NAME

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

KEYWORDS

       imagery, terrain, topographic correction

SYNOPSIS

       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]

   Flags:
       -i
           Output sun illumination terrain model

       -s
           Scale output to input and copy color rules

       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       input=name[,name,...]
           Name of reflectance raster maps to be corrected topographically

       output=name [required]
           Name (flag -i) or prefix for output raster maps

       basemap=name [required]
           Name of input base raster map (elevation or illumination)

       zenith=float [required]
           Solar zenith in degrees

       azimuth=float
           Solar azimuth in degrees (only if flag -i)

       method=string
           Topographic correction method
           Options: cosine, minnaert, c-factor, percent
           Default: c-factor

DESCRIPTION

       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:

           •   cos_i = cos(s) * cos(z) + sin(s) * sin(z) * cos(a - o)
       where,  i  is  the  incident  angle to be calculated, s is the terrain slope angle, z is the solar zenith
       angle, a the solar azimuth angle, o the 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: cosine
           •   ref_c = ref_o * cos_z / cos_i

   Method: minnaert
           •   ref_c = ref_o * (cos_z / cos_i) ^k
       where, k is obtained by linear regression of
       ln(ref_o) = ln(ref_c) - k ln(cos_i/cos_z)

   Method: c-factor
           •   ref_c = ref_o * (cos_z + c)/ (cos_i + c)
       where, c is a/m from ref_o = a + m * cos_i

   Method: percent
       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

           •   ref_c = ref_o * 2 / (cos_i + 1)

NOTES

       1      The illumination model (cos_i) with flag -i uses the actual region as limits and the resolution of
              the elevation map.

       2      The topographic correction use the full reflectance file (null remain null) and its resolution.

       3      The 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:

       # 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

           •   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 r.sunmask

AUTHOR

       E. Jorge Tizado  (ej.tizado unileon es)
       Dept. Biodiversity and Environmental Management, University of León, Spain

       Figure derived from Neteler & Mitasova, 2008.

SOURCE CODE

       Available at: i.topo.corr source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual