i.topo.corrgrass - Online in the Cloud

PROGRAM:

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.

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