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I've been following a tutorial (https://www.pointsnorthgis.ca/blog/how-to-calculate-surfacecut-volume-qgis-advanced/) in order to try to find volumes for various stockpiles that I took imagery of with a drone. I've managed to get a TIN of the underlying surface and applied that to get the elevation difference, essentially giving us the stockpile only.

After running that through the "Raster layer zonal statistics" I get the following:

enter image description here

Following the further steps (which tells us to get the volume by multiplying the m² and the sum together), my volume results in lots of negative numbers. It would appear that, in a lot of cases, my sum column is negative.

Any idea what would be causing this?

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    Agree with @ahmadhanb: I doubt that with a drone you can detect areas as little as 0.00109429 square meters. I would suggest to check the CRS Mar 25 at 8:58
  • I'll add a comment, but won't answer this myself. Basically, some of the stockpile was next to a high wall, which the vector doing the "draping" got the value of. I worked around it by getting points from my polygon vertices, and using them to sample the DSM. Then I had to manually correct any points that were too high because of the wall (and vehicles too, as I later found out).
    – user25730
    Mar 29 at 5:07

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