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I have a vector layer with classified slopes in percentage (three classes). I vectorized that layer from a raster.

I want to eliminate all the small polygons from certain surface. I selected the polygons after calculating the area. I have this selection.

enter image description here

After selection I run the tool "Eliminate selected polygons" and I select to merge them with the neighbour with biggest area. The result is very bad. It eliminates some big polygons and keep some of the selected ones.

enter image description here

There is not options in the tool so I don't know what else to do or why it happens. All the polygons are valid. Some idea?

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I think it is something related to the CRS that you are using, which is EPSG:3857. The WGS 84 / Pseudo-Mercator -- Spherical Mercator, is mainly used to render maps in Google Maps, OpenStreetMap, etc., and it is not designed for reliable area calculations or complex analysis.

Therefore, you need to change the CRS to something more reliable such as UTM if you want to to use meter unit, then load your projected data to a new QGIS project and repeat the same process that you have done in your question by selecting the area that you want to eliminate and run eliminate tool again.

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  • I tried changing the SRC of the project, and opening a new project with the layer SRC and same result
    – B RG
    Dec 11 '20 at 10:19
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I solved the issue fixing the geometries. The vectorizing tool made a lot of invalid geometries, that is why the elimination tool was not workong properly.

After fixing the geometry It workes perfectly.

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