Hi Bulent,
Yes, I think it is better to treat the sea area as NULL data. One way to interpolate the DEM is to use a mask corresponding to the land area, another way is to replace the values of the sea with null data in the DEM. Best, Edgar Pimiento ________________________________ From: Bulent Arikan <[email protected]> To: Edgar Pimiento <[email protected]> Sent: Mon, March 21, 2011 9:20:11 AM Subject: Re: [GRASS-user] interpolating images with coastal areas Hi Edgar, Thank you for the explanation and suggestion. If I understand you correctly, you suggest converting elevation values zero and below to NULL and use that 30m resolution imagery for interpolation. So, this map should be used as MASK. I hope I got it right? Best, Bulent On Sun, Mar 20, 2011 at 3:14 PM, Edgar Pimiento <[email protected]> wrote: Hi, > > >The problem is probably the data for the sea of the ASTER images. In some >cases >I have seen negative values for sea areas and extremely high values, errors >also, in mountainous areas. For interpolating I think it is better to treat >the >sea areas as no data values. Use the land area as mask in the interpolation. >You can also check the ASTER elevation data comparing with the corrected >version >of 90-m SRTM from http://srtm.csi.cgiar.org/ > > >Best, > Edgar Pimiento Chamorro >[email protected] > > > > > ________________________________ From: Bulent Arikan <[email protected]> >To: [email protected] >Sent: Mon, March 21, 2011 3:03:35 AM >Subject: [GRASS-user] interpolating images with coastal areas > > >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. > > >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! > >-- >BÜLENT > > > -- BÜLENT ARIKAN, PhD Postdoctoral Scholar Center for Social Dynamics and Complexity & School of Human Evolution and Social Change Arizona State University Tempe - AZ 85287-2402
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