[GRASS-user] Re: i.atcorr negative DEM

[Markus Metz]

On Tue, May 17, 2011 at 10:08 PM, Elena Mezzini <elena.mezzini at gmail.com> wrote:
>
> On Tue, May 17, 2011 at 9:22 PM, Elena Mezzini
> &lt;elena.mezzini at gmail.com&gt; wrote:
>> Hi all,
>> I'm once again in trouble with i.atcorr in GRASS6.5.svn.
>> I notice that my output reflectance images are strange: there are some
>> areas
>> that are really bright and other that are really dark. Both seem to match
>> with my DEM raster.
>> Could there be problems with negative values in DEM?? I'm in a coastal
>> area
>> and there are some real altitude negative values.
>
> i.atcorr (the 6S code it is using to be exact) should treat negative
> elevation values as elevation = 0. It is debatable if this is correct,
> but in theory i.atcorr should behave accordingly. Actually, I am not
> sure about it's behaviour for negative elevation values...
>
> You could set all negative elevation values to zero,
>
>
> Ok, how can I do it?
>
replace my_elevation with the name of your elevation map:

g.region rast=my_elevation
r.mapcalc "elevation_positive = if(my_elevation < 0, 0, <my_elevation>)"