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 flag -i and given an elevation map as basemap (UTM), i.topo.corr permits to obtain a simple illumination model from 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¶

Method: minnaert¶

where, k is obtained by linear regression of ln(ref_o) = ln(ref_c) - k ln(cos_i/cos_z)

Method: c-factor¶

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

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), 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. 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¶

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: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $ Full index © 2003-2011 GRASS Development Team