[Qgis-user] Precise area calculation with CRS 4326 (+ formula)

[Andrea Giudiceandrea]

> Gabor Szakacs gabor.szakacs at agreena.com
> Tue Feb 20 03:58:40 PST 2024

> 1) is CRS 4326 suitable to make precise area calculations of agriculture
field boundaries that vary from 2 to 100ha for the 2nd decimal of a 
hectare?.

 > The avg area difference of the sum of all fields was in the 9th 
decimal of a hectare between area calculation with CRS 4326 and area 
calculation with regional equal area CRS for each region.

Hi Gabor,
it depends on how you calculate the area: using geodesic calculation on 
the ellipsoid, or planimetric calculation. If you use geodesic 
calculation, the CRS doesn't matter: you will obtain the same value 
provided you are calculating the are on the same ellipsoid and obviously 
that the coordinates of the vertices of the polygons are the same on 
such ellipsoid.

 > It seems that QGIS and Python use formulas that correctly convert
degrees into metric area calculation and minimize the effect of different
latitudes. Any kind of insights would be greatly appreciated.

Which expressions / formulas / methods have you used? Which version of 
QGIS have you used?

AFAIK, geodesic area and length calculation on the ellipsoid are 
performed in QGIS, since version 3.20, using the PROJ library which in 
turns uses, like PostGIS, the routines coded in the geographiclib library.

For reference, see:

- Use the GeographicLib routines exposed through PROJ for all 
ellipsoidal calculations https://github.com/qgis/QGIS/pull/41726

- C. F. F. Karney, Algorithms for geodesics, J. Geodesy 87(1), 43–55 
(Jan. 2013) https://doi.org/10.1007/s00190-012-0578-z
- Errata / Addenda https://geographiclib.sourceforge.io/geod-addenda.html

- GeographicLib::PolygonAreaT< GeodType > Class 
https://geographiclib.sourceforge.io/C++/doc/classGeographicLib_1_1PolygonAreaT.html


Best regards.

Andrea