1i.topo.corr(1)                Grass User's Manual               i.topo.corr(1)
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NAME

6       i.topo.corr  - Computes topographic correction of reflectance.
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KEYWORDS

9       imagery, terrain, topographic correction
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SYNOPSIS

12       i.topo.corr
13       i.topo.corr --help
14       i.topo.corr  [-is]   [input=name[,name,...]]   output=name basemap=name
15       zenith=float    [azimuth=float]      [method=string]      [--overwrite]
16       [--help]  [--verbose]  [--quiet]  [--ui]
17
18   Flags:
19       -i
20           Output sun illumination terrain model
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22       -s
23           Scale output to input and copy color rules
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25       --overwrite
26           Allow output files to overwrite existing files
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28       --help
29           Print usage summary
30
31       --verbose
32           Verbose module output
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34       --quiet
35           Quiet module output
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37       --ui
38           Force launching GUI dialog
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40   Parameters:
41       input=name[,name,...]
42           Name of reflectance raster maps to be corrected topographically
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44       output=name [required]
45           Name (flag -i) or prefix for output raster maps
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47       basemap=name [required]
48           Name of input base raster map (elevation or illumination)
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50       zenith=float [required]
51           Solar zenith in degrees
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53       azimuth=float
54           Solar azimuth in degrees (only if flag -i)
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56       method=string
57           Topographic correction method
58           Options: cosine, minnaert, c-factor, percent
59           Default: c-factor
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DESCRIPTION

62       i.topo.corr is used to topographically correct reflectance from imagery
63       files, e.g. obtained with i.landsat.toar, using a sun illumination ter‐
64       rain  model. This illumination model represents the cosine of the inci‐
65       dent angle i, i.e. the  angle between the normal to the ground and  the
66       sun rays.
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68       Note:  If  needed,  the sun position can be calculated for a given date
69       with r.sunmask.
70       Figure showing terrain and solar angles
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72       Using the -i flag and given an elevation basemap (metric),  i.topo.corr
73       creates a simple illumination model using the formula:
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75           ·   cos_i = cos(s) * cos(z) + sin(s) * sin(z) * cos(a - o)
76       where, i is the incident angle to be calculated, s is the terrain slope
77       angle, z is the solar zenith angle, a the solar azimuth  angle,  o  the
78       terrain aspect angle.
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80       For each band file, the corrected reflectance (ref_c) is calculate from
81       the original reflectance (ref_o) using one of the four offered  methods
82       (one lambertian and two non-lambertian).
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84   Method: cosine
85           ·   ref_c = ref_o * cos_z / cos_i
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87   Method: minnaert
88           ·   ref_c = ref_o * (cos_z / cos_i) ^k
89       where, k is obtained by linear regression of
90       ln(ref_o) = ln(ref_c) - k ln(cos_i/cos_z)
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92   Method: c-factor
93           ·   ref_c = ref_o * (cos_z + c)/ (cos_i + c)
94       where, c is a/m from ref_o = a + m * cos_i
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96   Method: percent
97       We can use cos_i to estimate the percent of solar incidence on the sur‐
98       face, then the transformation (cos_i + 1)/2 varied from 0  (surface  in
99       the  side  in  opposition to the sun: infinite correction) to 1 (direct
100       exhibition to the sun: no correction) and the corrected reflectance can
101       be calculated as
102
103           ·   ref_c = ref_o * 2 / (cos_i + 1)
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NOTES

106       1      The  illumination  model  (cos_i)  with  flag -i uses the actual
107              region as limits and the resolution of the elevation map.
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109       2      The topographic correction use the full reflectance  file  (null
110              remain null) and its resolution.
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112       3      The  elevation map to calculate the illumination model should be
113              metric.
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EXAMPLES

116       First, make a illumination model from the elevation map  (here,  SRTM).
117       Then  make perform the topographic correction of e.g. the bands toar.5,
118       toar.4  and  toar.3  with  output  as  tcor.toar.5,  tcor.toar.4,   and
119       tcor.toar.3 using c-factor (= c-correction) method:
120
121       # first pass: create illumination model
122       i.topo.corr -i base=SRTM zenith=33.3631 azimuth=59.8897 output=SRTM.illumination
123       # second pass: apply illumination model
124       i.topo.corr base=SRTM.illumination input=toar.5,toar.4,toar.3 output=tcor \
125         zenith=33.3631 method=c-factor
126

REFERENCES

128           ·   Law K.H. and Nichol J, 2004. Topographic Correction For Differ‐
129               ential Illumination Effects On Ikonos Satellite Imagery. Inter‐
130               national  Archives of Photogrammetry Remote Sensing and Spatial
131               Information, pp. 641-646.
132
133           ·   Meyer, P. and Itten, K.I. and Kellenberger, KJ  and  Sandmeier,
134               S.  and  Sandmeier,  R., 1993. Radiometric corrections of topo‐
135               graphically induced effects on Landsat TM data in  alpine  ter‐
136               rain. Photogrammetric Engineering and Remote Sensing 48(17).
137
138           ·   Riaño, D. and Chuvieco, E. and Salas, J. and Aguado, I., 2003.
139               Assessment of Different Topographic Corrections  in  Landsat-TM
140               Data  for  Mapping  Vegetation Types. IEEE Transactions On Geo‐
141               science And Remote Sensing, Vol. 41, No. 5
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143           ·   Twele A. and Erasmi S, 2005. Evaluating topographic  correction
144               algorithms  for improved land cover discrimination in mountain‐
145               ous areas of Central Sulawesi. Göttinger Geographische Abhand‐
146               lungen, vol. 113.
147

SEE ALSO

149        i.landsat.toar, r.mapcalc, r.sun r.sunmask
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AUTHOR

152       E. Jorge Tizado  (ej.tizado unileon es)
153       Dept.  Biodiversity  and Environmental Management, University of León,
154       Spain
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156       Figure derived from Neteler & Mitasova, 2008.
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SOURCE CODE

159       Available at: i.topo.corr source code (history)
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161       Main index | Imagery index | Topics index | Keywords index |  Graphical
162       index | Full index
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164       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual
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168GRASS 7.8.2                                                     i.topo.corr(1)
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