Hi,
To complete Nicolas answer, you should check what are the measurements
options set in Project --> Project Properties --> General tab.
See also
http://docs.qgis.org/2.14/en/docs/user_manual/introduction/general_tools.html#measuring

2016-10-12 14:07 GMT+02:00 Nicolas Cadieux <nicolas.cadi...@archeotec.ca>:

> 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] <[hidden email]
> <http:///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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