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

GRASS GRASS user
1

I run r.slope.aspect on Aster DEM (reproject in greek grid). But I get some
strange results (like fishnet).
Here is a screenshot of Aspect :
http://www.geographer.gr/downloads/aspect.png
http://www.geographer.gr/downloads/aspect.png

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2

Hello,
are You certain that it's not just a problem with data?

Have You run Your analysis at DEM native resolution? Try to g.region
rast=MyDEM before calculating aspect.
Also how reprojection was done?

Maris.

2012/3/12 leonidas <leonidas_liakos@yahoo.gr>:

I run r.slope.aspect on Aster DEM (reproject in greek grid). But I get some
strange results (like fishnet).
Here is a screenshot of Aspect :
http://www.geographer.gr/downloads/aspect.png
http://www.geographer.gr/downloads/aspect.png

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3

Probably you changed resolution of the rasrter itself before runing r.aspect.slope.

Has Maris said, try to first run the module with DEM native resolution, and then, if you need a better resolution, set it in yout region and then run r.resample.

André

2012/3/13 Maris Nartiss <maris.gis@gmail.com>

Hello,
are You certain that it’s not just a problem with data?

Have You run Your analysis at DEM native resolution? Try to g.region
rast=MyDEM before calculating aspect.
Also how reprojection was done?

Maris.

2012/3/12 leonidas <leonidas_liakos@yahoo.gr>:

I run r.slope.aspect on Aster DEM (reproject in greek grid). But I get some
strange results (like fishnet).
Here is a screenshot of Aspect :
http://www.geographer.gr/downloads/aspect.png
http://www.geographer.gr/downloads/aspect.png

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4

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*
4. Cut out the raster in my area of interest: *gdal_translate -projwin 89749
4490072 296874 4303842 dem_temp.tif dem_GGRS.tif*
5. Import in Grass:* r.in.gdal -o -e input=dem_GGRS.tif
output=aster_dem_ggrs*
6. Set the region resolution: *g.region rast=aster_dem_ggrs*
7. Finally I run *r.slope.aspect elevation=aster_dem_ggrs@data slope=slope
aspect=aspect*

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5

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@gclements.plus.com>

6

Leonidas, you’re facing an issue that arises when you reproject a grid (in your case the DEM)
from a geographic coordinate system (lat-long) to UTM or other planar coordinate system.
When you reproject, the cells will shift and skew a bit, and this will create those ‘fishnet’
lines in your projected raster (you can also see them in a shaded relief).

I discussed it briefly in a IJGIS paper, and to me, the option is to convert your original (latlong) data
to vector points, then project the points to UTM (or other) and then interpolate the vector points
to create a new raster in the new coordinate system. This will eliminate those artifacts.

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…

Here is the paper:

Grohmann, C.H., Steiner, S.S., 2008. SRTM resample with Short Distance-Low Nugget Kriging.
International Journal of Geographical Information Science, 22 (8):895-906.

direct link (no subscription needed): http://igc.usp.br/pessoais/guano/downloads/Grohmann_Steiner_2008_IJGIS_uncorrected_proof.pdf

best

Carlos

On Thu, Mar 15, 2012 at 11:01, Glynn Clements <glynn@gclements.plus.com> wrote:

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@gclements.plus.com>

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–
Prof. Carlos Henrique Grohmann - Geologist D.Sc.
Institute of Geosciences - Univ. of São Paulo, Brazil

http://www.igc.usp.br/pessoais/guano
http://www.igc.usp.br/openstereo
http://lattes.cnpq.br/5846052449613692 (CV)

Twitter: @CarlosGrohmann
http://carlosgrohmann.tumblr.com/

Can’t stop the signal.

7

The problem was solved (at least visually removed the fishnet) using *-r
cubic* in gdalwarp.
Thank you for your information!

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