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In QGIS 3.16.6 I need to create a polygon shapefile of ellipses from points. I am doing the following:

Processing Toolbox -> Vector Geometry -> Geometry by expression

  • Input layer - points_in.shp

  • Output geometry type - polygon

  • Geometry expression - make_ellipse(transform(make_point("Lon-DD","Lat-DD"), 'EPSG:4269', 'EPSG:26913'), 635, 190, 90)

  • Modified geometry - ellipses_out.shp

Where "Lon-DD" and"Lat-DD" are fields in the point shapefile, the input CRS is GDD NAD83, the output CRS NAD83/UTM Zone 13N meters, and the semi-major and semi-minor axis are 635 meters and 190 meters and the azimuth is 90 degrees.

The Geometry by expression tool runs without complaints but the output shapefile is empty.

What am I doing wrong and how do I fix this?

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    Did you check the attribute table of ellipses_out layer? Did you zoom to the layer? Does "Empty" mean that you don't see ellipses in the map or that the attribute table of "ellipses_out" is empty?
    – Kadir Şahbaz
    Commented Apr 25, 2021 at 23:33
  • Zooming to the output layer is different than zooming to the input layer. The output layer is there, all 200 plus ellipses. So I know what the problem is now what is the solution? Again, transform('EPSG:4269', 'EPSG:26913') appears not to be working.
    – Steve Haymes
    Commented Apr 26, 2021 at 0:34

1 Answer 1

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The problem is that you expect that make_ellipse returns an ellipse in EPSG:26913 coordinates. Yes, it returns EPSG:26913 coordinates (big numbers like 500000 which are projected coordinates), but QGIS considers those are in source layer's CRS, EPSG:4269 (lat, long). If you check the CRS of ellipses_out, you will see its CRS is EPSG:4269.

For a solution, use Processing toolbox > Vector general > Assign projection tool for output layer. Choose EPSG:26913 as new CRS.

enter image description here

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  • Right. Thanks for identifying the problem, but what I asked was for a solution: "What am I doing wrong and how do I fix this?" Apparently transform('EPSG:4269', 'EPSG:26913') is not working. I assume that if I project the input points from GDD to UTM first, then this should work?
    – Steve Haymes
    Commented Apr 26, 2021 at 0:24

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