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

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=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

       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 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)

NOTES
       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
	      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
	   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.

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

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

       Figure derived from Neteler & Mitasova, 2008.

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

       Main index | Imagery index | Topics index | Keywords index |  Graphical
       index | Full index

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

GRASS 7.8.3							i.topo.corr(1)

NAME | KEYWORDS | SYNOPSIS | DESCRIPTION | NOTES | EXAMPLES | REFERENCES | SEE ALSO | AUTHOR | SOURCE CODE

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