[GRASS-user] i.points gps

[Hamish]

Charles wrote:
> I am in the process of geocoding a map that has been scan.  I have  
> read where it is possible to use global positioning system(gps) data  
> as reference points or ground control points to accomplish an image  
> rectification.  Is anyone familiar with the procedure or have a basic  
> idea how to use gps data to perform image rectification?  Thanks for  
> any information on this subject!


Have you collected GPS points at key points in the map? (can be a fun
afternoon)

With those you use the georectification tool (from the File menu) to add
the GCPs. Click on the image at a point you know the GPS position for,
then type in the coordinates.

You can also do the same with the old i.points module, and if you want to
set the points for raster map from another map you can use the i.vpoints
module, e.g. click on the known point on the already registered map, then
click on the corresponding point on your scanned map. QGIS has a
georectification tool you can use too. gdal_translate to add GCPs and
gdalwarp to rectify the image is another non-GRASS solution. 

To avoid unneeded losses it is useful to know the projection of the map
before starting, rather than rectify to WGS84 lat/lon then reproject
(r.proj) back to the original. In the case of a projected scanned map you
may want to reproject the GCPs with the m.proj module or PROJ.4's cs2cs
instead of sending the raster data on a round trip.


see on the wiki:
  http://grass.osgeo.org/wiki/Georeferencing



Hamish



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