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)

       Accessed: unknown

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