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-boun...@lists.osgeo.org] im Auftrag von Nicolas 
Cadieux [nicolas.cadi...@archeotec.ca]
Gesendet: Mittwoch, 12. Oktober 2016 14:07
An: qgis-user@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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