qgis - How to compare two DEMs and show their difference?

I want to compare my DEMs with each other (ASTER GDEM2 and SRTM4.1). I know, that the GDEM has some "holes" with 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.

Answer

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