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I have very large raster, whih is basically merge of smaller rassters : enter image description here

I would like to create separate polygons for each "group" of pixels with values, to get something like this (sorry for the quality of drawing, it suppose to be separate polygons): enter image description here

I have tried to do this with qgis tool "extract layer extent" , but it did not break the small rasters into polygons, but created large polygon based on the extent: enter image description here

As this didn't work, I was looking for solution like "difference" but between polygon and raster, however, I haven't found yet, and as the raster extent is very large it seems to not work.

I'm looking for other tools/plugins that I can use in order to get separate polygons for the not null grouped pixels.

Edit: for now I have made not very esthetic workaround,which is to generate small polygon around each part-raster, clip it (to break the big raster to small raster), then vectorize the small raster and then dissolve the pixels-polygons. However, I'm still looking for better methods to do that ,I'm sure there are.

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  • Are the purple areas where there werent any smaller raster? Do you have access to the smaller rasters?
    – BERA
    Nov 24, 2022 at 10:06
  • @BERA the purple area is the extent of the raster, the smaller rasters are what I want to get is polygons of the small rasters. There are more small rasters outside of the image I took which makes the extent larger.
    – ReutKeller
    Nov 24, 2022 at 12:50
  • Do you still have access to the original/smaller rasters?
    – BERA
    Nov 24, 2022 at 14:27

1 Answer 1

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Use the raster calculator to create a new binary raster, with 0 where the values of the original raster are NULL, 1 everywhere else. Then use polygonize to create a polygon layer which will have one polygon for each of your original rasters, and one (or more) for all the NULL values.

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    Raster calc expression would be something like: gis.stackexchange.com/a/428556/2856
    – user2856
    Nov 25, 2022 at 4:33
  • this is good option. The problem is that I still need to run calculations on very large area
    – ReutKeller
    Nov 28, 2022 at 8:58
  • Unless you have to do this often on many different rasters, just let your computer do the work. Who cares if it takes a while. It will be faster than waiting for a different answer.
    – Llaves
    Nov 29, 2022 at 16:24

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