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Net.Noise owner news at
Tue Mar 30 23:05:17 EST 1993

Newsgroups: info.grass.user
From: shapiro at (Michael Shapiro)
Subject: Re: topo index
Message-ID: <C4qJCq.L7x at>
Sender: news at (Net.Noise owner)
Organization: US Army Corps of Engineers Construction Engineering Research Labs
References: <9303281719.AA04780 at>
Date: Wed, 31 Mar 1993 04:05:14 GMT
Lines: 22

In <9303281719.AA04780 at> riker at (Dan Riker) writes:

If you are using r.mapcalc, you have to bve careful about integer vs floats.
The expression a/tanB would be integer divison since a and tanB are maps
(which are taken to be integer) and trucncated to integer before passed to ln().
The the relsuts of ln are again rounded to integer so that the result can
be sore in a raster map (which hold integers only).

Try r.mapcalc 'x = 1000 * ln(float(a)/tanB)'

>Hello there,
>I am still trying to determine the topographic index ln(a/tanB) and i do have some values.  they seem to me to be strange, in that there is a bimodal distribution, with peaks at 17 and 0, while the lows are at 21(the highest value) and 13.  Is this a common thing for rugged topography?
>If not, any suggestions as to what I am doing wrong?
>I think that it would really help if i had an example set of data, so if anyone could supply it I would be most appreciative.
>Dan Riker
>Duke University
Michael Shapiro               shapiro at
U.S. Army CERL                (217) 373-7277
P.O. Box 9005 
Champaign, Ill. 61826-9005

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