[GRASS-user] r.topidx - area of the hillslope per unit contour length

[Ing. Pierluigi De Rosa]

Dear All,
I should calculate  the area of the hillslope per unit contour length.
I saw r.topidx calculate topographic index (wetness 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). 

I should only have the numerator of topographic index. How can i get it?
Thanks
Pierluigi

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Ing. Pierluigi De Rosa (PhD)
Studio Associato GFOSSERVICES
                
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