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