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I have a DTM with 40cm resolution and a DSM with 2cm. Now I would like to compare the two so I can identify vegetation that is higher than a certain distance from the ground. I changed the pixel size from the DSM to 40 cm and wanted to use the raster calculator with a the simple calculation: Output = DSM - DTM I get the message "insufficient memory available for this operation" I have 16gb of ram and would consider my laptop as good. Even after splitting up the area, it would not calculate the difference.

What can be the issue? Is my equation correct?

  • How big are the individual rasters? – Mikkel Lydholm Rasmussen Aug 15 '16 at 10:48
  • Express size in pixels. – user30184 Aug 15 '16 at 10:55
  • The width is 1250 pixel and the length is simiar. Maybe I made a mistake when aligning the rasters (Raster -> Align Rasters). I checked the boxes CRS, cell size and grid offset. Pixel of both, DSM and DTM are aligned though – Sepp Haunerdinger Aug 15 '16 at 11:12
  • That would give that your study area is about 500 m on each side. Is that correct? I think there's something else causing your issue, because a couple of million pixels shouldn't be any problem at all on almost any reasonably new computer. At 2 cm resolution (about 600 million pixels for your area) I can imagine that there'll be memory insufficencies with only 16 GB RAM though. I have successfully worked with a raster half that size, but I have 40 GB RAM. And each calculation takes a while.. :) – Martin Aug 15 '16 at 11:34
  • it is 500m, yes. but since I changed the DSM to 40cm, I dont really understand why it shows that error message. The pixels are perfectly aligned and the raster calculator has an easy equation to calculate. So what can it be? – Sepp Haunerdinger Aug 15 '16 at 11:47

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