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-boun...@lists.osgeo.org] On Behalf Of Nyall 
Dawson
Sent: 14. oktober 2016 22:16
To: Andrew <amcani...@gmail.com>
Cc: qgis-user <qgis-user@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" 
<amcani...@gmail.com<mailto:amcani...@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.up...@gmail.com<mailto:sig.up...@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.daw...@gmail.com<mailto:nyall.daw...@gmail.com>>:
>>>
>>> On 12 Oct 2016 11:56 PM, "Carlos Cerdán" 
>>> <sig.up...@gmail.com<mailto:sig.up...@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 
>>> > <del...@gmail.com<mailto:del...@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.cadi...@archeotec.ca<mailto:nicolas.cadi...@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:
>>> >>>> http://osgeo-org.1560.x6.nabble.com/inconsistenty-when-calculating-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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