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   [-i]    [input=name[,name,...]]    output=name   basemap=name   zenith=float
       [azimuth=float]   [method=string]   [--overwrite]  [--verbose]  [--quiet]

   Flags:
       -i
           Output sun illumination terrain model

       --overwrite
           Allow output files to overwrite existing files

       --verbose
           Verbose module output

       --quiet
           Quiet module output

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

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

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

       zenith=float
           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.e. the  angle between the  normal  to
       the  ground and the sun rays. It can be obtained with r.sun (parameter incidout), and then
       calculating its cosine with float precision.

       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,  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:

       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, Spain

       Last changed: $Date: 2013-04-17 22:56:53 -0700 (Wed, 17 Apr 2013) $

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