I have a DSM and a DTM and want to create Normalized DSM by subtracting DTM from DSM. It is a simple calculation that I can do it in Raster Calculator with the DSM-DTM expression. However, the height values are generated different than I expected.

For example here DOM40_Clipped is my DSM and maximum height value is 629. The maximum value in DTM is 582 and I expect to the max value in N_DSM as around 47 but it looks like 124 and have the same problem in the minimum value.

What could be the reason and how to fix it?

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  • You should compare individual value not min/max value, also does your DSM/DTM have the same pixel size and extent ?
    – J.R
    Mar 21, 2023 at 9:37
  • 1
    The min/max may also be estimated (from a sample of cells) rather than the true values. This can be changed in the Min / Max Value Settings section of the Layer Styling pane.
    – Matt
    Mar 21, 2023 at 9:44
  • @J.R both have same resolution and extent, got from the same source so are in the same reference system as well. What did you mean exactly with individual value? Because it is just the first step then I need to use NDSM to create the Canopy Height Model but with this NDSM, the result was -110 to 120 for CHM and I also checked the histogram and there are many pixels in the negative values. Mar 21, 2023 at 10:04
  • @Matt you are right but I think Min/Max Value Settings are just for visualization. I am not sure if the layer is saved after min/max specification. Because I am going to use the NDSM in the next steps and as far as I know it always considers the generated values not the specified ones. Mar 21, 2023 at 10:07
  • You do get the values you expect in the layers panel if you change from estimated to true values, or does the problem persist?
    – Matt
    Mar 21, 2023 at 10:10


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