On 19 October 2016 at 02:22, Carlos Cerdán <sig.up...@gmail.com> wrote: > It was my fault. I was used to consider plane areas as "correct", forgetting > ellipsoidal areas. So, QGIS give us freedom to calculate planar or > ellipsoidal (geodetical) areas, but we must be careful about settings; Is > not it?. Sometimes, too much freedom is annoying! > > But, How can I calculate UTM (planar) area from a GEO layer in one step?
Under project properties, general tab - change "ellipsoid" to "None / Planimetric". I keep meaning to split up the concept of ellipsoidal measurement from OTF reprojection. In my view these are two different concepts - one is just about display of data, the other relates to measurement. Ie, you should be able to perform ellipsoidal calculations even if OTF rendering is disabled. I just keep forgetting to do it early in a release cycle for extensive testing... Nyall > > Thank you very much for your time and so sorry if I'm doing basic questions. > > Carlos > > > > 2016-10-16 17:59 GMT-05:00 Nyall Dawson <nyall.daw...@gmail.com>: >> >> On 16 October 2016 at 05:20, Carlos Cerdán <sig.up...@gmail.com> wrote: >> > Hi Nyall (and list) >> > >> > A small projecto with UTM and GEO layers can be downloaded from dropbox: >> > >> > https://www.dropbox.com/s/c4cfb48r5326vrk/Cajamarca.zip?dl=0 >> > >> > Attribute table has the correct area in AreaKm2 field (and in these >> > unities). "AreaOTFoff" is the area given by field calculator when "On >> > The >> > Fly transformation" is deactivated. "AreaOTF_on" is the area with it >> > activated. >> > >> > So: when I have a GEO layer, I can't calculate correct area directly in >> > that >> > layer: I've to reproject to UTM and deactivate automatic transformation >> > of >> > SRCs, to use field calculator expecting correct values. >> > >> > OS : Ubuntu 16.04 >> > QGIS : 2.16.3 >> > SRC settings: by default (ellipsoide WGS84 for measures) >> >> Thanks for sending this through, but again - I can't find any issue here. >> >> The different area calculations come from the difference between >> measuring the area using a flat plane (OTF off, no ellipsoid set) vs >> measuring the area using an ellipsoid (OTF on, ellipsoid set under the >> project "general" settings). When OTF is on QGIS calculates the same >> area regardless of whether the geographic or projected source layer is >> used. >> >> Hope that clarifies! >> >> Nyall >> >> >> >> > >> > Thank you for your interest >> > >> > Carlos Cerdán >> > >> > >> > 2016-10-14 15:14 GMT-05:00 Nyall Dawson <nyall.daw...@gmail.com>: >> >> >> >> On 15 Oct 2016 12:51 AM, "Carlos Cerdán" <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? >> >> >> >> Hi Carlos, >> >> >> >> Can you please share your file? Cut it down to just a few polygons and >> >> let >> >> me know what area you expect to see. Email direct to myself. >> >> >> >> Thanks! >> >> >> >> Nyall >> >> >> >> >> >> > >> >> > Regards from Peru >> >> > >> >> > Carlos >> >> > >> >> > >> >> > >> >> > 2016-10-12 18:06 GMT-05:00 Nyall Dawson <nyall.daw...@gmail.com>: >> >> >> >> >> >> On 12 Oct 2016 11:56 PM, "Carlos Cerdán" <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>: >> >> >> >> >> >> >> >> 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>: >> >> >> >>> >> >> >> >>> 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 >> >> >> >>> Qgis-user@lists.osgeo.org >> >> >> >>> List info: http://lists.osgeo.org/mailman/listinfo/qgis-user >> >> >> >>> Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> >> _______________________________________________ >> >> >> >> Qgis-user mailing list >> >> >> >> Qgis-user@lists.osgeo.org >> >> >> >> List info: http://lists.osgeo.org/mailman/listinfo/qgis-user >> >> >> >> Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user >> >> >> > >> >> >> > >> >> >> > >> >> >> > _______________________________________________ >> >> >> > Qgis-user mailing list >> >> >> > Qgis-user@lists.osgeo.org >> >> >> > List info: http://lists.osgeo.org/mailman/listinfo/qgis-user >> >> >> > Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user >> >> > >> >> > >> > >> > > > _______________________________________________ Qgis-user mailing list Qgis-user@lists.osgeo.org List info: http://lists.osgeo.org/mailman/listinfo/qgis-user Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user
