Gis Data Models

GIS DATA MODELS CONCEPTUAL MODEL OF SPATIAL INFORMATION 1. Introduction GIS does not store a map in any conventional sense, nor it stores a particular image or view of geographic area. Instead, a GIS stores the data from which we can draw a desired view to suit a particular purpose known as geographic data. There are two types of data in GIS; spatial data and non-spatial data (Attribute data). Non-spatial data include information about the features. For example, name of roads, schools, forests etc., population or census data for the region concerned etc. Non-spatial or attribute data is that qualifies the spatial data. It describes some aspects of the spatial data, not specified by its geometry alone. A geographical information system essentially integrates the above two types of data and allows user to derive new data for planning. Spatial models are important in that way in which information is represented affects the type of analysis that can be performed and the type of graphic display that can be performed and the type of analysis that can be obtained. In GIS systems there is a major distinction between what are usually referred to as vector GIS and raster GIS. These two approaches to spatial data processing, often to be found in the same GIS package, reflect two different methods of spatial modeling: the former focusing on discrete objects that are to be described, and the latter concerned primarily with recording what is to be found at a predetermined set of locations that may be grid cells or points. 2. Spatial Information Spatial characteristics of information can be broadly distinguished between those that describe where things are, using locations consisting of reference positions, spatial units and spatial relationships; those that describe the form of phenomena, using qualitative and quantitative descriptions of shape and structure; and those that describe associations and interactions between different phenomena. All geographical data can be reduced to three basic geographical phenomenon can in principle be represented by a point, line or area plus a label saying what it is. So an oil well could be represented by a point entity consisting of a XY coordinate; a road could be represented by a series of XY coordinates; a floodplain could be represented by an area entity covering a set of XY co-ordinates plus the label ‘floodplain’. The labels could be the actual names as given here, or they could be special symbols. The essential features of any data storage system are that they should be able to allow data to be accessed-and cross-referenced quickly. There are several ways of achieving this, some of which are more efficient than others. Unfortunately, there seems to be no one 'best' method that can be used for all situations. This explains in part the massive investment in labour and money in effective database management systems, which are the computer programs that control data input, output, storage, and retrieval from a digital database. Figure 1: Real world phenomena represented as three basic entities 3. Layers and Coverages The Common requirement to access data on the basis of one or more classes has resulted in several GIS employing organizational schemes, in which all data of a particular level of classification, such as roads, rivers or vegetation types, are grouped into layers or coverages (refer Figure 3.). GIS organize spatial data into layers. Typical layers represent information belonging to particular classes. The layers can be combined with each other in various ways to create new layers that are a function of the individual ones. Any layer does not contain any areal regions that are overlapping, therefore it is possible for each region to have multiple attributes corresponding to multiple perspectives on the meaning of that region. Figure 2. Layers and Coverages 2 4. Data Model In order to represent the spatial information and their attributes, a data model – a set of logical definitions or rules for characterizing the geographical data is adopted. The data model represents the linkages between the real world domain of geographical data and the computer and GIS representation of these features. As a result, the data model, not only helps in organizing the real-world geographical features into a systematic storage/retrieval mechanism, but also helps in capturing the user’s perception of these features. The model : a) Structures the data to be amenable to computer storage/retrieval and manipulation. The data structure is the core of the model and it is based upon this that features of real world are represented. The ability of the data structure to totally represent the real world determines the success of the model. b) Abstracts the real world into properties, which is perceived by a specific application. For example, a Landuse map is perceived to be made up of different classes with symbols and legends. The district information is perceived to be made up of district maps and different attribute tables. c) Helps organize a systematic file structure, which is the internal organization of real world data in a computer. 5. Conceptual Models of Spatial Information There are different models, which have influenced the way in which data are organized and processed within GIS. They are based respectively on objects, networks and fields. Object-based model: Object-based spatial models emphasize individual phenomena that are to be studied in isolation or in terms of their relationships with other phenomena. Any phenomena however big or small may be designated as an object, provided that it can be separated conceptually from neighboring phenomena (refer figure 4). Objects may be composed from other objects and they may have specific relationships with other separate objects. An object-based view is appropriate, though not confined, to phenomena that have well defined boundaries. Hence it is suited to human made phenomena, such as buildings, roads, utilities and administrative regions. Some natural phenomena, such 3 as lakes, rivers, islands and forests, are often represented in object based models because they need to be treated as discrete phenomena for some purposes. Figure 3. The Object based Conceptual View Networks Model: Network based spatial models share some aspects of the object based model in that they often deal with discrete phenomena, but the essential characteristic is the need to consider interactions between multiple objects, often along discrete paths or routes that connect them. The exact shape of the phenomena may not be of much importance. What is more important is some measure of distance, or the impedance, between specified phenomena. Typical examples of applications for which a network model is appropriate are studies of traffic on road sea and air route; and analysis of flow of water, gas and electricity through pipelines and cables (refer figure 4). Fields Model: The field based view is appropriate for modeling phenomena that are regarded as continuously variable across some region of space. Examples of phenomena that may be treated as fields include the concentration of pollutants in the atmosphere, temperature of the ground surface, the moisture levels in soil, and the speed and direction of flow in bodies of air and water. Fields may represent either two or three spatial dimensions, depending on the application. A field based spatial model is often adopted when the data to be modeled are not known in sufficient detail to provide precise boundaries, even though at some resolution they could be said to exist (refer figure 4). 4 5 Figure 4. Conceptual models of spatial Data 6. Spatial Data models for GIS: Representation in computer When Geographical data are entered into a computer the user will be most at ease if the geographical information system can accept the phenomenological data structures that he has always been accustomed to using. But computers are not organized like human minds and must be programmed to represent phenomenological structures appropriately. Moreover, the way the geographical data are visualized by the user is frequently, not the most efficient way to structure the computer database. Finally, the data have to be written and stored on magnetic devices that need to be addressed in a specific way. Geographical information systems provide methods for representing spatial data that allow the user to adopt conceptual models resembling to a large extent the three classes of model as discussed above. There are two broad categories of spatial data models. These are vector data model and raster data models. The data base concept is central to a GIS and is the main difference between a GIS and drafting or computer mapping systems, which can produce only good graphic output. All contemporary Geographic Information system incorporates a data base management system. Data base systems provide the means of storing a wide range of geographic information and Updating it without the need to rewrite program. In GIS, the spatial data models handle where the features are and Non- spatial data models or Data base management system handle the feature description and how each feature is related to other. Two approaches or models have been widely adopted for representing the spatial data within GIS ; The cartographic map model and the geo-relational model. Each of these approaches is based on a specific spatial data model. The Composite Map Model is usually based on a tessellated (raster) representation of space and the Geo-relational model is usually associated with a vector representation of space. Vector data model represents phenomena in terms of the spatial primitives, or components, consisting of point, line, polygon, surfaces and volumes. Raster data model represents phenomena as occupying the cells of a predefined, grid shaped tessellation. 6.1 Raster Spatial Data Model 6 Typically the grid-cell tessellation, widely known as raster structure is the most commonly adopted structure in a GIS package.

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