[Qgis-user] inconsistenty when calculating area depending on file type or projection?

[Blumentrath, Stefan]

Hei Martina, 

When you compare area measurments in QGIS with area measurements in Mapinfo you are probably comparing geodesic measurements (in QGIS)  with cartesian measurements (in Mapinfo). 
Thus,  you get a difference. 
A bit confusing indeed, but a design choice... 
See: https://hub.qgis.org/issues/12057

Cheers, 
Stefan
________________________________________
Von: Qgis-user [qgis-user-bounces at lists.osgeo.org] im Auftrag von Nicolas Cadieux [nicolas.cadieux at archeotec.ca]
Gesendet: Mittwoch, 12. Oktober 2016 14:07
An: qgis-user at lists.osgeo.org
Betreff: Re: [Qgis-user] inconsistenty when calculating area depending on file type or projection?

Hi,
You may be calculating square degrees and not metres.  It can depend on the crs depending on the tools you are using.
Nicolas

Le 11 oct. 2016 à 08:54, Martina Schäfer [via OSGeo.org<http://OSGeo.org>] <[hidden email]</user/SendEmail.jtp?type=node&node=5290417&i=0>> a écrit :

I experienced some confusion with calculation of area using the field calculator in QGIS version 2.16.3. Since I'm using MapInfo Professional as well, I mainly use tab-files that I can open in both programmes, but occasionally I save as shapefile since this used to be the default in QGIS.

When comparing files, I coincidently realized that there was a mismatch in calculated area for the shapefile and the tab-file for exactly the same polygons! I used the field calculator in the attribute table in both cases, but for the shapefile the resulting areas were almost doubled in area compared to the tab-file. Any idea why this is happening?

I also realized similar differences when calculating area in a file where projection has been converted from SWEREF99TM (a Swedish national projection) to WGS84. There differences occurred in both the tab and shapefile compared to the area calculated for the same tab-file in MapInfo. Again I find this very confusing!

I need to rely on the area-calculations thus I really hope someone here can explain to me what is happening!

Thanks in advance,
Martina




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