i.topo.corr(1grass) GRASS GIS User's Manual i.topo.corr(1grass)
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 ter-
rain model. This illumination model represents the cosine of the inci-
dent 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 sur-
face, 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 re-
gion 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 Differ-
ential Illumination Effects On Ikonos Satellite Imagery. Inter-
national 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 topo-
graphically induced effects on Landsat TM data in alpine ter-
rain. 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 Geo-
science And Remote Sensing, Vol. 41, No. 5
• Twele A. and Erasmi S, 2005. Evaluating topographic correction
algorithms for improved land cover discrimination in mountain-
ous areas of Central Sulawesi. Göttinger Geographische Abhand-
lungen, 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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