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I want to get a vector layer that would be water area around Europe. Seems easy enough, I got world shoreline, cleaned it up a bit, created a scratch layer on which I put a rectangular polygon to be my bounds:

enter image description here

Now, I go Vector -> Geoprocessing Tools -> Difference. Unfortunately, the result is always somehow wrong:

enter image description here

No matter what I do, how I draw the rectangle, the northern part is always wrong, sometimes the south and the west are slightly outside of what I set as the bounding rectangle. Do you have any idea what might be causing it? Both layers are set to EPSG:4326 while the project uses ETRS89-extended

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  • It can't be wrong. Change the project CS to ESPG:4326
    – Ash
    Commented Nov 29, 2021 at 22:31
  • you can try clipping the shoreline polygons first, or to add more vertices to the bounding box polygon
    – JGH
    Commented Nov 30, 2021 at 13:59

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It can't be a QGIS failure. Change the project CS to ESPG:4326 prior to the process. then it will be OK.

If you desire the project CS to be ETRS89-extended, after the processing.

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  • +1 for working in a single projection, though QGIS and underlying libraries are not bug-free either :-)
    – JGH
    Commented Nov 30, 2021 at 14:04
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    I reprojected the layers to ETRS89, as I wanted the rectangle to be rectangle in that project and it works! And I guess it's logical, considering on Mercator it looked like this i.imgur.com/CBcTkre.png
    – Michał Sadowski
    Commented Nov 30, 2021 at 20:49

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