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I just used a tool from the "Shape Tools" plugin but the results were nonsense. The CRS of the point layer and my project CRS were EPSG:4326 - WGS 84.

Here is an image of the result: The correct result should be an ellipse around the point.

enter image description here

I realized if I change both the project CRS and point layer CRS to EPSG:25832 - ETRS92/UTMzone 32N. The tools of the plugin work perfectly fine.

That's the correct ellipse.

enter image description here

Any explanaition why this is the case?

  • 1
    Can you describe what you mean by "nonsense", or post a screenshot? – Erik Nov 12 '19 at 13:34
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I can't say for absolutely certain as it will depend on the underlying code and I am not the author. However, it is probably down to EPSG:4326 having a distance unit of decimal degrees and the plugin using distance units in linear measurements (such as meters). Therefore, when you swap to a CRS that uses meters as its linear unit, the code works perfectly. Mixing CRSs with different units of measurement is a common 'gotcha' in GIS in general (not just QGIS) and is best avoided, even with improvements in reprojecting on-the-fly.

  • Thanks, that sounds like a good explanation. Can I ask, how do I recognize wether a CRS uses decimal degrees or linear measurements? – DGIS Nov 12 '19 at 14:20
  • Thanks, that sounds like a good explanation. Can I ask, how do I recognize wether a CRS uses decimal degrees or linear measurements? Could it be that a projected CRS is always in linear measures and an unprojected in decimal degrees? – DGIS Nov 12 '19 at 14:34
  • Firstly, if you want to make a change to a comment and are in time, there is an edit button, otherwise just do something like "...and also" if you miss the window to save yourself typing so much. WRT distance units, all geographic coordinate systems will be in degrees and all projected coordinate systems will use something else (m, ft, km, miles etc). Look in the CRS details in QGIS and you will see a Proj4 string. Look for something like "+units=m". If you don't see "+units=" then the units by default are degrees, otherwise they are whatever is specified by "+units". – MappaGnosis Nov 12 '19 at 15:01

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