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

Fri Oct 14 13:34:24 PDT 2016

Hi,

As far as I know, Webmercator (EPSG:3857) is not suitable for area measurements at all. So no surprise that you get “wrong” results with that CRS.
See for example: http://blog.geogarage.com/2014/09/advisory-notice-on-web-mercator.html

Cheers
Stefan

From: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Nyall Dawson
Sent: 14. oktober 2016 22:16
To: Andrew <amcaninch at gmail.com>
Cc: qgis-user <qgis-user at lists.osgeo.org>
Subject: Re: [Qgis-user] inconsistenty when calculating area depending on file type or projection?

On 15 Oct 2016 1:50 AM, "Andrew" <amcaninch at gmail.com<mailto: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<mailto: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<mailto:nyall.dawson at gmail.com>>:
>>>
>>> On 12 Oct 2016 11:56 PM, "Carlos Cerdán" <sig.upagu at gmail.com<mailto: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<mailto: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<mailto: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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