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

Sun Oct 16 15:41:54 PDT 2016

On 15 October 2016 at 01:49, 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

Andrew,

There's no issue here. The values calculated by QGIS match within
expected tolerances (ie < 0.2%) to those calculated by PostGIS. You
need to compare QGIS' ellipsoidal (OTF) areas with the results of the
PostGIS expression:

ST_Area(ST_Transform(geom,4326)::geography)

By just comparing against ST_Area(geom) you are comparing the
cartesian area in the projected EPSG3857 CRS vs the ellipsoidal area,
and as mentioned previously you NEVER want to calculate areas in 3857.
Basically doing anything except viewing web map tiles in EPSG3857 is a
Bad Idea!

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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>>> >>>> discussion below:
>>> >>>>
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>>> >>>> NAML
>>> >>>
>>> >>>
>>> >>> ________________________________
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>>> >>> depending on file type or projection?
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