt. Command line instructions are faster to execute, but typing errors can cause delays. Most modern GIS are Windows-based, but also allow the user to work from a command line interface if desired.
Completeness: Datasets do not always cover an entire geographical area of interest. They may not, therefore, be complete. This also relates to the amount of information recorded within a dataset.
Computer Aided Design (CAD): CAD systems are used to create maps and plans at architectural scales. They are closely related to GIS systems, as many GIS systems, such as Intergraph and AutoCAD, have their origin in CAD software.
Concurrency management: A database management procedure for preserving a system’s security against accidental or deliberate data loss, which supports simultaneous access by multiple users. This type of access regime allows only one user at a time to change the content of the database (ie, have write-access), while remaining users will have read-only access. The next user wanting write-access to the database will have to wait until the first person has completed their transaction. All database changes are logged, showing the time of transaction and the name of the operator.
Conditional operator: An SQL (Structured Query Language) keyword specifying how to compare values. Conditional operators are used to query a database. Examples include = , >, LIKE and CONTAINING.
27 Connectivity: A topological property relating to how geographical features relate to one another spatially. The topological identification of connected lines are established by recording the ‘from’ and ‘to’ nodes for each line. Lines that share a common node are said to be topologically linked. Connectivity is useful in network analysis.
Contiguity: The topological definition of adjacent polygons, by recording the left and right polygons of each edge of the enclosed polygon.
Continuous surface data: A surface for which each location has a specified or derivable value, such as DEM's (Digital Elevation Models), as distinct from continuous attribute data.
Contour: A line on a surface model representing points of equal height value.
Contouring: The interpolation of contour lines from height data.
Contour interval: The difference in surface values between successive contours.
Conversion: The process of transforming data derived from existing records and maps to a digital database, or from one digital form to another.
Coordinate Geometry (COGO): A data input method similar to the way land surveyors enter their survey data. Input begins at a point, moves along a given bearing for a set distance, and continues in the same fashion until the geographical feature (such as a pond) is completely outlined. Unlike the error-prone process of tablet digitisation, COGO data entry generally establishes more accurate locations and boundaries.
Coordinate: A set of numbers that designate a location in a given geographical reference system, such as (x, y) in a two-dimensional coordinate system or (x, y, z) in three-dimensional terrain models.
Coordinate system: A reference system used to measure horizontal and vertical distances on a planimetric (two-dimensional) map. A common coordinate system (such as the Ordnance Survey’s National Grid) is used to reference the (x, y) location of geographical objects.
Copyright: The exclusive right to make copies and to control original work, granted by law for a specific number of years. 28 Currency: The period of time when a dataset is valid. This relates to the date when the data was originally collected and/or updated.
Dangle: A line with a loose end: a line with one end (or both) that does not connect to any other feature in the map. A dangle occurs when the end of the line undershoots or overshoots the map feature to which it is meant to be attached.
Data Access Language (DAL): The standard adopted by Apple computers to facilitate access to relational databases.
Data integrity: Generally speaking, the term data integrity is used to refer to the internal consistency of the data kept in a database. For example, the presence of characters in a column that is meant to hold only numbers indicates poor data integrity.
Data model: In GIS terms, a data model is a user-defined, abstract representation of data describing the behaviour of the geographical entities represented by the data. For example, the terrain of a geographical location can be represented with an (x, y, z) data model known as Digital Terrain Model (DTM), where x and y represent the horizontal plane and z represents spot heights at the respective x, y co- ordinates.
Data set: A collection of logically related information arranged in a specified way.
Data type: The quantitative or alphanumeric characteristics of variables that define the type of data used. Examples include character, integer and floating point (numbers with decimals, or real numbers).
DataBase Management System (DBMS): Computer software used to enter, store, organise, retrieve and query information in a database. A DBMS with limited GIS capability is said to be equipped with a spatial component (or a "spatial cartridge" in the case of the major DBMS Oracle™).
Database: A collection of organised information, usually stored on magnetic tape or disk. A GIS database includes data about the location and the attributes of geographical features that have been coded as points, lines, polygons or grid cells (pixels).
29 Data dictionary: A database in which information is stored about objects (and their relationships) in a GIS.
Datum: The singular for data. Specifically, the term datum is often used in GIS circles to refer to geodetic datum. See Geodetic datum.
De facto standard: These are standards which are accepted due to their common usage in a particular applications context. They have no standing in law.
De jure standard: These are standards which have some basis in law either in some States in the US, or internationally.
Demography: The study of population characteristics. In a GIS context this usually entails statistical analysis.
Disaggregated data: Data collected about individuals or objects. See also Aggregated data.
Dissolve operations: The process of merging two or more adjacent areas by removing common boundaries. See also Merge.
Digital Chart of the World (DCW): A widely available 1:1,000,000 digital base map of the world, containing vector data digitised by the US Defense Mapping Agency.
Digital Elevation Model (DEM): See Digital Terrain Model (DTM).
Digital Terrain Model (DTM): A database with (x, y, z) coordinates, used to represent landform digitally. Also known as Digital Elevation Model. See Data model.
Digital: See Binary code and binary files.
Digitisation: The process of converting analogue map data into digital codes stored and processed by computers. Digitising commonly involves tracing map features into a computer using a digitising tablet or mouse, or a scanner.
Directory: A partitioned set of data files, stored on computer disk. Operating systems use directories and subdirectories to organise data. The location of a directory is specified with a pathname, for example C:\LIS\PROPERTIES\. Under Windows-based systems, directories are visually represented with a folder icon. 30 Discrete data: Geographical features containing well-defined represented by points, lines or polygons boundaries. Unlike raster (grid cell) GIS, vector GIS represent geographical features as discrete (sharply defined) features.
Dots Per Inch (DPI): Measure of the resolution of graphic displays and printers, representing the number of pixels per inch.
Drape: A perspective visualisation of surface features superimposed on a Digital Elevation/Terrain Model. See Digital Terrain Model (DTM).
Drawing Exchange Format (DXF): File exchange format developed by Autodesk. DXF files are ASCII (text) records of the objects in a drawing file. DXF is widely used for exchanging map files.
Dynamic segmentation: In ARC/INFO, the term refers to analysing events on network routes, based on ‘event tables’ for which distance and impedance attributes are listed. An ESRI (Arc/Info) specific term.
Edge Matching: Unmatched polygons often occur when neighbouring map tiles are misaligned as they are laid edge-to-edge, producing a "sliver". These misalignment errors occur when features run across the boundaries of their respective map sheets. Sliver polygons are not always a consequence of this. The errors often originate during digitisation, generalisation of map features, or map projection. Warping (rubber-sheet correction) and the automatic elimination of (user-defined) tiny sliver polygons are two major methods of mitigating edge-matching errors. See Rubber sheeting.
Element: Basic geographical unit of information, such as a point/node, line/arc, area/polygon or pixel/cell. Also known as map entity.
Ellipsoid: An elliptic form of geodetic model of the Earth. See Geodetic datum.
Embedded SQL: SQL statements that are embedded in a programming language (such as AML: ARC/INFO Macro Language). See Structured Query Language (SQL).
Encapsulated PostScript (EPS): See Postscript. 31 Entity-relationship model (ER model): A method developed in the 1990's to describe real world relationships in a logical form. Generally takes the form of a diagram and can be used to conceptualise elements of a proposed database.
Enumeration district: In the UK, the basic area or unit of approximately 150 households used by the Office for National Statistics for the planning and carrying out of population counts and surveys.
Environment: In computing jargon, the system’s configuration which defines the various display, editing, and data manipulation processes handled by GIS.
Ethernet: A local-area network (LAN) protocol used for high-speed communication between linked up computers.
Event: An additional geographical feature occurring on or along a linear feature, such as a route.
Extended character set: Extended character sets support additional languages which require double-byte characters, such as Arabic or Greek.
Field: In its most popular use in GIS, a field is a common synonym for column in a database.
File Transfer Protocol (FTP): A common protocol for copying or moving data across a computer network or the Internet.
File: A collection of related data (textual or graphical) in a computer. Files are the basic units managed by the computer's operating system.
Filter: A grid of weighted pixel values, used to remove superfluous (noise) features, or to generalise spatial patterns from a raster data set.
Format: The method in which data are organised and stored in a computer, for transmission between computers or between a computer and a device. Most GIS have proprietary formats used to store and process geographical information.
Fuzzy boundaries: Indeterminate loosely-defined boundaries of geographical objects, such as soil groups.
32 Gazetteer: A work of geographical reference that supplies place names and location information. In the UK, an address standard known as BS7666 has been adopted since 1984, based on two sets of identifiers known as BLPU (Basic Land and Property Unit) and UPRN (Unique Property and Reference Number). See BLPU and URPN.
Generalisation: In its basic form, the process simplifying a shape by reducing the number of vertices, according to a pre-specified tolerance level or algorithm.
Geocoding: The process of identifying or designating the coordinates of a geographical object, given its address. Also known as address geocoding.
Geodesy: The study of the size and shape of the Earth, and the determination of exact longitude and latitude positions on it.
Geodetic Datum: A three dimensional (more specifically ellipsoidal) model used to represent the shape of the Earth in a specified region. A geodetic datum is the basis for the geographical coordinate system adopted by a country. For example, the National Grid of the United Kingdom is based on a geodetic datum representing the shape of the Earth in the UK. A national geodetic datum gives a more accurate representation of the shape of Earth at local level, because global models are quite generalised.
Geodetic framework: A spatial framework of points whose positions have been precisely determined on the surface of the Earth. The UK’s National Grid is the British geodetic framework. Also known as geodetic network. See Geodetic datum.
Geographical Resource Analysis Support System (GRASS): A public-domain raster GIS developed by the US Army Corp of Engineers Research Laboratory (USACERL).
Geomatics: A description of Geography and computing functions and facilities, usually used in relation to land information systems and location-aware applications.
Georelational database: Geometry stored using a numeric identifier as an attribute in a database.
Georeferencing: Establishing the location of a given geographical object, according to an agreed system of map coordinates such as the National Grid. 33 GIS Output: This can take the form of maps, tables, reports or digital information. (The first three can be instances of the last.)
Global Positioning System (GPS): A system of Earth-orbiting satellites, each providing precise time and position information which enables GPS receiving devices to compute positions on the Earth. According to the US-based system which is most widely in use nowadays, signals must be received from at least three satellites in order to establish the receiver’s position in latitude and longitude, or according to a national coordinate system provided by the equipment. Measurements from a fourth satellite are required to calculate altitude (height) position.
Graphical User Interface (GUI): A monitor, menu-based method of controlling how a user interacts with a computer. Instead of issuing commands at a command-line prompt, the user performs the required tasks by using a mouse, or selecting a menu item. See Command line interface.
Graphics display terminal: A computer unit used to view and manipulate graphic information.
Gravity model: An analytical technique used in geographical research to analyse the geographical pattern of economic behaviour. The underlying assumption of the model is that the influence of population density (or any other quantifiable economic parameter) on one another is inversely proportional to the distance (or resistance effect) between them. The term gravitational was borrowed from Newton’s theory of gravitation.
Grey scales: Levels of brightness used in displaying information on monochrome display devices, or on non-colour printers.
Grid cell: A unit that represents a level of resolution, or scale, on an array of equally sized (square) cells arranged in rows and columns. Each grid cell is referenced by its geographical (x, y) location. Also known as pixel in raster GIS.
Grid Reference: A precise description of geographical position in terms of the planar co-ordinate system. Often associated with map co- ordinates. Some countries such as the UK have developed their own National Grid system. See also National Grid and Planar co-ordinates.
34 Grid: A regular set of lines used to depict the planar co-ordinate system. See also Co-ordinate and Array.
Hard-copy: A paper copy.
Hardware: The physical components of a computer system, such as the computer and its attached devices: digitisers, plotters, printers, etc.
Heuristic: A computational method that uses trial and error to approximate a solution, for problems that would otherwise be intractable.
Image analysis: The processing and interpretation of raster information that are held in digital form (such as satellite images). An image is usually stored as values which represent the intensity of reflected light, or other values in the electromagnetic spectrum.
Image resampling: See Rectification.
Index coverage/layer: The polygon coverage that is used as an index (reference layer) for the structure of other layers of geographical features overlaid onto the index layer.
Information: Data that has undergone some form of evaluation. Maps are information because they are cartographic interpretations of survey data. However, data can also be mapped without addition of interpretation.
INFORMIX: A corporate (organisation-wide) relational DBMS.
Infrastructure: In its simplest form, a reference to the basic utility structures that support a local economy, such as roads, electricity pylons, water and drain pipes, etc. GIS are used to hold information about these structures, and map out their location.
INGRES: A corporate relational DBMS.
Input device: A hardware component for data entry, such as a digitiser, keyboard, scanner, mouse, disk drive, etc.
Institute of Electrical and Electronics Engineers (IEEE). An important US organisation, which has played a major role in setting standards for many engineering and GIS related applications.
Integer: A whole number (a number without decimals). 35 Interactive Graphics Design Software (IGDS): An Intergraph GIS file format.
Inter-Application Communication (IAC): The capability of different computer software systems to communicate with one another. With IAC, several computer programs can execute commands simultaneously, share data, and make requests of each other. See Interoperability.
Interface: A hardware and software link that connects two computer systems, or a computer and its peripherals. Also used as a reference to a Graphic User Interface (GUI).
Internet: An international consortium of Wide Area Networks (WAN's) that operate using a standard set of communication protocols. The Internet grew out of a US Defense Advanced Research Projects Agency (DARPA) research project in the early 1970s. Private and public networks have joined the Internet since then.
Internet GIS: Internet-based GIS is a growing area of software development and application. The technology enables Internet users to see and query a GIS map, using a Web browser such as Netscape Navigator or Internet Explorer. Major GIS developers now provide special Internet Server software that allows a GIS (such as ARC/INFO or Intergraph) to be browsed over the Internet by remote users.
Interoperability: The ability of different GIS software and data format to work with one another, without having to standardise the format of the data which any one system uses. Interoperability has received greater momentum with the advent of Open Systems. In the past, emphasis was placed mainly on standardising GIS data, but increasing GIS software conforming to Open Systems standards can now work with different data formats without having to translate them first into a standard format. See Open Systems GIS.
Interpolation: Estimating the value of a geographical object (eg a point, contour or area) from measurements made at surrounding points. A basic example is the estimation of z (height) values of a surface model, using the known z values of surrounding points.
Intersection: The topological integration of two spatial data sets within the area common to both data sets.
36 Intranet: An web based networked facility operating mainly within an organisation. It may be accessible in different geographical locations but remains as an internal resource to a particular organisation. Usually it is not connected to the Internet.
Isoline: A line on a surface connecting points of equal value.
Isopleth map: A map showing the distribution of data as lines connecting points of equal value.
Item indexing: A means of accelerating logical queries by creating an index for key terms in the database table. The benefits of this approach are similar to finding information quickly in a book by consulting its index.
Item: A column of information in an attribute table.
Land Information System (LIS): A GIS-based system concerned with capturing, storing, analysing and retrieving land and cadastral information. See Cadastre.
Land line: Property boundary maps, in digital form, created by the Ordnance Survey.
Land parcel: A piece of land representing a legal estate. See Cadastre.
Large scale: A large scale map shows small areas e.g. 1:20, 1:500. At a large scale it would be possible, for example, to identify the individual buildings of Kingston University.
Latitude: Angular distance, expressed in degrees, along a line that is parallel to the Equator. Also known as parallel.
Latitude-longitude: A global coordinate system used to measure locations on the Earth's surface. Latitude and longitude are angles measured from the Earth's centre to locations on the Earth's surface. The Greenwich Meridian (passing through Greenwich in England) marks the prime (zero) longitude; while the Equator marks the prime (zero) latitude circumventing the Earth. Latitude measures angles in a north-south direction. Longitude measures angles in an east-west direction.
Lattice: A surface representation with a rectangular array of grid points spaced at constant sampling intervals in the x and y directions. 37 Layer: A thematic plane of GIS features containing geographically and logically related data (such as vegetation types). Layers are the basic components of overlay operations in GIS.
Least-cost path: The path between two points which has the lowest travel cost, where cost is a function of time, distance, or other impedance factors.
Legend: The part of the drawn map explaining the meaning of the symbols and colours used to encode the geographical elements. Also known as map key.
Lifestyle questionnaire: Created by geodemographic companies, these are designed to collect information about our lifestyles or consumer habits. Questions may vary from choice of car to our shopping preferences (say favoured retails outlet, frequency of purchases, spending patterns).
Line: A basic geographical element, defined by two or more points with known (x, y) coordinates. Examples include motorways, streams and cable paths.
Lineage: The history of a dataset including its origin and the processes by which it was derived from that origin.
Line on polygon analysis: A key operation where lines in one layer or coverage are intersected with polygons in another layer or coverage.
Liveware: GIS personnel. People are an important component of any GIS. See also hardware, software.
Local Area Network (LAN): A network arrangement for connecting computers, within one site. The networked computers on a LAN can share data and peripheral devices, such as magnetic storage devices, printers and plotters.
Logical consistency: This term is associated with the relationship between data elements within a dataset. It also relates to the maintenance of those relationships.
Logical operator: One of several words, including AND, OR, NOT and XOR, used to build complex logical expressions in a query. Also known as Boolean operator. 38 Logical order: The order in which operations are performed. Conventionally operations in brackets are performed first.
Longitude: The angular distance east or west from a standard meridian (such as the Greenwich line) expressed in degrees.
Look-up table: A set of data values that can be accessed by a computer programme to convert data from one form to another (for example from numerical values to colours or symbols).
Macro: A script (text) file containing a sequence of commands that can be executed as one command. Macros can be built to perform frequently used or complex operations.
Many-to-one relate: A relational database arrangement in which many records in a table are related by a single record in another table.
Map extent: The rectangular limits of a map, in terms of monitor or printer coordinates.
Map generalisation: The process of reducing detail on a map as a consequence of reducing the map scale. See Generalisation.
Map projection: A geometric model that transforms the locations of features on the Earth's surface to corresponding locations on a two- dimensional surface. It is impossible to project a spheroid perfectly onto a plane; but some projections can preserve shape, while others preserve area, distance, or direction.
Map scale: The relationship between distance on a map and the corresponding distance on the Earth's surface. Map scale is often expressed as a fraction of distance, such as 1:50,000 (where one unit of distance on the map represents 50,000 of the same units of distance on Earth surface). Map scale can also be expressed as a statement of equivalence using different units, for example, 1 metre = 3 kilometres.
Map: An abstract representation of geographical features using recognisable symbols. Maps often highlight (generalise) features, depending on what they are intended to illustrate. For example, a road atlas may not show navigable canals, and a motorway map may not show all side roads.
Merge operations: The process of aggregating areas by removing common boundaries. See also Dissolve. 39 Meridian: A line running vertically from the North to the South Pole. The Prime Meridian runs through Greenwich, England, where measures of longitude are negative to the west and positive to the east of Greenwich. See Latitude and Longitude.
Metadata: Data that describe the nature of a data set, in terms of accuracy, extent, copyright and restrictions. There is growing interest in metadata, because of increasing interest in data quality.
Metes and Bounds: A method of land surveying in which the boundary lines of land parcels are entered in terms of relative bearing and distance (from one station to the next). See Coordinate Geometry (COGO).
Model: An simplified representation of reality used to describe, analyse, predict or simulate a process, using a set of rules and procedures.
Module: A distinct piece of software that can be called from other parts of a computer program. Modules are designed to reduce the bulk of large software systems, and reduce running times.
Mouse: A hand-controlled computer device for interacting with a computer terminal or entering data.
Multispectral scanner (MSS): A scanning system on some satellites used for imaging the earth. A MSS image will have data recorded by the scanner from several wavebands of the electromagnetic spectrum.
National Grid: 1. a superimpsoed grid, with a known origin, developed for particular regions or countries. It allows the precise description of any position in it. See also Grid. 2. A term used to describe the electricity transmission lines in a country.
National Transfer Format (NTF): British Standard BS 7567, which defines an exchange format for the transfer of vector data. NTF is the main format used by the Ordinance Survey.
Network: A system of connected geographical features, such as roads, pipes, or cables. A network structure enables the calculation of optimal routes or the simulation of flow through rivers or pipes.
Network analysis: A set of techniques to study the properties of networks in order to simulate flows of traffic or water. Network 40 analysis is often employed to calculate shortest (or lowest cost) paths between two points in a network.
Node: The location at which two or more lines connect. Nodes can carry information about the topology of a network or adjacent polygons.
Normalisation: A conceptual database task designed to avoid data redundancy. A table in a relational database is said to be in normal form if it satisfies certain constraints. There are five generally accepted steps of normalisation. First Normal Form eliminates repeating groups by putting each into a separate table and connecting them with a one-to-many relationship. Second Normal Form eliminates functional dependencies on a partial key by putting the fields (columns) in a separate table from those that are dependent on the whole key. Third Normal Form eliminates functional dependencies on non-key fields by putting them in a separate table. At this stage, all non-key fields are dependent on the key, the whole key and nothing but the key. Fourth Normal Form separates independent multi valued facts stored in one table into separate tables. Fifth Normal Form breaks out data redundancy that is not covered by any of the previous normal forms. See Redundancy.
Null value: The equivalent of saying that there is no value specified.
Object Linking and Embedding (OLE): A Microsoft standard, which allows objects from one application to be embedded within another (for example, taking a Map Object and embedding it into a Windows application like Excel).
Object Oriented: A data created approach to database modelling based on objects and object classes. Object oriented data are encapsulated with operations and commands. See also State.
One-to-many: A relate in which one record in a table is related to many records in another table, within a relational DBMS.
Online access: Direct access to data that does not involve file transfer. In practice, this means real-time interaction between a server and end user, or between a computer and a device (such as a printer) on the network.
Open Database Communication (ODBC): A Microsoft API (Application Programme Interface) which enables communication between different database systems. 41 Open Systems GIS. See Interoperability.
Open Systems GIS (OGIS): The Open Systems GIS Consortium (OSGC) is an association of GIS developers, academics and government organisations, set up to define and encourage open systems standards for the GIS industry. The consortium’s main objective is to facilitate OGIS (Open Geodata Interoperability Specification) between GIS software and data formats. See Interoperability.
Open Look: A GUI (Graphical User Interface) for the X Windows system developed by Sun Microsystems. The interface is used mainly to emulate UNIX-based work on PC’s. See X Windows.
Open Software Foundation (OSF): An international consortium which promotes the UNIX operating system, and defines its technical standards.
Operating system (OS): Computer software designed to allow communication between the computer and the user, and between the computer and the devices attached to it. The operating system controls the organisation of files and directories, the flow of data between the computer and other hardware and the execution of commands and software. DOS (Disk Operating System) and Windows are the two most popular operating systems for PC's. Other operating systems include VM and MVS (for IBM mainframes), VMS (for DEC machines), UNIX (including AIX, Ultrix, and UNICOS), OS/2 (for PC's), and Apple OS.
Optical Character Recognition (OCR): The automatic recognition of text, using scanning devices.
Optical disk: A storage technology that uses optical media to store data. Optical disks are slower than magnetic disks, but store more data and cost less per unit of stored data. Optical disks are often used to store very large amounts of data.
ORACLE: A popular, corporate relational DBMS.
Ordnance Survey (OS): Great Britain's national public mapping agency.
42 Overlay: Another name for a layer or coverage. More specifically, an overlay refers to an additional layer superimposed on the base layer. See Layer.
Overlaying: The process of stacking data layers on top of each other, so that geographical position in one theme layer can be analysed in relation to other themes for the same position.
Packed binary: A method of data indexing and compression used with raster data. Another description of run length encoded.
Panning: Moving the viewable portion of a displayed map around the screen, in order to display map areas which lie outside the viewing window.
Parcel: A basic cadastral (property) unit. See Cadastre.
Path: A continuous set of network links and nodes.
Pathfinding: The process of finding a path between an origin and destination, on a network.
Peripheral device: A hardware device that is not part of the main computer unit, such as digitisers, printers, zip drives and external modems.
Photogrammetry: The use of stereo pairs of aerial photographs (or satellite images) to produce geometrically correct maps.
Pixel: A shortened word for picture element, the basic grid-cell unit in raster image files (images). See Grid cell.
Planar co-ordinate system: Two dimensional (Cartesian) representation of spatial data.
Platform: A synonym for computer, especially powerful computers capable of complex processing.
Plotter: A printing device used to print out maps with high geometric precision in terms of coordinates, especially for engineering purposes.
Point: A geographical object, with a single (x, y) coordinate, used to designate such features as schools, wells, trees, etc.
43 Point in polygon analysis: A key operation in GIS where calculation involves assessing whether points fall within polygons.
Polygon on polygon analysis: A key operation in GIS where polygons in one layer or coverage are intersected with polygons in another layer or coverage. The calculation is made using geometric techniques.
Polygon: A feature used to represent areas defined by a boundary and an associated identifier located within the polygon. Attributes may be related to the identifier.
Positional accuracy: This refers to the closeness of locational information (usually co-ordinates) to their real world position.
Precision: A measure of the tendency of digitised or mapped geographical objects to cluster, measured by root mean square error (RMSE) or the standard deviation (SD). Note that high precision does not necessarily indicate high accuracy, because precisely repeated spatial errors would not improve accuracy no matter how often they have recurred. The term precision is also used to refer to the number of significant digits used to store numbers and map coordinate values. See Accuracy
Projection: See Map projection.
Protocol: A set of rules or standards which control communication between computers and between computers and peripherals.
Quadtree: A compact and efficient spatial indexing and data compression system based upon the recursive division of cells into quadrats.
Query: A statistical or logical expression used to highlight or analyse a set of data. See Logical operator.
Raster: A regular grid of cells covering an area, usually in the form of a scanned image (such as a satellite image or scanner-captured aerial photograph).
Raster-vector conversion: The process of converting a raster image into a vector map.
RDBMS: See Relational Database Management System.
44 Real number: A numbers with decimal digits. See Integer.
Real time systems: GIS can be used to display specific locations as they occur at current points in time. A GPS could be fitted to a vehicle, for example, and the location of that vehicle could be recorded in the GIS as it moved on its journey. See also GPS.
Record: An individual or set of related data entries within a database. Attribute tables are made up of a number of data records.
Rectification: The process of geometrically correcting an image, according to a set of real-world coordinates. Rectification usually involves the rotation and re-scaling of the original image. The process is also known as image resampling.
Redundancy: The (unnecessary) duplication of data in a database.
Reference ellipsoid: See Geodetic datum.
Relate key: The common attribute (column) used to relate two databases.
Relate: An operation that establishes a connection between corresponding records in two tables, in a relational database.
Relational DataBase Management System (RDBMS): A database management system based on using several tables, connected through one or more common columns in the tables to be linked. Therefore, the data in a relational database must be structured as collections of tables that are logically associated with each other by shared attributes. A relational database arrangement allows separate tables to be linked as and when the need arises, thus eliminating the necessity of storing all attribute data in one very large table.
Remote sensing: Acquiring information about an object at a distance, using aerial photography, radar, and satellite imagery.
Resampling: See Rectification, Rubber sheeting.
Resolution: The size of the smallest feature that can be detected or mapped on a scanned or digitised image (or monitor display). The larger the map scale of the image, the higher the resolution. Also known as visual definition.
Root mean square error (RMSE): See Precision. 45 Route: A continuous set of links in a network.
Rubber sheeting: A computer-based geometric process used to edge match mis-aligned image or map tiles. The term is also used to refer to the warping or resampling of images. See Rectification.
Run Length Encoding (RLE): A method of data index and compression used in raster GIS. Data are stored in ‘runs’ of identical cell values. See also Packed Binary.
Satellite data: Data gathered by space borne sensors that detect the Earth’s surface reflectance across the electro-magnetic spectrum. See also LANDSAT, SPOT.
Scale: The relationship between the size of an object on a map or image, and its size in the real world.
Scanner: A data capture device for converting images or vector shapes into digital form automatically. Some scanners are also able to convert raster data into vector data.
Scanning: The process of converting images into digital form automatically, using a scanner. It is usually part of the digitising process.
Segment: Part of a line that is described by a set of co-ordinates. See also Vertex.
Sensitivity analysis: A set of procedures used in a prototype GIS project to estimate the level of accuracy which can be achieved by a GIS, relative to a set of test objectives. The term is also used in a different context in exploratory data analysis.
Shape file: A proprietary ARC/INFO file format.
Single precision: A level of accuracy based on the number of significant digits that can be stored for each coordinate. Single- precision numbers store up to 7 significant digits for each data entry.
Sliver polygon: A small overlap area, or gap, between the borders of polygons, which results from errors in overlaying or edge-matching several map tiles.
46 Slope: A measure of change in surface value over distance, expressed in degrees or as a percentage.
Small scale: A small scale map shows large areas e.g. 1: 625,000. A t a small scale Kingston University buildings would not be visible, rather the town of Kingston-upon-Thames would be represented (probably by a symbol).
Snapping: The process of getting two geographical features, which are meant to be linked (such as a node and a line), to touch one another.
Spaghetti data: A series of lines and points which are not topologically structured or organised as objects. No relationships are recorded. See also Unlinked data.
Spatial data: Data about the location of geographical features. Postcode is an example of spatial data.
Spatial query: See Map query.
Spike: An overshot line, or an anomalous data point, created erroneously during digitisation.
Spline: A mathematically calculated curve to smooth an abrupt change in the direction of a continuous line.
SPOT: A common abbreviation for Système Probatoire de l’Observation de la Terre, a reconnaissance satellite system with high (10-30 metre) resolution launched by France in 1986.
State: A term used in Object Oriented GIS to describe an object. The state may contain information about an object’s local variables as well as a set of operations or methods that are associated with that object.
String: A series of alphanumeric characters.
Structured Query Language (SQL): A computing language for defining and manipulating data from a relational database, which has become an industry standard for querying in most DBMS.
Surface data: Geographical data can also represent surface information. Usually it is recorded as continuous data using a model built from regular or irregular spaced sampling points. See also TIN, DEM. 47 Surface model: See Digital Elevation/Terrain Model.
SYBASE: A commercial relational DBMS.
Symbol: A graphic pattern used to represent a feature in GIS and cartography.
Table: A set of data elements organised into rows and columns, within a DBMS.
TCP/IP: The common acronym for Transmission Control Protocol / Internet Protocol, which are the communication protocols that enable computers to send and receive data across a network (including the Internet).
Temporal GIS: This describes a GIS that has been used to measure or record information over time. See also Real time GIS.
Terminal: A computer unit acting as part of a network that is served by a more powerful (server) computer.
Terrain analysis: See Digital Terrain Model (DTM), Spatial analysis/modelling.
Tessellation. The process of splitting a map region into several (matching) tiles.
The Object Management Group (OMG): A computing industry group interested in promoting object-oriented interoperability among different computing systems. OMG has also developed the specifications required to make object-oriented interoperability work, known as Common Object Request Broker Architecture (CORBA).
Thematic map: A map displaying information relating to specific geographical themes, such as soil types, population density, etc.
Thiessen polygons: The boundaries of Thiessen polygons are defined by the perpendicular bisectors of the lines between all neighbouring points. Thiessen polygons are generated from a set of irregular points, and are designed to interpolate a continuous surface from an incomplete set of spatial data.
TIFF: The common abbreviation for Tagged Interchange File Format, an industry standard graphics data format. 48 Tile: A subset of a GIS database or map that contains information about one subset (usually, one sub-area) of the entire map, designed to save access and processing time.
Tolerance: A distance used, generally, to adjust data errors within a GIS. See also Undershoot, Overshoot errors.
Topographic map: A terrain (relief) map, often in the form of a contour map.
Topology: The spatial relationship between geographical objects. Topological properties include order, connectivity and neighbourhood (adjacency). Topological relationships are built from simple elements into complex elements: points (simplest elements), lines (sets of connected points), polygons (areas enclosed by connected lines), and routes (sets of route sections). Topology is an important prerequisite for sophisticated geographical analysis, such as network analysis and contiguity analysis.
Total station: A survey instrument able to measure distances and angles using infrared beams bounced off reflectors that can store the data in a digital format (eg as points and line co-ordinates).
Transformation: The process of converting coordinates from one coordinate system to another, which usually entails changing the orientation, scale or projection of a mapped area.
Translation: The process of converting data or commands from one GIS format to another, for example from DXF to shapefile.
Triangulated Irregular Network (TIN): A surface representation derived from irregularly spaced sample points and features, marked by Thiessen polygons. See Thiessen polygons.
Tuple: A row in a relational database.
Undershoot: A digitised line that does not intersect with another node or line, because it is too short. See Dangle.
Union: An overlay of two data layers, where all features from both layers are retained.
Unique Property Reference Number (UPRN): A subset of the of BLPU (Basic Land and Property Unit) reference system, used in 49 the address gazetteer adopted in the UK since 1984 (known as BS7666). See Gazetteer, Basic Land and Property Unit (BLPU).
Universal Transverse Mercator (UTM): Global coordinate System and map projection which provides locational descriptions accurate to 1 metre (except the north and south poles).
Unix: A general-purpose, multi-user computer operating system.
Unlinked data: Vector data stored as a collection of lines and points with no recorded relationships. See also Spaghetti data.
USGS: United States Geological Survey, one of the US’ main mapping agencies.
USGS digital elevation models: Surface models produced by the Survey Branch of the United States Department of the Interior.
Vector: A coordinate (x,y) based data structure commonly used to represent linear geographical features. In vector format, spatial objects are defined by points and lines. See Discrete data.
Vertex: One part ,or segment, of a line usually recorded as a set of co-ordinates. See also Segment.
Viewshed: This can be used by a GIS visualise a given area from a particular place, or point, on the digital map.
Visibility analysis: The process of creating viewsheds or representations of surface data for the purposes of visibility. Used extensively in telecommunication and military organisations.
Warping: See Rectification.
Wide area network (WAN): Computer network that connects computers at remote sites (not in the same location), often through telephone links.
Wide-Area Information Server (WAIS): A networked client/server software system that provides sophisticated file indexing based on all text entries in each file. Searching can include Boolean expressions.
Workspace: A GIS directory containing path references to the set of maps and tables that a user is working on. 50 World Wide Web: A global computer-based network, developed by the European Laboratory for Particle Physics (CERN) in Switzerland. See Internet.
X Windows/Motif: A system that allows applications to be shared among different workstations and terminals (irrespective of the operating system used on the individual computers). Also known as OSF (after the Open Systems Foundation).
Zooming: Enlarging a visual display on a computer monitor.
Z-value: Usually a reference to the value of surface height in a Digital Elevation/Terrain Model at a particular (x, y) location.
51 Section 3
Major GIS Acronyms
52 ACSM: American Congress on Surveying and Mapping AGI: Association for Geographic Information AI: Artificial/Amplified Intelligence AM/FM: Automatic Mapping / Facilities Management AML: Arc Macro Language ANSI: American National Standards Institute ASCII: American Standard Code for Information Interchange AV: ArcView (ESRI software) AVL: Automatic Vehicle Location
BIOS: Basic Input / Output System BLOB: Binary Large OBject BLPU: Basic Land and Property Unit BM: Bench Mark BMP: BitMaP BSI: British Standards Institute
CAD: Computer Aided Drafting, Design, or Delivery CAL: Computer Aided Learning CAM: Computer Aided Mapping/Manufacturing CASE: Computer Aided Software Engineering CCT: Computer Compatible Tape CD: Compact Disk CD-ROM: Compact Disk-Read Only Memory CEN: Comité European Normalisation CGI: Common Gateway Interface CLIENT: A software application which interacts with a server computer COBOL: COmmon Business Oriented Language COGO: COordinate GeOmetry COORDS: x,y coordinates (or x,y,z for three dimensional models) CPU: Central Processing Unit
DB: DataBase DBMS: Database Management System DCM: Digital Cartographic Model DDE: Dynamic Data Exchange DEI: Data Extraction Interface DEM: Digital Elevation Model DIGEST: Digital Geographic Information Exchange DIME: Dual Independent Map Encoding System DIR: DIRectory DLG: Digital Line Graph DLL: Dynamically Linked Library 53 DLM: Digital Landscape Model DN: Digital Number DOS: Disk Operating System DPI: Dot Per Inch DTM: Digital Terrain Model
ED: Enumeration District EIA: Environmental Impact Assessment EIS: Environmental Impact Statement EISA: Extended Industry Standard Architecture EOS: Earth Observation Satellite EPS: Encapsulated PostScript ES: Expert System ESRI: Environmental Systems Research Institute Inc (developers of ARC/INFO) EXE: EXEcutable file
FORTRAN: FORmula TRANslation (programming language) FTP: File Transfer Protocol
GB: GigaByte GIF: Graphics Interchange Format GIS: Geographical Information Systems GPS: Global Positioning System GUI: Graphical User Interface
HMLR: Her Majesty’s Land Registry HP: Hewlett-Packard HTML: Hyper Text Markup Language HTTP: Hyper Text Transport Protocol HW: HardWare
I/O: Input/Output IE: Internet Explorer (Microsoft browser) IEEE: Institute of Electrical and Electronics Engineers IMS: Information Management System IP: Internet Protocol ISDN: Integrated Services Digital Network ISO: International Standards Institute ISPRS: International Society of Photogrammetry and Remote Sensing
KB: Kilo-Byte KCGIS: Kingston Centre for GIS
54 LAN: Local Area Network LANDSAT: Land Satellite LIS: Land Information System LUT: Look Up Table
MB: Mega-Byte MHz: Mega-Hertz MIS: Management Information System MO: MapObject MODEM: MOdulator-DEMolulator MOIMS: MapObjects Internet Map Server
NA: Network Analysis NG: National Grid NC: Network Computer NDVI: Normalised Difference Vegetation Index NET: NETwork NIC: Network Interface Card NFS: Network File System NLIS: National Land Information Service NOAA: National Oceanographic and Atmospheric Administration NSF: National Science Foundation NT: New Technology (in Windows NT) NTF: National Transfer Format (UK)
OCR: Optical Character Recognition ODBC: Open DataBase Connectivity OGIS: Open Geodata Interoperability Specification OLE: Object Linking and Embedding OLTP: OnLine Transaction Processing ONS: Office for National Statistics (UK) OO: Object Oriented OODBMS: Object Oriented Data Base Management System OS: Operating System, Ordnance Survey (UK)
PAF: Postcode Address File (UK) PC: Personal Computer PERL: Practical Report and Extraction Language (in UNIX) PIXEL: Picture element PS: PostScript
RAM: Random Access Memory RDBMS: Relational DataBase Management System RGS: Royal Geographical Society (UK) RIMS: Road Information Management System 55 RGB: Red, Green, Blue RISC: Reduced Instruction Set Computing
SAS: Statistical Analysis System SCSI: Small Computer System Interface SDE: Spatial Database Engine SIMS: Survey Information Management System SIS: Spatial Information System SPOT: Système Probatoire d'Observation de la Terre. Also, Societé Pour l’Observation de la Terre SPSS: Statistical Package for the Social Sciences SQL: Structured Query Language STORET: The Storage and Retrieval System SVGA: Super Video Graphics Adapter SW: SoftWare
TCP/IP: Transport Control Protocol / Internet Protocol TELNET: TELetype NETwork TIFF: Tagged Image File Format TIGER: Topologically Integrated Geographic Encoding and Referencing (US) TM: Thematic Map(ping) Also LANDSAT Thematic Mapper UI: User Interface UPRN: Unique Property Reference Number (UK) URL: Uniform Resource Locator UTM: Universal Transverse Mercator
VB: Visual Basic VGA: Video Graphics Adapter VR: Virtual Reality VRML: Virtual Reality Modelling Language WAN: Wide Area Network WWW: World Wide Web
56 Section 4
Recommended Further Reading
57 Paul A. Longley, David J. Maguire, Michael F. Goodchild, David W. Rhind (Editors) (1999). Geographical Information Systems : Principles, Techniques, Management and Applications. Wiley, John & Sons, Incorporated.
The second edition of the definitive GIS reference work. The highly-successful first edition has been completely restructured and rewritten by nearly 100 world authorities from academic, commercial, and government backgrounds. This book takes a contemporary look at the key issues in GIS, identifying and tackling the important challenges in implementing GIS and applying it in the real world.
D. Ian Heywood Sarah Cornelius Steve Carver Ian Heywood (1998). An Introduction to Geographical Information Systems. Prentice Hall
An up-to-date introduction to the world of Geographical Information Systems. It is designed to be easy to use for students at all levels, from undergraduates to professionals retraining in GIS. The book focuses on the practical applications of GIS, and considers how the technology works. Each chapter addresses a specific theme and is backed up with examples, case studies, directed reading and additional. The book can be used to give the reader a quick tour through the world of GIS, to help the reader develop a thorough understanding of GIS or as a source of reference information. Introduction to Geographical Information Systems is an ideal text for undergraduates, postgraduates and professionals following courses in GIS.
Molenaar, Martien (1998). An Introduction to the Theory of Spatial Object Modelling for GIS. Taylor & Francis
This book provides a background view of the object approach, related geometric aspects, the different level of data modelling and the role of geo-information theory. The rest of the book builds upon these concepts describing the applicability of broader aspects such as topology, thematic object classes and class hierarchies, object aggregation and generalisation and uncertainty.
Jan Van Sickle (1996). GPS for Land Surveyors. Sleeping Bear Press
58 An introduction to the Global Positioning System (GPS) and a practical guide to the techniques used in a GPS survey. Van Sickele, a surveyor himself, simplifies the mathematics while still presenting the most important elements of the GPS design and technology, covering biases and solutions, receivers and methods, coordinates, planning a survey, observation, and postmission processing.
Karen Steede-Terry, Jack Dangermond (2000). Integrating GIS and the Global Positioning System with CD-ROM. Environmental Systems Research Institute (ESRI)
The integration of Global Positioning System (GPS) with Geographic Information System (GIS) is an information revolution that gives unprecedented accuracy in measurement and completeness of coverage to anyone who works with geographic data of any kind. This book examines the questions involved in combining these two systems. It provides information about how GPS works, the pros and cons of implementing GPS use, what is the best GPS system for specific applications, and how to integrate the technology and equipment needed to use GPS with existing GIS systems. Real-life case studies give users the opportunity to consider every aspect of systems integration before the process is started. Includes CD-ROM.
Bon A. DeWitt Paul R. Wolf (2000). Elements of Photogrammetry (with Applications in GIS). McGraw-Hill Higher Education
This book incorporates recent changes on the subject of streamlining from advances in computers. Their ever increasing speed and storage capabilities have directly led to an entire new approach in photogrammetric mapping known as “Soft-Copy” photogrammetry. Digital Imaging systems, including those used in modern satellite programs, scanners for digitising photographic images, and digital image processing techniques are new topics to be covered that are fundamental to soft copy photogrammetry.
Andrew Skidmore, Hendrik Prins (Editors) (2001). Environmental Modeling with GIS and Remote Sensing. Routledge
59 This book derives from a training course run by ITC (Holland) for professionals and managers in the environmental sciences, detailing the applications of remote sensing and GIS for environmental monitoring, modelling and assessment. It sets out current research results and provides operational methods for environmental mapping and monitoring.
J. Stillwell, S. Geertman, S. Openshaw (Editors) (1999). Geographical Information and Planning. Springer-Verlag
This book examines developments in the use of information systems and modelling methods in different spatial/geographical planning contexts. It provides a review of the 1990s and offers detailed insights into the application of geographical information technology in urban, physical, environmental and socio-economic planning. Important advances in the use of the Internet for access to data and to GIS for planning are also considered. Perspectives are provided by a wide range of contributors from academia, planning practice and the business world.
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