[GRASS-user] interpolating images with coastal areas

Micha Silver micha at arava.co.il
Sun Mar 20 14:51:00 EDT 2011

On 03/20/2011 08:03 PM, Bulent Arikan wrote:

> Dear List,
> I am working on interpolating 30m ASTER DEMs to 10m resolution. I am 
> using GRASS 6.4.1 svn on Mac OS 10.6.6. Since the imagery covers a 
> large area, I crop the 30m DEM into individual tiles in order to make 
> the project more manageable. The interpolation process (using 
> v.surf.rst) has been working fine and the values in resultant 10m DEMs 
> have been quite accurate, especially for inlands. However, I have some 
> problems with interpolating tiles where coastline is present. In these 
> tiles, the range of values in 10m DEMs go below zero. In some cases I 
> have values such as -21 meters whereas the original –30m– tiles have 
> only zero. Obviously, I am concerned about the accuracy of the end 
> product.

Interesting problem.
AFAIK the ASTER DEM does not have bathymetry information so the coverage 
ends at the coastline. I suppose that when there's a slope down to the 
coast, the interpolation algorithm continues that slope beyond the 
coastline into the "nodata" cells thus creating negative values.
If you don't need the below sea level elevations, why not just set 
everything beyond the coastline to NULL?

> Do I get high negative values because of the cells with zero value?
> Should I zoom further in and avoid having zero-value cells? (which is 
> a problem with coastlines where they are not straight)
> Does any one have a suggestion about how to deal with this problem?
> * I sample the original imagery at 50% in r.random,
> * I use the default parameter values in v.surf.rst (tension= 40.) and 
> use the 30m DEM as MASKMAP,
> * I align region to resolution, using g.region.
> Thanks!
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Micha Silver
Arava Development Co. +972-52-3665918

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