Hi, I wonder what method QGIS is using when it computes ellipsoidal area with $area function. I made a test with this EPSG:4326 polygon: POLYGON (( 20.13293641 59.95688345, 26.94617837 60.47397663, 29.74782155 62.56499443, 27.45254202 68.7065034, 23.75771765 68.24937206, 25.42698984 65.27444593, 21.51545237 63.10353609, 21.4056276 61.12318104, 19.41123592 60.40477513, 20.13293641 59.95688345 ))
I checked that the CRS of the project and layer were both EPSG:4326. The ellipsoid in the Project-Settings was WGS 84 (EPSG:7030). The area that $area returns is 249566957499.7546 As a references I used PostGIS and web site https://geographiclib.sourceforge.io/scripts/geod-calc.html#area. With my test polygon they both give this result: 251199344354.4308 PostGIS returns bigger area. The difference is 0.654% so not huge but not negligible either. A third test with SpatiaLite 5.0 gives a result that is very close to PostGIS (difference 0.001%). 251195856999.549927 As a conclusion I have decided to trust in PostGIS because it gives the same results than the web site https://geographiclib.sourceforge.io/scripts/geod-calc.html that feels scientifically sound. However, I wonder if I have used QGIS in a correct way or if there is anything I could do for getting areas to match better with PostGIS for example by densifying the long segments in my test polygon. For getting testing easy I attach directly runnable SQL for PostGIS and Spatialite (version 5.0 with RTTopo is needed) as well as a coordinate list for the web app. PostGIS select st_area(st_geogfromtext('POLYGON (( 20.13293641 59.95688345, 26.94617837 60.47397663, 29.74782155 62.56499443, 27.45254202 68.70650340, 23.75771765 68.24937206, 25.42698984 65.27444593, 21.51545237 63.10353609, 21.40562760 61.12318104, 19.41123592 60.40477513, 20.13293641 59.95688345))') , true); Spatialite select st_area(st_geomfromtext('POLYGON (( 20.13293641 59.95688345, 26.94617837 60.47397663, 29.74782155 62.56499443, 27.45254202 68.70650340, 23.75771765 68.24937206, 25.42698984 65.27444593, 21.51545237 63.10353609, 21.40562760 61.12318104, 19.41123592 60.40477513, 20.13293641 59.95688345))') ,true); Web site (notice lat-lon order) 59.95688345 20.13293641 60.47397663 26.94617837 62.56499443 29.74782155 68.70650340 27.45254202 68.24937206 23.75771765 65.27444593 25.42698984 63.10353609 21.51545237 61.12318104 21.40562760 60.40477513 19.41123592 59.95688345 20.13293641 -Jukka Rahkonen-
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