Leonidas, you're facing an issue that arises when you reproject a grid (in your case the DEM)<div>from a geographic coordinate system (lat-long) to UTM or other planar coordinate system.</div><div>When you reproject, the cells will shift and skew a bit, and this will create those 'fishnet'</div>
<div>lines in your projected raster (you can also see them in a shaded relief).</div><div><br></div><div>I discussed it briefly in a IJGIS paper, and to me, the option is to convert your original (latlong) data</div><div>
to vector points, then project the points to UTM (or other) and then interpolate the vector points </div><div>to create a new raster in the new coordinate system. This will eliminate those artifacts.</div><div><br></div>
<div>
I also remember that Dylan Baudette had a page showing the outputs from gdalward, and you could see the artifacs there as well, but it seems that page is offline...</div><div><br></div><div>Here is the paper:</div><div><br>
</div><div><span style="color:rgb(64,64,64);font-family:Verdana,Tahoma,Arial,sans-serif;font-size:12px;line-height:18px;background-color:rgb(255,255,255)">Grohmann, C.H., Steiner, S.S., 2008. SRTM resample with Short Distance-Low Nugget Kriging.</span><br style="color:rgb(64,64,64);font-family:Verdana,Tahoma,Arial,sans-serif;font-size:12px;line-height:18px;background-color:rgb(255,255,255)">
<span style="color:rgb(64,64,64);font-family:Verdana,Tahoma,Arial,sans-serif;font-size:12px;line-height:18px;background-color:rgb(255,255,255);font-style:italic">International Journal of Geographical Information Science,</span><span style="color:rgb(64,64,64);font-family:Verdana,Tahoma,Arial,sans-serif;font-size:12px;line-height:18px;background-color:rgb(255,255,255)"> 22 (8):895-906.</span><br>
</div><div>direct link (no subscription needed): <a href="http://igc.usp.br/pessoais/guano/downloads/Grohmann_Steiner_2008_IJGIS_uncorrected_proof.pdf">http://igc.usp.br/pessoais/guano/downloads/Grohmann_Steiner_2008_IJGIS_uncorrected_proof.pdf</a><br>
<div><br></div><div><br></div><div>best </div><div><br></div><div>Carlos</div><div><br></div><div><br></div><div><br></div><div><br><br><div class="gmail_quote">On Thu, Mar 15, 2012 at 11:01, Glynn Clements <span dir="ltr"><<a href="mailto:glynn@gclements.plus.com">glynn@gclements.plus.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im"><br>
leonidas wrote:<br>
> Thank you for your answers!<br>
<br>
> I did g.region rast=aster_dem_ggrs and then I run again r.slope.aspect but<br>
> the problem still remains.<br>
><br>
> Here is the whole process.<br>
> 1. I download the tiles from <a href="http://www.gdem.aster.ersdac.or.jp/search.jsp" target="_blank">http://www.gdem.aster.ersdac.or.jp/search.jsp</a><br>
> 2. Merge them: *gdalwarp ASTG* mosaic.tif*<br>
> 3. Reproject the raster to greek grid: *gdalwarp -s_srs EPSG:4326 -t_srs<br>
> EPSG:2100 mosaic.tif dem_temp.tif*<br>
<br>
</div>This is the problem. Resampling introduces artifacts which affect the<br>
gradient calculation. Ideally, you should calculate the slope and<br>
aspect on the original data, then re-project the result. Failing that,<br>
either use the -r switch to select a more suitable resampling method<br>
(such as cubic, cubicspline or lanczos) or filter the data with e.g.<br>
r.mfilter or r.neighbors prior to running r.slope.aspect.<br>
<span class="HOEnZb"><font color="#888888"><br>
--<br>
Glynn Clements <<a href="mailto:glynn@gclements.plus.com">glynn@gclements.plus.com</a>><br>
</font></span><div class="HOEnZb"><div class="h5">_______________________________________________<br>
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</div></div></blockquote></div><br><br clear="all"><div><br></div>-- <br>Prof. Carlos Henrique Grohmann - Geologist D.Sc.<br>Institute of Geosciences - Univ. of São Paulo, Brazil<br>---<br><a href="http://www.igc.usp.br/pessoais/guano" target="_blank">http://www.igc.usp.br/pessoais/guano</a><br>
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---<br>Twitter: @CarlosGrohmann<br><a href="http://carlosgrohmann.tumblr.com/" target="_blank">http://carlosgrohmann.tumblr.com/</a><br>________________<br>Can’t stop the signal.<br>
</div></div>