[GRASS-user] Re: Strange r.slope.aspect output of Aster dem

[Glynn Clements]

leonidas wrote:
> Thank you for your answers!

> I did g.region rast=aster_dem_ggrs and then I run again r.slope.aspect but
> the problem still remains.
> 
> Here is the whole process.
> 1. I download the tiles from http://www.gdem.aster.ersdac.or.jp/search.jsp
> 2. Merge them: *gdalwarp ASTG* mosaic.tif*
> 3. Reproject the raster to greek grid: *gdalwarp -s_srs EPSG:4326 -t_srs
> EPSG:2100 mosaic.tif dem_temp.tif*

This is the problem. Resampling introduces artifacts which affect the
gradient calculation. Ideally, you should calculate the slope and
aspect on the original data, then re-project the result. Failing that,
either use the -r switch to select a more suitable resampling method
(such as cubic, cubicspline or lanczos) or filter the data with e.g. 
r.mfilter or r.neighbors prior to running r.slope.aspect.

-- 
Glynn Clements <glynn at gclements.plus.com>