|
NAMEi.topo.corr - Computes topographic correction of reflectance.KEYWORDSimagery, terrain, topographic correctionSYNOPSISi.topo.corri.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] Flags:
Parameters:
DESCRIPTIONi.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.
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). Method: cosine
Method: minnaert
ln(ref_o) = ln(ref_c) - k ln(cos_i/cos_z) Method: c-factor
Method: percentWe 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
NOTES
EXAMPLESFirst, 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 REFERENCES
SEE ALSOi.landsat.toar, r.mapcalc, r.sun r.sunmaskAUTHORE. Jorge Tizado (ej.tizado unileon es)Dept. Biodiversity and Environmental Management, University of León, Spain Figure derived from Neteler & Mitasova, 2008. SOURCE CODEAvailable at: i.topo.corr source code (history)Main index | Imagery index | Topics index | Keywords index | Graphical index | Full index © 2003-2021 GRASS Development Team, GRASS GIS 7.8.6 Reference Manual
Visit the GSP FreeBSD Man Page Interface. |