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? 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 <[email protected]>: > On 16 October 2016 at 05:20, Carlos Cerdán <[email protected]> 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 <[email protected]>: > >> > >> On 15 Oct 2016 12:51 AM, "Carlos Cerdán" <[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? > >> > >> 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 <[email protected]>: > >> >> > >> >> On 12 Oct 2016 11:56 PM, "Carlos Cerdán" <[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]>: > >> >> >> > >> >> >> 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]>: > >> >> >>> > >> >> >>> 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] > >> >> >>> 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] > >> >> >> 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] > >> >> > List info: http://lists.osgeo.org/mailman/listinfo/qgis-user > >> >> > Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user > >> > > >> > > > > > >
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