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.

       Last changed: $Date: 2014-12-09 19:39:37 +0100 (Tue, 09 Dec 2014) $

SOURCE CODE

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

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       © 2003-2018 GRASS Development Team, GRASS GIS 7.4.0 Reference Manual

GRASS 7.4.0                                                                                  i.topo.corr(1grass)