i.topo.corr(1grass) | GRASS GIS User's Manual | i.topo.corr(1grass) |
i.topo.corr - Computes topographic correction of reflectance.
imagery, terrain, topographic correction
i.topo.corr
i.topo.corr --help
i.topo.corr [-is] [input=name[,name,...]]
output=name basemap=name
zenith=float [azimuth=float]
[method=string] [--overwrite] [--help]
[--verbose] [--quiet] [--ui]
i.topo.corr is used to topographically correct reflectance from imagery files, e.g. obtained with i.landsat.toar, using a sun illumination terrain model. This illumination model represents the cosine of the incident angle i, i.e. the angle between the normal to the ground and the sun rays.
Note: If needed, the sun position can be calculated for a given
date with r.sunmask.
Figure showing terrain and solar angles
Using the -i flag and given an elevation basemap (metric), i.topo.corr creates a simple illumination model using the formula:
For each band file, the corrected reflectance (ref_c) is calculate from the original reflectance (ref_o) using one of the four offered methods (one lambertian and two non-lambertian).
where, k is obtained by linear regression of
ln(ref_o) = ln(ref_c) - k ln(cos_i/cos_z)
where, c is a/m from ref_o = a + m * cos_i
We can use cos_i to estimate the percent of solar incidence on the surface, then the transformation (cos_i + 1)/2 varied from 0 (surface in the side in opposition to the sun: infinite correction) to 1 (direct exhibition to the sun: no correction) and the corrected reflectance can be calculated as
First, make a illumination model from the elevation map (here, SRTM). Then make perform the topographic correction of e.g. the bands toar.5, toar.4 and toar.3 with output as tcor.toar.5, tcor.toar.4, and tcor.toar.3 using c-factor (= c-correction) method:
# first pass: create illumination model i.topo.corr -i base=SRTM zenith=33.3631 azimuth=59.8897 output=SRTM.illumination # second pass: apply illumination model i.topo.corr base=SRTM.illumination input=toar.5,toar.4,toar.3 output=tcor \
zenith=33.3631 method=c-factor
i.landsat.toar, r.mapcalc, r.sun r.sunmask
E. Jorge Tizado (ej.tizado unileon es)
Dept. Biodiversity and Environmental Management, University of León,
Spain
Figure derived from Neteler & Mitasova, 2008.
Available at: i.topo.corr source code (history)
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