Georeference

From Wikipedia, the free encyclopedia

To georeference something means to define its existence in physical space. That is, establishing a relation between raster or vector images to map projections or coordinate systems. This procedure is thus imperative to data modeling in the field of geographic information systems (GIS) and other cartographic methods. When data from different sources need to be combined and then used in a GIS application, it becomes essential to have a common referencing system. This is brought about by using various georeferencing techniques.

[edit] Need

Georeferencing is crucial to making aerial and satellite imagery, usually raster images, useful for mapping as it explains how other data, such as the above GPS points, relate to the imagery.
Very essential information may be contained in data or images that were produced at a different point of time. It may be desired either to combine or compare this data with that currently available. The latter can be used to analyze the changes in the features under study over a period of time.
Different maps may use different projection systems. Georeferencing tools contain methods to combine and overlay these maps with minimum distortion.
Using georeferencing methods, data obtained from surveying tools like Total Stations may be given a point of reference from topographic maps already available.

[edit] Methods

There are various GIS tools available that can transform image data to some geographic control framework, like ArcMap or ERDAS Imagine. One can georeference a set of points, lines, polygons, images, or 3D structures. For instance, a GPS device will record latitude and longitude coordinates for a given point of interest, effectively georeferencing this point. A georeference must be a unique identifier. In other words, there must be only one location for which a georeference acts as the reference.

To georeference an image, one first needs to establish control points, input the known geographic co-ordinates of these control points, choose the co-ordinate system and other projection parameters and then minimize residuals. Residuals are the difference between the actual co-ordinates of the control points and the co-ordinates predicted by the geographic model created using the control points. They provide a method of determining the level of accuracy of the georeferencing process.