.TH i.topo.corr 1grass "" "GRASS 6.4.2" "Grass User's Manual"
.SH NAME
\fI\fBi.topo.corr\fR\fR  - Computes topographic correction of reflectance.
.SH KEYWORDS
imagery, terrain, topographic correction
.SH SYNOPSIS
\fBi.topo.corr\fR
.br
\fBi.topo.corr help\fR
.br
\fBi.topo.corr\fR [-\fBi\fR]  [\fBinput\fR=\fIname\fR[,\fIname\fR,...]]  \fBoutput\fR=\fIname\fR \fBbasemap\fR=\fIname\fR \fBzenith\fR=\fIfloat\fR  [\fBazimuth\fR=\fIfloat\fR]   [\fBmethod\fR=\fIstring\fR]   [--\fBoverwrite\fR]  [--\fBverbose\fR]  [--\fBquiet\fR] 
.SS Flags:
.IP "\fB-i\fR" 4m
.br
Output sun illumination terrain model
.IP "\fB--overwrite\fR" 4m
.br
Allow output files to overwrite existing files
.IP "\fB--verbose\fR" 4m
.br
Verbose module output
.IP "\fB--quiet\fR" 4m
.br
Quiet module output
.PP
.SS Parameters:
.IP "\fBinput\fR=\fIname[,\fIname\fR,...]\fR" 4m
.br
Name of reflectance raster maps to be corrected topographically
.IP "\fBoutput\fR=\fIname\fR" 4m
.br
Name (flag -i) or prefix for output raster maps
.IP "\fBbasemap\fR=\fIname\fR" 4m
.br
Name of input base raster map (elevation or illumination)
.IP "\fBzenith\fR=\fIfloat\fR" 4m
.br
Solar zenith in degrees
.IP "\fBazimuth\fR=\fIfloat\fR" 4m
.br
Solar azimuth in degrees (only if flag -i)
.IP "\fBmethod\fR=\fIstring\fR" 4m
.br
Topographic correction method
.br
Options: \fIcosine,minnaert,c-factor,percent\fR
.br
Default: \fIc-factor\fR
.PP
.SH DESCRIPTION
\fIi.topo.corr\fR is used to topographically correct reflectance
from imagery files, e.g. obtained with \fIi.landsat.toar\fR, 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 \fIr.sun\fR
(parameter incidout), and then calculating its cosine with float precision.
.PP
Using flag -i and given an elevation map as basemap (UTM),
\fIi.topo.corr\fR permits to obtain a simple illumination model from the formula:
.RE
cos_i = cos(s) * cos(z) + sin(s) * sin(z) * cos(a - o) 
.RE
where,
\fIs\fR is the terrain slope angle, \fIz\fR is the solar zenith angle,
\fIa\fR the solar azimuth angle, \fIo\fR the terrain aspect angle.
.PP
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).
.SS Method: cosine
.RS
.IP
 ref_c = ref_o * cos_z / cos_i 
.RE
.SS Method: minnaert
.RS
.IP
ref_c = ref_o * (cos_z / cos_i) ^k
.RE
where,
\fIk\fR is obtained by linear regression of
.br
ln(ref_o) = ln(ref_c) - k ln(cos_i/cos_z)
.SS Method: c-factor
.RS
.IP
ref_c = ref_o * (cos_z + c)/ (cos_i + c)
.RE
where,
\fIc\fR is a/m from ref_o = a + m * cos_i
.SS 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
.RS
.IP
ref_c = ref_o * 2 / (cos_i + 1)
.RE
.SH NOTES
..IP
.IP \fB1\fR
The illumination model (cos_i) with flag -i uses the actual region
as limits and the resolution of the elevation map.
.IP \fB2\fR
The topographic correction use the full reflectance file (null remain
null) and its resolution.
.IP \fB3\fR
The elevation map to calculate the illumination model should be metric.
.PP
.SH EXAMPLES
First, make a illumination model from the elevation map (here, SRTM), and
then make topographic correction of 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.
.PP
\fC
.DS
.br
i.topo.corr -i base=SRTM zenith=33.3631 azimuth=59.8897 out=SRTM.illumination
.br
i.topo.corr base=SRTM.illumination input=toar.5,toar.4,toar.3 out=tcor \(rs 
.br
  zenith=33.3631 method=c-factor
.br
.DE
\fR
.SH REFERENCES
.RS
.IP
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.
.IP
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).
.IP
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
.IP
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.
.RE
.SH SEE ALSO
\fI
i.landsat.toar,
r.mapcalc,
r.sun
\fR
.SH AUTHOR
E. Jorge Tizado  (ej.tizado unileon es)
.br
Dept. Biodiversity and Environmental Management, University of León, Spain
.PP
\fILast changed: $Date: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $\fR
.PP
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.PP
© 2003-2011 GRASS Development Team