1i.topo.corr(1) GRASS GIS User's Manual i.topo.corr(1)
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6 i.topo.corr - Computes topographic correction of reflectance.
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9 imagery, terrain, topographic correction
10
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 Flags:
19 -i
20 Output sun illumination terrain model
21 -s
23 Scale output to input and copy color rules
24 --overwrite
26 Allow output files to overwrite existing files
27 --help
29 Print usage summary
30 --verbose
32 Verbose module output
33 --quiet
35 Quiet module output
36 --ui
38 Force launching GUI dialog
39 Parameters:
41 input=name[,name,...]
42 Name of reflectance raster maps to be corrected topographically
43 output=name [required]
45 Name (flag -i) or prefix for output raster maps
46 basemap=name [required]
48 Name of input base raster map (elevation or illumination)
49 zenith=float [required]
51 Solar zenith in degrees
52 azimuth=float
54 Solar azimuth in degrees (only if flag -i)
55 method=string
57 Topographic correction method
58 Options: cosine, minnaert, c-factor, percent
59 Default: c-factor
60
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.
67 Note: If needed, the sun position can be calculated for a given date
69 with r.sunmask.
70 Figure showing terrain and solar angles
71 Using the -i flag and given an elevation basemap (metric), i.topo.corr
73 creates a simple illumination model using the formula:
74 • 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.
79 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).
83 Method: cosine
85 • ref_c = ref_o * cos_z / cos_i
86 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)
91 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
95 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 • ref_c = ref_o * 2 / (cos_i + 1)
104
106 1 The illumination model (cos_i) with flag -i uses the actual re‐
107 gion as limits and the resolution of the elevation map.
108 2 The topographic correction use the full reflectance file (null
110 remain null) and its resolution.
111 3 The elevation map to calculate the illumination model should be
113 metric.
114
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 # 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
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 • 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 • 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
142 • 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.
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149 i.landsat.toar, r.mapcalc, r.sun r.sunmask
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152 E. Jorge Tizado (ej.tizado unileon es)
153 Dept. Biodiversity and Environmental Management, University of León,
154 Spain
155 Figure derived from Neteler & Mitasova, 2008.
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159 Available at: i.topo.corr source code (history)
160 Accessed: Saturday Jan 21 21:16:50 2023
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165 © 2003-2023 GRASS Development Team, GRASS GIS 8.2.1 Reference Manual
167GRASS 8.2.1 i.topo.corr(1)