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Georeferencing Georeferencing How do we make sure all our data layers line up ? Georeferencing: = linking a layer or dataset with spatial coordinates Registration: = lining up layers with each other Rectification: The process by which the geometry of an image is made planimetric Georeferencing • ‘To georeference’ the act of assigning locations to atoms of information • Is essential in GIS, since all information must be linked to the Earth’s surface • The method of georeferencing must be: – Unique, linking information to exactly one location – Shared, so different users understand the meaning of a georeference – Persistent through time, so today’s georeferences are still meaningful tomorrow Georeferencing Based on Data Types • Raster and Raster • Vector and Vector • Raster and Vector Geocoding Concepts and Definitions Definition of Geocoding • Geocoding can be broadly defined as the assignment of a code to a geographic location. Usually however, Geocoding refers to a more specific assignment of geographic coordinates (latitude,Longitude) to an individual address.. UN Report Definition of Geocoding • What is Geocoding • Geocoding is a process of creating map features from addresses, place names, or similar textual information based on attributes associated with a referenced geographic database, typically a street network that has address ranges associated with each street segment or 'link' running from one intersection to the next. Definition of Geocoding • What is Geocoding • Geocoding typically uses Interpolation as a method to find the location information about an address. – (If the address along one side of a block range from 1 to 199, then Street Number = 66 is about one-third of the way along that side of the block.) • Data required: – Reasonably clean, consistent list of legal addresses (i.e. not too many typos, addresses really exist, etc.) – Address range attributes on a linear street network » Most commonly from Census » More current/cleaner data available from private vendors • Georeferencing vs Geocoding • Georeferencing – Aligning geographic data to a known coordinate system so it can be analyzed, viewed, and queried with other geographic data • Geocoding – The process of assigning geographic codes to features in a digital database (including the GIS operation for converting street addresses into spatial data that can be displayed as features on a map) Data Collection Methods • Two main methods: – Direct Collection Approach – Matching Approach Direct Collection Approach • Digitizing from available topographic maps • Direct collection using field techniques (ex.GPS) Digitizing from a topographic map Global Positioning System (GPS) Areas, Street, Dwelling Matching approach • Using an Address locator database and street network database in a GIS • Joining an address database to an existing spatial database for the area of interest Street Network Avenue First Street Segment First Avenue First Left of Street Left of Street #1 #51 #99 #1 address number #99 Second Avenue Second Main Street Avenue Second #2 #32 #100 #2 #100 Right of Street Right of Street Nodes Address Databases Address Database –Another Geocoding Definitions • Address Geocoding - Assigning X,Y coordinates to tabular data such as street addresses or zip codes so that they may be displayed as points on a map. • Address Parity - Evenness or oddness. In address geocoding, parity is used to locate an address on the correct side of the street. – Such as, odd numbers on the left side; even on the right... Street Address Locations (Lines) • Single-Field Range Geocoding Style • Dual Range (with Parity) Address Geocoding • Street Centerlines are typically attributed with corresponding Address Ranges and Street Name. Zip Code Locations (Points) • Postal Codes such as 5-digit zip codes can be matched with address databases. • The centroid of the Zip Code area may be used to determine the X,Y coordinate. • Interpolation of +4 Zip Codes along street centerlines Place Names or Addresses (Polygons) • Tax Assessor or Parcel Databases • Place Names such as City or State names may also be used to “Locate” non-graphic attribute records. • Map labels are common “Locators” for other data. The Postal Address • Single most common form of geographic information • May be “geocoded” to an X,Y coordinate location using a “Locator File” containing points, lines or polygons with similar address “ranges” or attributes. • Street Centerlines are “interpolated” based upon Address Range (and Parity) to determine a single X, Y location. Locating Addresses • Place Names, Buildings (or Vanity Addresses) are located by linking the non- graphic table to the “spatially referenced” or graphic data. • Correct Spelling and/or sensitivity settings are important when “matching attributes.” • Street intersections are also common “Locators” Network Analysis What is a network? • A network doesn't have to be streets or roads, although they are probably the networks you are most familiar with. Networks can represent rivers, pipelines, and utilities. The route doesn't actually have to exist in the real world as a set of linear features. An airline route or a course charted for a ship can also be represented as a network. • Because a network is a set of interconnected lines, you can model the movement of goods, services, energy, information, or even people throughout the network. What is a network? What is a network? Any system of interconnected linear features What is network analysis? •Solving problems involving networks •Goal is efficiency – Saving time and money What it can do? Preparing Network dataset Find the optimal path • Using Network Analyst, you can find the path that will reach specified locations in the most efficient order. The path can be the shortest path or the fastest path. Network Analyst also gives you the option to return to the point of origin when defining a path. • Network Analyst provides you with directions for navigating a route. The directions can be customized to include different units, such as time or distance, or landmarks to help you find your way while navigating the route. Modeling accessibility • Accessibility is a measure, usually in units of distance or time, of how easy it is to get to a location or locations. • For example, 12,350 people may be able to drive to the hospital emergency room within 12 minutes; • 2,125 potential customers may live within a 1.5 kilometer (walking) distance of a shopping center; and a fire truck can reach 3,000 homes and businesses within 7.5 minutes. • The simplest way to model accessibility is to use a straight-line distance or radius that extends outward from a specific location. • While this may be useful if you're in an airplane or a boat, it doesn't reflect the way people move from one location to another on land. Finding the Best Route • Finds the route that minimizes travel cost through a series of stops • Options • Cost Attribute (kms, minutes, etc) • Find best order • Time windows • Cost on stops • Directions Finding the Closest Facility Finds the routes that minimizes travel cost between incidents and multiple facilities Options • Cost Attribute • Number of facilities to find • Cut-off value • Direction of travel • Costs on incidents and facilities • Directions Finding Service Areas Find the area or lines that can be traversed within a specified cost • Create polygons around specified locations • Create service area lines Options •Cost Attribute •Multiple Break values •Direction of travel •Polygon generation options •Line generation options More Network Analysis Options • Other parameters include • Barriers • U-Turn policy • Restrictions • Exact route vs. Hierarchical route Exact route Hierarchical route (major roads) Finding an Origin-Destination Cost Matrix Generates an “OD” matrix of the cost from each origin location to each destination location. Options • Cost Attribute • Cut-off value • Number of destinations to find.
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