Q: How to change map/image projections, datums, etc in GRASS GIS?
- create an import location in the projection and datum of the map/image you want to import
- import the map/image into the location
- create a destination location in the projection and datum you want to reproject the map/image into
- working in the destination location, use r.proj or v.proj (depending on whether the map/image is raster or vector) to reproject the map/image from the import location to the destination location.
If your raster data set is too big (i.e, 2 gb or so), r.proj will crash. Instead, export as a geotif, use gdalwarp to change the project and then import the new geotif file. If you have installed gdal, you almost certainly have gdalwarp as well. Directions for gdalwarp are at http://www.remotesensing.org/gdal/gdalwarp.html. The options for gdalwarp are a bit confusing. Here is an example of projecting a geotif based on srtm elevation data to utm 37N. -t_srs is the output file projection. AfricaHornElev.tif is the input file, AfricaHornElev37n.tif is the output file.
gdalwarp -t_srs '+proj=utm +zone=37 +ellps=WGS84 +datum=WGS84 +units=m +no_defs' -rcs AfricaHornElev.tif AfricaHornElev37n.tif
Another (similar) strategy frequently needed when maps need to be imported using, e.g. v.in.ogr or r.in.ogr, is to tell these programs to create a new location (argument "location"). If the imported data contains proper PROJ_INFO files, they can be reprojected into the current mapset, using the the projection in the current location/mapset.