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

[Nyall Dawson]

On 15 Oct 2016 1:50 AM, "Andrew" <amcaninch at gmail.com> wrote:
>
> Nyall,
>
> in 2.14.6 I get incorrect areas in a layer with CRS 3857 but not with a
layer CRS 26910(UTM 10N).
>
> With the UTM layer I get different areas with OTR on and off, but the
difference is small and I assume it is just due to the different ellipsoids.
>
> When calculating area in the field calculator with a layer whose CRS is
3857 I get the correct area in these cases: 1) OTR is turned off or
2)ellipsiod is set to None/planimetric.
>
> If the default ellipsoid for the project CRS is kept I get incorrect area
values with 2 different project CRS': 3857 and 26910(UTM 10N).
>
> the difference in values is large and variable, for instance:
> correct vs incorrect
> 20243 sqm vs 612695 sqm
> 1333 sqm vs 721 sqm

Hi Andrew,

Can you please share your file? Cut it down to just a few polygons and let
me know what area you expect to see for each. Email direct to myself.

Thanks!

Nyall

>
> Andrew
>
> On Fri, Oct 14, 2016 at 7:51 AM, Carlos Cerdán <sig.upagu at gmail.com>
wrote:
>>
>> Hi Nyall
>>
>> I'm afraid that 2.16 has still this issue. I've loaded an UTM-17 south
layer (my zone) and a Lat-long layer and:
>>
>> 1. SRC was seted in UTM
>>
>> 2. In UTM layer, if OTF SRC transformation is active, I get different
area than if it's deactivated. Correct value is the last one.
>>
>> 3. In Lat-long layer, if OTF SRC transformation is active, calculated
area is same as the wrong value of first layer. I can't get the correct
value in this layer, so I have to reproject into a new one and do step 2
(with OTF deactivated).
>>
>> What about a general option to set the prefered SRC to calculate areas
and lengths with OTF active?
>>
>> Regards from Peru
>>
>> Carlos
>>
>>
>>
>> 2016-10-12 18:06 GMT-05:00 Nyall Dawson <nyall.dawson at gmail.com>:
>>>
>>> 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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