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

[Nyall Dawson]

On 12 Oct 2016 11:56 PM, "Carlos Cerdán" <sig.upagu at gmail.com> wrote:
>
> AFAIK, It also is needed to turn off "on the fly SRC transformation" to
get correct area values.... Or QGIS has fixed this point?

Everything should be fixed in recent versions, and I very (VERY) much want
to know if any issues are still encountered.

Calculating area/length is a core task for a GIS and we need to make sure
it's rock solid. (Which it should be since 2.16!)

Nyall


>
> If you can't get correct area values, check out about it....
>
>
> 2016-10-12 7:40 GMT-05:00 DelazJ <delazj at gmail.com>:
>>
>> 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.cadieux at 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]> 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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>>>
>>>
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