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

[Gabor Szakacs]

Hi Bernie and Andrea,

thank you very much for your feedback.

Andrea, regarding your question, which QGIS version and which method we are
using, this is the method we are using in our programming pipeline...
https://pyproj4.github.io/pyproj/dev/_modules/pyproj/geod.html#Geod.polygon_area_perimeter

I am using QGIS version 3.28 LTR.

Best, Gabor

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On Tue, 20 Feb 2024 at 13:28, Andrea Giudiceandrea via QGIS-User <
qgis-user at lists.osgeo.org> wrote:

> > 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
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Dr. Gabor Szakacs
Geospatial Analyst in Regenerative Agriculture

+55 11 989319305

gabor.szakacs at agreena.com

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