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I want to compare my DEMs (ASTER GDEM2 and SRTM4.1). I know, that the GDEM has some extreme "holes" with totaly wrong elevations (I found 3 of them in my working area). Now I need to know, if there are more of them to eventually correct them with the SRTM. I use SAGA, GRASS and QGIS.

Thanks!

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compare them with what? with each other? one is you baseline and the other is faulty? –  nickves Sep 11 '12 at 13:07
    
right! I just want to draw the difference between them. –  MAP Sep 11 '12 at 13:10
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2 Answers

up vote 2 down vote accepted

Let's say Raster1 is the correct, And Raster2 is the one you suspect it has faults.

They also both are in the same units (since you say both rasters are DEMs the z value should be in meters - I assume we're correct and we compare same things)

You can use rast calc from either package (qgis, grass, saga) to compare them eg:

deviation_rast  = abs( (raster1 - raster2) / raster1 * 100 ) 

each cell from the resulting raster will display the percentage which raster2 deviates from raster1.

After that you can reclassify deviation_rast based on your needs: eg

red -> deviation_rast_value > 75
orange -> deviation_rast_value > 50
yellow ->  deviation_rast_value > 25
green -> deviation_rast_value  > 0

to identify problematic areas - and to choose if you want to intervene or not

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The ratio is not a good idea for DEMs with elevations near sea level: you will get many bogus results and completely undefined results wherever raster2 == 0. The right approach, for all DEMs, is just to subtract one raster from the other and screen for outliers (which can be done in many, many ways: consult stats.stackexchange.com for some methods). –  whuber Sep 11 '12 at 14:25
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You could start by getting the difference of the two DEMs. QGIS has a raster calculator tool that should come in handy. Just get tiles of both DEMs that cover the same area and subtract the values of one DEM from the other. That should get you a nice raster layer that shows the differences in elevation between the two DEMs.

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Exactly, just subtract One DEM from the other. I would then represent it in QGIS or even reclassify it, using some tolerance around Zero. Something like If -5m < Result < 5m then consider that DEM's have the same value in that particular cell. –  Alexandre Neto Sep 11 '12 at 13:20
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(+1) That's a good start--and simple, too. Errors in DEMs are a little complicated: they result from elevation errors and from positional errors. If there is no positional error, then typically a screen based on a constant tolerance is good--but it shouldn't necessarily be around zero, because one DEM might have a (small) constant vertical shift relative to another. Use a tolerance around the median difference instead. In hilly areas, the positional error adds more variation where slopes are steeper, so the tolerance needs to be increased roughly in proportion to the tangent of the slope. –  whuber Sep 11 '12 at 14:28
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