I confirm what I wrote before. Measurements on ellipsoid is different then on a planar projection. This is not an "error", this is how geodesy is :)
giovanni PS: and this is why there's the option to choose the ellipsoid you want to work with... 2012/7/11 Harish <harry.om2...@gmail.com> > So, to elaborate the tool and discussion, I have prepared a brief document > in word format for *Export/Add geometry columns* tool for better > understanding of tool and question. > > Paragraph written in bold and italic in document give hint to the query I > am discussing. > > Hope this will sufficient to clear the problem/query from my side. > > Editing/comments on document are welcome. > > Regards > > harish > > > > > On Wed, Jul 11, 2012 at 2:26 PM, G. Allegri <gioha...@gmail.com> wrote: > >> I'm sorry Harish but I cannot understand what are the steps you're doing. >> Probably a language problem :) >> >> First question. You added the area/perimeter fields and then did you >> filled them with the Field calculator (Geometry->$area method)? >> The Ellipsoid for measurment must be set to WGS84. >> You are measuring with the Layer CRS, WGS84, WGS84 with on the fly >> projection? >> And, how do you measure area precisely with the Measure Tool??? >> >> giovanni >> >> >> >> >> 2012/7/11 Harish <harry.om2...@gmail.com> >> >>> OK, This is not a big issue but we use *'Ellipsoidal' *measurements >>> in*'Measure tool' >>> * available with* Attribute tool bar,* in all projections. >>> >>> This is to be checked later, what effect it may put on precise >>> measurements and choices of measurement methods. >>> >>> Regards >>> Harish >>> >>> On Wed, Jul 11, 2012 at 11:58 AM, G. Allegri <gioha...@gmail.com> wrote: >>> >>>> So, the differences are between using/not using the Ellipsoid option? >>>> It's normal. Measurements on the ellipsoid are different then on a >>>> projected CRS. The first use geodesic metrics while the latter is a planar >>>> metrics. >>>> >>>> Giovanni >>>> >>>> Inviato da dispositivo mobile >>>> Il giorno 11/lug/2012 07.40, "Harish" <harry.om2...@gmail.com> ha >>>> scritto: >>>> >>>> Shape file is attached as Zipped to use. >>>>> I have used "*Vector>Geometry Tools>Export/Add Geometry colum*n" on >>>>> this file using LayerCRS, ProjectCRS (Both are UTM Zone 43N/WGS84 code >>>>> 32643), and Ellipsoid option which is also WGS84 in >>>>> *settings>options>maptools>ellipsoid >>>>> for distance calculations*. Result (area, perimeter) by using project >>>>> and layer CRS is same (it should be), but result from using ellipsoid >>>>> option is different than these two. Is this difference justifiable or not? >>>>> (brad is providing a justification). >>>>> Regards >>>>> Harish >>>>> >>>>> On Tue, Jul 10, 2012 at 7:27 PM, Brad Nesom < >>>>> kidsmake6until2...@gmail.com> wrote: >>>>> >>>>>> All crs are mathematic approximations of a surface. Each with an >>>>>> acceptable distortion of either measurement (with aesthetic distortion) >>>>>> or >>>>>> acceptable aesthetic distortion (with measured distortion). For these >>>>>> reasons you should not expect two different projection systems to produce >>>>>> exact duplicate measurements. That is. Unless both systems are designed >>>>>> for >>>>>> accuracy. >>>>>> >>>>>> Brad Nesom GISP >>>>>> 580-616-2029 >>>>>> >>>>>> >>>>>> On Jul 10, 2012, at 2:30 AM, "G. Allegri" <gioha...@gmail.com> wrote: >>>>>> >>>>>> Hi Harish, >>>>>> how did you calculate the area/perimeter values? >>>>>> Could you provide us an excerpt of your data to test it? >>>>>> >>>>>> giovanni >>>>>> >>>>>> 2012/7/10 Harish <harry.om2...@gmail.com> >>>>>> >>>>>>> Yes, when ellipsoid is same, why there is a difference. >>>>>>> >>>>>>> Regards >>>>>>> >>>>>>> On Tue, Jul 10, 2012 at 10:57 AM, Alister Hood < >>>>>>> alister.h...@synergine.com> wrote: >>>>>>> >>>>>>>> Hi Harish, >>>>>>>> It's a little unclear - are you asking for an explanation of why >>>>>>>> there is a difference? >>>>>>>> >>>>>>>> Regards, >>>>>>>> Alister >>>>>>>> >>>>>>>> > Date: Mon, 9 Jul 2012 13:03:05 +0530 >>>>>>>> > From: Harish <harry.om2...@gmail.com> >>>>>>>> > To: Qgis-user@lists.osgeo.org >>>>>>>> > Subject: [Qgis-user] Difference in Area/Perimeter calculations >>>>>>>> > Message-ID: >>>>>>>> > <CAKCjEX8-H++RBnVFF5trsKMv= >>>>>>>> ldqoo43bbivapod6jox2nb...@mail.gmail.com> >>>>>>>> > Content-Type: text/plain; charset="iso-8859-1" >>>>>>>> > >>>>>>>> > There is difference between area/perimeter calculations with >>>>>>>> ellipsoid and >>>>>>>> > project CRS. Attached is a screenshot in which first area and >>>>>>>> perimeter >>>>>>>> > columns are generated with tool 'Export/Add geometry columns' >>>>>>>> using >>>>>>>> > 'Ellipsoid' and second area and perimeter with using 'Project >>>>>>>> CRS' (used >>>>>>>> > 32643: UTM zone 43 North/WGS1984). >>>>>>>> > >>>>>>>> > Ellipsoid/Spheroid used in both cases is WGS84. >>>>>>>> > >>>>>>>> > QGIS1.8, Windows7. >>>>>>>> > >>>>>>>> > Regards >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>> >>>>>>> >>>>> >>> >>> >> > > > -- > Harish Kumar Solanki, Assistant Professor > National Institute of Rural Development (NIRD), > Ministry of Rural Development, Government of India > Jaipur, Rajasthan, India- Pin-302006 > > Email- hksola...@nird.gov.in, harry.om2...@gmail.com, > Tele-Fax-0141-2209905, Mo.- 09214446732, 09414291732 > Website- http://www.nird.org.in, > http://watershed.rajasthan.gov.in/<http://www.nird.org.in> > LinkedIn: www.linkedIn.com/in/hksolanki > <http://www.nird.org.in> >
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