## NAME

r.topidx - Creates topographic index, ln(a/tan(beta)), map from elevation map.

## SYNOPSIS

r.topidx
r.topidx help
r.topidx [-v] input=string output=string

### Flags:

-v
  Output verbosely

### Parameters:

input=string
  Elevation map
output=string
  Topographic index ln(a/tanB) map

## DESCRIPTION

r.topidx creates topographic index, ln(a/tan(beta)), map from elevation map

where

  a: the area of the hillslope per unit contour length that drains through any point

  tan(beta): the local surface topographic slope (delta vertical) / (delta horizontal)

  Elevation map need not to be rectangular if study basin was already determined. Non-interesting region may have NULL values. Study basin map can be taken from r.water.outlet then the following r.mapcalc statements will create study basin extracted elevation map.

  r.mapcalc "belev = if(isnull(basin), basin, elev)"

  r.stats -Anc prints out averaged statistics for topographic index.

  ## SEE ALSO

  r.topmodel, r.water.outlet, r.mapcalc

  ## AUTHORS

  Main algorithm sources are rewritten from GRIDATB.FOR.
  GRIDATB.FOR is available from www.es.lancs.ac.uk/hfdg/topmodel.html.
  Thanks to Keith Beven.

  GRASS port by Huidae Cho
  Hydro Laboratory, Kyungpook National University, South Korea

  Last changed: $Date: 2005/03/01 06:58:58 $

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