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I have a lot of my projects that are located in mountains. When I draw my polygones, I can easily calculate $area, of course. But in many cases, I underestimate the surface of my polygons due to the slope.

My question, is it a simple way, as an expression for example, to calculate $area but taking in count the DEM?

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  • You'd need to get a DEM first. Do you have one?
    – Erik
    Aug 24, 2021 at 7:52
  • Yes, I have one.
    – katagena
    Aug 24, 2021 at 8:25
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    It guess DEM is what OP means by "MNT": modèle numérique de terrain / MNT = DEM. @Stocker Antoine - if I'm right, it would be good to edit your question and replace MNT by DEM as this is an english-language site
    – Babel
    Aug 24, 2021 at 8:25
  • You are right... I modified it!
    – katagena
    Aug 24, 2021 at 8:27
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    Have a look here: gis.stackexchange.com/q/139957/88814
    – Babel
    Aug 24, 2021 at 8:29

1 Answer 1

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I found myself a quiet good solution, which allowed to handle with a lot of polygones.

First I convert my DEM with the tool from SAGA called "Real surface area". After that, I have a raster with the real surface for each pixels, on band 1.

Than I simply use "zonal statistics" with my polygones layer, to get the 'sum' of band 1... and that's it!

It works perfectly, and it's correct, I tested on different known surfaces!

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  • A similar solution seems to be GRASS r.surf.area, see: grass.osgeo.org/grass78/manuals/r.surf.area.html As it is stated there, "calculation is heavily dependent on data resolution: (...) the more resolution the more detail, the more area". So this might be something to consider in your case, too.
    – Babel
    Sep 16, 2021 at 14:55
  • As I mentioned, GRASS never workes for me... I always have python error!!!
    – katagena
    Sep 16, 2021 at 15:00
  • And you can't use it to calculate on different polygones!
    – katagena
    Sep 16, 2021 at 15:15

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