A georeference is a service object which stores the relation between rows and columns of a raster map (row,col) and coordinates (X,Y). A georeference is needed for raster maps. A georeference uses a coordinate system. It is advised that raster maps of the same area use the same georeference.
- For rasterized vector maps (usually North-oriented), use a georef corners.
- For other raster maps, for example satellite images, which are not North-oriented and in which the pixels do not represent exactly square areas on the ground, use a georef tiepoints.
- To add coordinates to a scanned photograph while using a Digital Terrain Model (DTM), create a georef direct linear.
- To add coordinates to a scanned aerial photograph with fiducial marks for which you have DTM, create a georef orthophoto.
- To obtain three dimensional views of maps using a DTM, create a georef 3D.
To combine raster maps with different georeferences, for instance in MapCalc or Cross, first use the Resample operation, so that all maps will use one georeference.
