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:[email protected]] On Behalf Of Nyall Dawson Sent: 14. oktober 2016 22:16 To: Andrew <[email protected]> Cc: qgis-user <[email protected]> Subject: Re: [Qgis-user] inconsistenty when calculating area depending on file type or projection? On 15 Oct 2016 1:50 AM, "Andrew" <[email protected]<mailto:[email protected]>> 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 > <[email protected]<mailto:[email protected]>> 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 >> <[email protected]<mailto:[email protected]>>: >>> >>> On 12 Oct 2016 11:56 PM, "Carlos Cerdán" >>> <[email protected]<mailto:[email protected]>> 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 >>> > <[email protected]<mailto:[email protected]>>: >>> >> >>> >> 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 >>> >> <[email protected]<mailto:[email protected]>>: >>> >>> >>> >>> 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 >>> >>>> >>> >>>> >>> >>>> >>> >>>> >>> >>>> ________________________________ >>> >>>> If you reply to this email, your message will be added to the >>> >>>> 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? >>> >>> Sent from the Quantum GIS - User mailing list archive at Nabble.com. >>> >>> >>> >>> _______________________________________________ >>> >>> Qgis-user mailing list >>> >>> [email protected]<mailto:[email protected]> >>> >>> List info: http://lists.osgeo.org/mailman/listinfo/qgis-user >>> >>> Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user >>> >> >>> >> >>> >> >>> >> _______________________________________________ >>> >> Qgis-user mailing list >>> >> [email protected]<mailto:[email protected]> >>> >> List info: http://lists.osgeo.org/mailman/listinfo/qgis-user >>> >> Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user >>> > >>> > >>> > >>> > _______________________________________________ >>> > Qgis-user mailing list >>> > [email protected]<mailto:[email protected]> >>> > List info: http://lists.osgeo.org/mailman/listinfo/qgis-user >>> > Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user >> >> >> >> _______________________________________________ >> Qgis-user mailing list >> [email protected]<mailto:[email protected]> >> List info: http://lists.osgeo.org/mailman/listinfo/qgis-user >> Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user > > > > _______________________________________________ > Qgis-user mailing list > [email protected]<mailto:[email protected]> > List info: http://lists.osgeo.org/mailman/listinfo/qgis-user > Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user
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