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I have two similar rasters of the Mount Vesuvius and surroundings: one spans 20 kilometers, and the other one 50 kilometers. The two rasters are overlapping:

enter image description here

What I would like to do is to create a new raster file which will show the difference in elevation between these two rasters. I tried using the Raster calculator with the following expression:

"vesuvius_radius_20KM@1" - "vesuvius_radius_50KM@1"

enter image description here

But for some reason it does not work. After I press OK, all I get is some sort of black raster file which does not have any kind of elevation data in it, nor it has any number of pixels:

enter image description here

What am I doing wrong, and how can I create a raster file which will show the difference between the upper mentioned two raster files?

Here are those two raster files:

https://www.dropbox.com/s/98hcftjsnmrqs8p/vesuvius_radius_20KM.tif?dl=0 https://www.dropbox.com/s/a68921e6tpszt0f/vesuvius_radius_50KM.tif?dl=0

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2 Answers 2

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Oh will you feel silly! ;-)

The result is actually correct. The two rasters are identical where they overlap. Where they do not, the result is NaN because you cannot do math with a non-existent value.

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  • 3
    Thank you for the quick reply and explanation bugmenot123. I do feel silly.
    – marco
    May 7, 2016 at 23:49
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I´m not quite sure,if what bugmeont suggested is correct. I guess if both layers are equal the outcome of the new one should be a layer with zeros and not with no-data values.

edit: also yourpicture shows 0 there, where there is no overlap and NaN where you have overlap, which also does not fit to the explanation.

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  • Hi @Philipp Dahlem. Thank you for the reply. I am not sure I understood the first part of your reply. Can you try to download the both .tif files please? As for the second one: you are correct. It's definitively a bug in QGIS: the 'nan' should be depicted with white rectangle, and '0' should be depicted with black rectangle
    – marco
    May 8, 2016 at 16:13

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