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<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">I wonder what method QGIS is using when it computes ellipsoidal area with $area function. I made a test with this EPSG:4326 polygon:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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 ))<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">249566957499.7546<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">As a references I used PostGIS and web site
<a href="https://geographiclib.sourceforge.io/scripts/geod-calc.html#area">https://geographiclib.sourceforge.io/scripts/geod-calc.html#area</a>. With my test polygon they both give this result:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">251199344354.4308<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">PostGIS returns bigger area. The difference is 0.654% so not huge but not negligible either.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">A third test with SpatiaLite 5.0 gives a result that is very close to PostGIS (difference 0.001%).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">251195856999.549927<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">As a conclusion I have decided to trust in PostGIS because it gives the same results than the web site
<a href="https://geographiclib.sourceforge.io/scripts/geod-calc.html">https://geographiclib.sourceforge.io/scripts/geod-calc.html</a> 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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">PostGIS<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">select st_area(st_geogfromtext('POLYGON ((<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">20.13293641 59.95688345,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">26.94617837 60.47397663,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">29.74782155 62.56499443,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">27.45254202 68.70650340,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">23.75771765 68.24937206,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">25.42698984 65.27444593,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">21.51545237 63.10353609,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">21.40562760 61.12318104,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">19.41123592 60.40477513,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">20.13293641 59.95688345))') , true);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Spatialite<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">select st_area(st_geomfromtext('POLYGON ((<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">20.13293641 59.95688345,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">26.94617837 60.47397663,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">29.74782155 62.56499443,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">27.45254202 68.70650340,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">23.75771765 68.24937206,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">25.42698984 65.27444593,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">21.51545237 63.10353609,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">21.40562760 61.12318104,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">19.41123592 60.40477513,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">20.13293641 59.95688345))') ,true);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Web site (notice lat-lon order)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">59.95688345 20.13293641 <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">60.47397663 26.94617837 <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">62.56499443 29.74782155 <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">68.70650340 27.45254202 <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">68.24937206 23.75771765 <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">65.27444593 25.42698984 <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">63.10353609 21.51545237 <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">61.12318104 21.40562760 <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">60.40477513 19.41123592 <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">59.95688345 20.13293641<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">-Jukka Rahkonen- <o:p></o:p></span></p>
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