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<p style="margin-bottom: 0cm; margin-top: 0pt;">On 03/20/2011 08:03
PM, Bulent Arikan wrote:<br>
</p>
<blockquote
cite="mid:AANLkTi=m2skzq5BH1TssuAvs2ZPjxq4fvRDpkcViok9q@mail.gmail.com"
type="cite">Dear List,
<div><br>
<div>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. </div>
<div><br>
</div>
</div>
</blockquote>
<br>
Interesting problem.<br>
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. <br>
If you don't need the below sea level elevations, why not just set
everything beyond the coastline to NULL?<br>
<br>
<blockquote
cite="mid:AANLkTi=m2skzq5BH1TssuAvs2ZPjxq4fvRDpkcViok9q@mail.gmail.com"
type="cite">
<div>
<div>Do I get high negative values because of the cells with
zero value?</div>
<div><br>
</div>
<div>Should I zoom further in and avoid having zero-value cells?
(which is a problem with coastlines where they are not
straight)</div>
<div><br>
</div>
<div>Does any one have a suggestion about how to deal with this
problem? </div>
<div><br>
</div>
<div>* I sample the original imagery at 50% in r.random,</div>
<div><br>
</div>
<div>* I use the default parameter values in v.surf.rst
(tension= 40.) and use the 30m DEM as MASKMAP,</div>
<div><br>
</div>
<div>* I align region to resolution, using g.region.<br
clear="all">
<br>
</div>
<div>Thanks!</div>
<div><br>
-- <br>
BÜLENT <br>
<br>
</div>
</div>
<br>
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</blockquote>
<br>
<br>
<pre class="moz-signature" cols="72">--
Micha Silver
Arava Development Co. +972-52-3665918
<a class="moz-txt-link-freetext" href="http://surfaces.co.il">http://surfaces.co.il</a>
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