[Qgis-user] Different values for $area and 'Add Geometry Attributes'

[Nigel Berjak - General]

Hi Jochen

The alternative to $area is to use area($geometry), which then applies 
the area based upon the CRS of the layer and the specific geometry, 
instead of the project. Perhaps this is what you are needing to apply.

---
Regards,

Nigel Berjak
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On 2025-03-14 19:48, Andrea Giudiceandrea via QGIS-User wrote:
> Il 14/03/2025 15:38, Jochen Albrecht ha scritto:
>> Thank you, Andrea. But let me reiterate that the layer is projected, 
>> so the default should in either case be to apply the algorithm.
>> $area does not allow me to specify projection parameters. But by 
>> playing with that option for the other tool, I found that the 
>> erroneous value is based on ellipsoidal calculations - regardless of 
>> whether a planimetric project CRS has been defined or not.
>> Can you think of a way to override this behavior for $area?
> 
> Hi Jochen,
> not sure to understand your question.
> 
> Area and length measurements can be performed either on the Cartesian 
> plane of the CRS or on the ellipsoid.
> 
> If a valid ellipsoid is the for the project [1], then the "$area" 
> function calculates the area on such ellipsoid, regardless of whether 
> the layer's or the project's CRS is a geographic or projected one.
> If the ellipsoid is set to "None/Planimetric", then the "$area" 
> function will return the value of the area measured on the layer's CRS 
> Cartesian plane (regardless of whether the layer's or the project's CRS 
> is a geographic or projected one).
> 
> Instead, the "area" function always calculates the value of the 
> planimetric area measured on the layer's CRS Cartesian plane 
> (regardless of whether the layer's or the project's CRS is a geographic 
> or projected one and regardless a valid ellipsoid is the for the 
> project).
> 
> Thus, if you want to calculate the planimetric area on the Cartesian 
> plane of the layer's CRS using an expression, then you can just use the 
> "area" function. For the same purpose, you can also use the "$area" 
> function, ensuring the project's ellipsoid is set to 
> "None/Planimetric".
> 
> Regards.
> 
> Andrea
> 
> 
> [1] 
> https://docs.qgis.org/3.40/en/docs/user_manual/introduction/qgis_configuration.html#general-properties
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