Input maps may have NULL values. For example, if you have a MASK for a watershed (basin map from r.water.outlet), the following command will create a masked elevation map (belev):
r.mapcalc "belev = if(isnull(basin), basin, elev)"
r.stats -Anc prints out averaged statistics for topographic index.
Unprojected lat/long locations are not supported. If data is not projected in a lat/long location, please create a new location in a projected coordinate system and reproject the data into the new projected location. Please run r.topidx from that location.
g.region raster=elevation -p r.topidx input=elevation output=topidx_elevation
Moore, I.D., R.B. Grayson, and A.R. Ladson, 1991. Digital Terrain Modeling: A Review of Hydrological, Geomorphological, and Biological Applications. Hydrological Processes 5:3-30.
Based on GRIDATB.FOR by Keith Beven.