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**NAME**

i.topo.corr- Computes topographic correction of reflectance.

**KEYWORDS**

imagery, terrain, topographic correction

**SYNOPSIS**

i.topo.corri.topo.corrhelpi.topo.corr[-i] [input=name[,name,...]]output=namebasemap=namezenith=float[azimuth=float] [method=string] [--overwrite] [--verbose] [--quiet]Flags:-iOutput sun illumination terrain model--overwriteAllow output files to overwrite existing files--verboseVerbose module output--quietQuiet module outputParameters:input=name[,name,...] Name of reflectance raster maps to be corrected topographicallyoutput=nameName (flag -i) or prefix for output raster mapsbasemap=nameName of input base raster map (elevation or illumination)zenith=floatSolar zenith in degreesazimuth=floatSolar azimuth in degrees (only if flag -i)method=stringTopographic correction method Options:cosine,minnaert,c-factor,percentDefault:c-factor

**DESCRIPTION**

i.topo.corris used to topographically correct reflectance from imagery files, e.g. obtained withi.landsat.toar, using a sun illumination terrain model. This illumination model represents the cosine of the incident angle, i.e. the angle between the normal to the ground and the sun rays. It can be obtained withr.sun(parameter incidout), and then calculating its cosine with float precision. Using the-iflag and given an elevation basemap (metric),i.topo.corrcreates a simple illumination model using the formula: cos_i = cos(s) * cos(z) + sin(s) * sin(z) * cos(a - o) where,sis the terrain slope angle,zis the solar zenith angle,athe solar azimuth angle,othe terrain aspect angle. 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:cosineref_c = ref_o * cos_z / cos_iMethod:minnaertref_c = ref_o * (cos_z / cos_i) ^k where,kis obtained by linear regression of ln(ref_o) = ln(ref_c) - k ln(cos_i/cos_z)Method:c-factorref_c = ref_o * (cos_z + c)/ (cos_i + c) where,cis a/m from ref_o = a + m * cos_iMethod: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 ref_c = ref_o * 2 / (cos_i + 1)

**NOTES**

1The illumination model (cos_i) with flag -i uses the actual region as limits and the resolution of the elevation map.2The topographic correction use the full reflectance file (null remain null) and its resolution.3The elevation map to calculate the illumination model should be metric.

**EXAMPLES**

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: i.topo.corr -i base=SRTM zenith=33.3631 azimuth=59.8897 out=SRTM.illumination i.topo.corr base=SRTM.illumination input=toar.5,toar.4,toar.3 out=tcor \ zenith=33.3631 method=c-factor

**REFERENCES**

Law K.H. and Nichol J, 2004. Topographic Correction For Differential Illumination Effects On Ikonos Satellite Imagery. International Archives of Photogrammetry Remote Sensing and Spatial Information, pp. 641-646. Meyer, P. and Itten, K.I. and Kellenberger, KJ and Sandmeier, S. and Sandmeier, R., 1993. Radiometric corrections of topographically induced effects on Landsat TM data in alpine terrain. Photogrammetric Engineering and Remote Sensing 48(17). Riaño, D. and Chuvieco, E. and Salas, J. and Aguado, I., 2003. Assessment of Different Topographic Corrections in Landsat-TM Data for Mapping Vegetation Types. IEEE Transactions On Geoscience And Remote Sensing, Vol. 41, No. 5 Twele A. and Erasmi S, 2005. Evaluating topographic correction algorithms for improved land cover discrimination in mountainous areas of Central Sulawesi. Göttinger Geographische Abhandlungen, vol. 113.

**SEE** **ALSO**

i.landsat.toar,r.mapcalc,r.sun

**AUTHOR**

E. Jorge Tizado (ej.tizado unileon es) Dept. Biodiversity and Environmental Management, University of León, SpainLastchanged:$Date:2013-04-1722:56:53-0700(Wed,17Apr2013)$Full index © 2003-2013 GRASS Development Team