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 >>>> > >>>> > >>>> > >>>> >>> >
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