[Qgis-user] Smooth DEMs

[richard burcher]

Hi Clemens,

To smooth dems, use the following:

Install  http://www.saga-gis.org/en/index.html saga gis  & install the
sextante plugin. Under the Analysis menu, open the Sextante options &
configuration, click on saga. Sextante should find your install of saga (I'm
assuming a windows platform here) at something like c:/program
files/saga-gis/. Ensure the "use min covering grid system for resampling" is
checked.

Click Analysis menu -- Sextante Toolbar. This opens the toolbox, you can
type filter in the text search. Alternatively, just expand saga -- grid
filter, you will have ~ 10 filter choices. Not sure of the filtering
application (cartographic or analytical) you can employ the "simple" filter
with its default smooth using 11*11 for ~ 990m output. If maintaining slopes
are important, review the multi-directional filter as it implements a lee
based filter.

I'm pretty sure the input raster must be tif.

#-----

Moving the srtm 90m? to 1km resolution is resampling the dataset. You can
use the built in gdal tools--  http://www.gdal.org/gdalwarp.html warp 
dialog for this. In the dialog after you have entered the information click
the pencil to allow editing of the config text, add the following argument
(before the -r method) to allow resample to 1km -tr 1000 1000. Ensure
resample method is not near but bicubic.

#-----

You can empose spatial scale on your orginial srtm90m by filtering with a
large filter say 11*11 (~990m support area) but the dem posting will remain
90m. This avoids resampling & may be a better way forward, I've used the
aforementioned approach for some soil tools. 


Cheers,

Richard

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