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I have two raster layers of elevation data. One, of a larger extent, - 'warped' - is to 25m definition. The other, of much smaller extent - '194555-1_25cm_DSM' - but within the bounds of the larger raster, is to a definition of 25 cm. Using QGIS, I want to create one raster layer the size of the larger raster, that uses the elevation values of the "194555-1_25cm_DSM layer within the bounds of that smaller area, but uses elevation values from 'warped' in the area outside the smaller layer.

I have already made the cell sizes in both raster layers the same by using warp, so all cell sizes are now 0.25 in both layers. I have tried various things, but none have given me the result I need.

I did try the raster calculator with an IF clause as follows

if ( "194555-1_25cm_DSM@1", "194555-1_25cm_DSM@1" * 1,"Warped@1" * 1 ) 

but that gave me 194555-1_25cm_DSM values in that area, and no_data values elsewhere.

I think maybe I need something like a 'difference' operation, for raster layers.

Here's a screenshot of the smaller raster lying on the larger. enter image description here

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  • What software are you using?
    – John Polo
    Mar 18, 2023 at 23:56
  • Sorry, should have said. QGIS 3.24.2 Tisler. Mar 19, 2023 at 7:23

1 Answer 1

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The problem are the "no data" values! Try the following:

  1. Replace the "no data" values by e.g. -9999 with the tool "Fill NoData cells" from the toolbox

  2. Use the raster calculator with an IF clause as follows

    if ( "194555-1_25cm_DSM@1" > -9999 , "194555-1_25cm_DSM@1" * 1,"Warped@1" * 1 )

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  • That worked well, thank you oekoplaner. Of course it was the 'no data' cells, which equate I suppose to NULL in the if test, which got in the way. In fact I ended up with 3 calculated raster layers, because each operation required output to a new layer. But essentially using the principal of making sure every cell had a value, and then testing for those value ranges got me the result I required. I am left with the next problem to solve - where the high res area meets the low res area surrounding it, there is a significant jump in cell values, or elevation. I need to smoothe these somehow. Mar 20, 2023 at 10:36

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