3

There is online service of GeographicLib, by Karney, where one can calculate the area of ellipsoidal polygon without need of reprojection.

Can one calculate the area of geometry in QGIS so as ellipsoidal polygons (in order to get the most accurate area), and not with $area that gives planar area after a equal-area projection of a geometry?

-1

Simple answer: no. QGis is a purely cartographic programme, all the computations it performs are carried on the Cartesian plane. Moreover, QGis erroneously assumes the counter-domain of any cartographic projection to be an infinite plane, which may invalidate its geometric computations in the vicinity of counter-domain edges (what are sometimes called "interruptions").

  • 3
    QGIS uses an ellipsoid for area calculations. Doesn't that mean it assumes the earth's surface is not flat, but three-dimensional? Please provide some references to support your assertion that QGIS calculates area on a Cartesian plane. – csk Dec 7 '18 at 19:02
  • @csk It is up to you to prove that QGis is a geodetic programme, rather than geographic. Open the manual and try to find a section on Geodesy. – Luís de Sousa Dec 9 '18 at 14:04
  • 1
    Doesn't this suggest, directly from Martin Dobias, that QGIS does calculate on an ellipsoid? – Gabriel C. Feb 5 at 16:27
  • @Gabriel C. Why don't you give it a try? Download the world administrative borders form Natural Earth. Open that dataset in QGis with Marinus of Tyre's projection and check for instance the area of Greenland. Then reproject the layer with an equal area projection, say the Sinusoidal. Open the reprojected layer in QGis and query again the Greenland feature. – Luís de Sousa Feb 6 at 8:48
  • I just did and I'm still unconvinced: I calculated Greenland's area 3 times from that shapefile. Once while project CRS was set as equidistant cylindrical (ellipsoid automatically set as none/planimetric) which results in 8229000.9328 sq.km. I then set the project CRS to earth sinusoidal (ellipsoid set to Earth 2000) which results in 2171418.4078 sq.km, only 5000 km off from the official figure which could easily be explained by generalization of the coastline. Using same projection but setting ellipsoid to none/planimetric, the result was 8229000.9328 sq.km. – Gabriel C. Feb 6 at 14:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.