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<br>I looked around for information about the artefacts in the aster data. The Quality Assessment file that accompanies the data mentions:<BR>Quote<BR>The automated cloud masking and statistical approach used to select data for stacking<br>are not totally effective in avoiding anomalous elevations values, and anomalies may<br>remain in the GDEM where the stack number is three or less, particularly. Where<br>available, existing DEMs were used to replace anomalous GDEM values, including<br>adjusting for offsets between the ASTER GDEM and the reference DEM data.<br>UnQuote<BR>following the link:<BR><a href="http://www.ersdac.or.jp/GDEM/E/image/ASTERGDEM_ValidationSummaryReport_Ver1.pdf">http://www.ersdac.or.jp/GDEM/E/image/ASTERGDEM_ValidationSummaryReport_Ver1.pdf</a><BR>At page ~20 the chapter "Artifacts Related to Irregular Stack Number Boundaries." has additional information and visual displays equal to the ones you link to at //yepka..<BR>It would be interesting if the artefacts reveal some secret about the landscape, but so far I've treated as a measurement/processing-error.<BR> <BR>I assume that 'stack-number' originates from overlapping measurement-swats<BR> <BR>Carsten<BR><hr id="stopSpelling">Sylv wrote:<BR> <BR>I used a re-sampling to 30m of SRTM-DEM (made by Brazilian INPE) and observe a visual superposition between GRASS-r.watershed river network output and 2001 Landsat7 deforested areas (the nearest date after SRTM) (see <a href="http://yepca.org/wp3/?p=349" target="_blank">http://yepca.org/wp3/?p=349</a>)<br>
As you suggest, I'll verify if CGIAR SRTM-DEM is better in such flat area with forest/cultivation patchwork.<br>In first analyze, ASTER GDEM gave better river network, even with the theses 20-30m holes or larges areas artifacts!... but more job is necessary to verify and understand why!<br>
<br clear="all">Sylv<br><br><BR> </div></body>
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