3 Geocoding a scanned map (4 corner points): Topographic map 1:24000

GRASS can be used to geocode unreferenced (e.g., scanned) maps by defining ground control points. Such a scanned map should be imported into a XY location without projection information. This location can be automatically created when using the r.in.gdal command within another location. Using the ``location'' parameter the command will not only import the map, but also write it to a new location (see section 3.1.5). Then GRASS must be restarted with the new XY location containing the scanned map. We now quickly outline the procedure without going into too many details:

• The scanned map has to be inserted into an image group (i.group; even if it is just a single map).

• The group is targeted to a reference location (i.target).

• The user opens a GRASS monitor and graphically places corresponding ground control points (i.points or i.vpoints). The unreferenced map is loaded into the left side of the graphical display, and the reference map is loaded into the right side. For an internally undistorted map four corner points should suffice.

• The ungeocoded map is rectified into the reference location (i.rectify). The polynomial order for a 4-point rectification is 1.

Once the rectification is done, GRASS has to be left and restarted with the reference location. Now the result can be validated.