Hi Jochen,
as described in the user manual, the "Add geometry attributes"
processing algorithm [1] "generates a new vector layer ... containing
geometric measurements based on a selected CRS.", i.e. it measures
Cartesian planimetric length and area on the CRS's cartographic plane.
The "$area" function [2] outputs the value of the area measured on the
ellipsoid set for the project, instead.
You can obtain the same area value calculated by the "Add geometry
attributes" processing algorithm using the "area" function [3].
Regards.
Andrea Giudiceandrea
[1]
https://docs.qgis.org/3.40/en/docs/user_manual/processing_algs/qgis/vectorgeometry.html#add-geometry-attributes
[2]
https://docs.qgis.org/3.40/en/docs/user_manual/expressions/functions_list.html#area
[3]
https://docs.qgis.org/3.40/en/docs/user_manual/expressions/functions_list.html#id25
Jochen Albrecht jochen.albrecht at gmail.com
Thu Mar 13 13:34:41 PDT 2025
When I use the Add Geometry Attributes function, the sum of the
calculated areas is less than a square millimeter off. When I use instead
the $area variable in the field calculator, the sum is some 670 square
meters off.
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