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.
7
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 and time 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,
77 • i is the incident angle to be calculated,
79 • s is the terrain slope angle (from r.slope.aspect),
81 • z is the solar zenith angle (i.e., 90° - solar horizon angle
83 from r.sunmask),
84 • a the solar azimuth angle (from r.sunmask),
86 • o the terrain aspect angle (from r.slope.aspect).
88 For each band file, the corrected reflectance (ref_c) is calculate from
90 the original reflectance (ref_o) using one of the four offered methods
91 (one lambertian and two non-lambertian).
92 Method: cosine
94 • ref_c = ref_o * cos_z / cos_i
95 Method: minnaert
97 • ref_c = ref_o * (cos_z / cos_i) ^k
98 where, k is obtained by linear regression of
99 ln(ref_o) = ln(ref_c) - k ln(cos_i/cos_z)
100 Method: c-factor
102 • ref_c = ref_o * (cos_z + c)/ (cos_i + c)
103 where, c is a/m from ref_o = a + m * cos_i
104 Method: percent
106 We can use cos_i to estimate the percent of solar incidence on the sur‐
107 face, then the transformation (cos_i + 1)/2 varied from 0 (surface in
108 the side in opposition to the sun: infinite correction) to 1 (direct
109 exhibition to the sun: no correction) and the corrected reflectance can
110 be calculated as
111 • ref_c = ref_o * 2 / (cos_i + 1)
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115 1 The illumination model (cos_i) with flag -i uses the actual re‐
116 gion as limits and the resolution of the elevation map.
117 2 The topographic correction use the full reflectance file (null
119 remain null) and its resolution.
120 3 The elevation map to calculate the illumination model should be
122 metric.
123
125 First, make a illumination model from the elevation map (here, SRTM).
126 Then make perform the topographic correction of e.g. the bands toar.5,
127 toar.4 and toar.3 with output as tcor.toar.5, tcor.toar.4, and
128 tcor.toar.3 using c-factor (= c-correction) method:
129 # first pass: create illumination model
131 i.topo.corr -i base=SRTM zenith=33.3631 azimuth=59.8897 output=SRTM.illumination
132 # second pass: apply illumination model
133 i.topo.corr base=SRTM.illumination input=toar.5,toar.4,toar.3 output=tcor \
134 zenith=33.3631 method=c-factor
135
137 • Law K.H. and Nichol J, 2004. Topographic Correction For Differ‐
138 ential Illumination Effects On Ikonos Satellite Imagery. Inter‐
139 national Archives of Photogrammetry Remote Sensing and Spatial
140 Information, pp. 641-646.
141 • Meyer, P. and Itten, K.I. and Kellenberger, KJ and Sandmeier,
143 S. and Sandmeier, R., 1993. Radiometric corrections of topo‐
144 graphically induced effects on Landsat TM data in alpine ter‐
145 rain. Photogrammetric Engineering and Remote Sensing 48(17).
146 • Riaño, D. and Chuvieco, E. and Salas, J. and Aguado, I., 2003.
148 Assessment of Different Topographic Corrections in Landsat-TM
149 Data for Mapping Vegetation Types. IEEE Transactions On Geo‐
150 science And Remote Sensing, Vol. 41, No. 5
151 • Twele A. and Erasmi S, 2005. Evaluating topographic correction
153 algorithms for improved land cover discrimination in mountain‐
154 ous areas of Central Sulawesi. Göttinger Geographische Abhand‐
155 lungen, vol. 113.
156
158 i.landsat.toar, r.mapcalc, r.sun r.sunmask
159
161 E. Jorge Tizado (ej.tizado unileon es)
162 Dept. Biodiversity and Environmental Management, University of León,
163 Spain
164 Figure derived from Neteler & Mitasova, 2008.
166
168 Available at: i.topo.corr source code (history)
169 Accessed: Saturday Oct 28 18:19:11 2023
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174 © 2003-2023 GRASS Development Team, GRASS GIS 8.3.1 Reference Manual
176GRASS 8.3.1 i.topo.corr(1)