<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:arial, helvetica, sans-serif;font-size:12pt"><div style="color: black; font-family: arial, helvetica, sans-serif; font-size: 12pt; "></div><div style="color: black; font-family: arial, helvetica, sans-serif; font-size: 12pt; ">Hi,</div><div style="color: black; font-family: arial, helvetica, sans-serif; font-size: 12pt; "><br></div><div><font class="Apple-style-span" face="arial, helvetica, sans-serif" size="3">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 </font><font class="Apple-style-span" face="arial, helvetica, sans-serif">mountainous</font><font class="Apple-style-span" face="arial, helvetica, sans-serif" size="3"> 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 </font><a href="http://srtm.csi.cgiar.org/">http://srtm.csi.cgiar.org/</a><br><font class="Apple-style-span" face="arial, helvetica, sans-serif" size="3"><br></font></div><div><font class="Apple-style-span" face="arial, helvetica, sans-serif" size="3">Best,</font></div><div><font class="Apple-style-span" face="arial, helvetica, sans-serif" size="3"> </font></div><font class="Apple-style-span" face="arial, helvetica, sans-serif" size="3">Edgar Pimiento Chamorro</font><br><font class="Apple-style-span" face="arial, helvetica, sans-serif" size="3">e_pimiento@yahoo.com</font><div style="color: black; font-family: arial, helvetica, sans-serif; font-size: 12pt; "><br></div><div style="font-family: arial, helvetica, sans-serif; font-size: 12pt; color: black; "><br><div style="font-family:times new roman, new york,
times, serif;font-size:12pt"><font size="2" face="Tahoma"><hr size="1"><b><span style="font-weight: bold;">From:</span></b> Bulent Arikan <bulent.arikan@gmail.com><br><b><span style="font-weight: bold;">To:</span></b> grass-user@lists.osgeo.org<br><b><span style="font-weight: bold;">Sent:</span></b> Mon, March 21, 2011 3:03:35 AM<br><b><span style="font-weight: bold;">Subject:</span></b> [GRASS-user] interpolating images with coastal areas<br></font><br>
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>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>
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