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

[Jochen Albrecht]

I am working with a projected vector layer (EPSG:31287) that, after
dissolving polygons, has just two areas with no more than 20 vertices each.
The total area should be exactly one million square meters.
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.
What is the difference in the underlying algorithm of the two operations
involved?
Cheers,
       Jochen

Dr. Jochen Albrecht, GISP (he/him/his)

Professor for Computational and Theoretical Geography

Department of Geography and Environmental Science
<http://www.geo.hunter.cuny.edu/>

Hunter College CUNY

695 Park Avenue

New York, NY 10065

Immediate Past-President, GIS Certification Institute
<https://www.gisci.org/>
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