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

[Markus Metz]

On Tue, May 17, 2011 at 10:34 PM, Elena Mezzini <elena.mezzini at gmail.com> wrote:
>
> On Tue, May 17, 2011 at 10:08 PM, Elena Mezzini
> &lt;elena.mezzini at gmail.com&gt; 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>)"
>
>
>
> g.region rast=dem_sross_srtm
> r.mapcalc "dem_sross_srtm_pos = if(dem_sross_srtm < 0, 0, <dem_sross_srtm>)"
>
> Syntax error!
> As you know I'm not a programmer... where is the error?

remove the <> (actually I forgot to remove them in the template...):
r.mapcalc "dem_sross_srtm_pos = if(dem_sross_srtm < 0, 0, dem_sross_srtm)"