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i.topo.corr(1)		    GRASS GIS User's Manual		i.topo.corr(1)

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

       imagery,	terrain, topographic correction

       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]

	   Output sun illumination terrain model

	   Scale output	to input and copy color	rules

	   Allow output	files to overwrite existing files

	   Print usage summary

	   Verbose module output

	   Quiet module	output

	   Force launching GUI dialog

	   Name	of reflectance raster maps to be corrected topographically

       output=nameA [required]
	   Name	(flag -i) or prefix for	output raster maps

       basemap=nameA [required]
	   Name	of input base raster map (elevation or illumination)

       zenith=floatA [required]
	   Solar zenith	in degrees

	   Solar azimuth in degrees (only if flag -i)

	   Topographic correction method
	   Options: cosine, minnaert, c-factor,	percent
	   Default: c-factor

       i.topo.corr is used to topographically correct reflectance from imagery
       files, e.g. obtained with i.landsat.toar, using a sun illumination ter-
       rain  model. This illumination model represents the cosine of the inci-
       dent 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:

	   o   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
	   o   ref_c = ref_o * cos_z / cos_i

   Method: minnaert
	   o   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
	   o   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 sur-
       face, 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

	   o   ref_c = ref_o * 2 / (cos_i + 1)

       1      The  illumination	model (cos_i) with flag	-i uses	the actual re-
	      gion 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

       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

	   o   Law K.H.	and Nichol J, 2004. Topographic	Correction For Differ-
	       ential Illumination Effects On Ikonos Satellite Imagery.	Inter-
	       national	 Archives of Photogrammetry Remote Sensing and Spatial
	       Information, pp.	641-646.

	   o   Meyer, P. and Itten, K.I. and Kellenberger, KJ  and  Sandmeier,
	       S.  and	Sandmeier,  R.,	1993. Radiometric corrections of topo-
	       graphically induced effects on Landsat TM data in  alpine  ter-
	       rain. Photogrammetric Engineering and Remote Sensing 48(17).

	   o   RiaA+-o,	 D.  and  Chuvieco,  E.	 and Salas, J. and Aguado, I.,
	       2003.  Assessment of Different Topographic Corrections in Land-
	       sat-TM  Data for	Mapping	Vegetation Types. IEEE Transactions On
	       Geoscience And Remote Sensing, Vol. 41, No. 5

	   o   Twele A.	and Erasmi S, 2005. Evaluating topographic  correction
	       algorithms  for improved	land cover discrimination in mountain-
	       ous areas of Central Sulawesi. GA<paragraph>ttinger Geographis-
	       che Abhandlungen, vol. 113.

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

       E. Jorge	Tizado	(ej.tizado unileon es)
       Dept.  Biodiversity and Environmental Management, University of LeA^3n,

       Figure derived from Neteler & Mitasova, 2008.

       Available at: i.topo.corr source	code (history)

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       A(C) 2003-2020 GRASS Development	Team, GRASS GIS	7.8.3 Reference	Manual

GRASS 7.8.3							i.topo.corr(1)


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