problems with r.mapcalc
Ann (Cinnamon) McManamon
am at snow.nohrsc.nws.gov
Tue Dec 13 12:02:35 EST 1994
Grass only stores integers (for now???) So it's truncating all your
values to either 0 or 1. To preserve decimal places, multiply your
equation by say 1000 to retain 3 decimal places of information. Then
you'll need to divide any subsequent use of that raster by 1000 to use
the data correctly. I've also found that you should include a decimal
point after 1000 to make sure that the calculations are done as floating
point values rather than integers before truncation.
Your revised equation might look something like this:
r.mapcalc tan.0 = "tan(@slope.0) * 1000."
This technique has worked for me. Hope it helps you.
Ann McManamon
NOHRSC -NWS
6301 34th Ave. South
Minneapolis MN 55450
(612) 725 -3007
am at snow.nohrsc.nws.gov
On Tue, 13 Dec 1994, John Stamm wrote:
>
> Hello. I have used r.slope.aspect to generate a raster map of
> slopes. The map has labeled each point with slope in degrees.
> I would like to get slope as the tangent of the degrees. I used
> r.mapcalc, and used the equation:
>
> tan.0 = tan(@slope.0)
>
> where tan.0 is my output map and slope.0 is the raster map with
> slopes. However, I get tangent values equal to either zero or
> one, and no fractional values. Can anyone help?
>
> John Stamm
>
>
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