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

[Andrew]

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

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/g
>> eneral_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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>> lating-area-depending-on-file-type-or-projection-tp5290228.html
>> >>>> To start a new topic under Quantum GIS - User, email [hidden email]
>> >>>> To unsubscribe from Quantum GIS - User, click here.
>> >>>> NAML
>> >>>
>> >>>
>> >>> ________________________________
>> >>> View this message in context: Re: inconsistenty when calculating area
>> depending on file type or projection?
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>> >>>
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