Now let us go deeper. I tried one polygon of the same layer having area of around 1300 hectares. I keep layer in Projected form as 32643 and used area calculate tool from Attribute tool bar. I keep snapping of 20 pixels for the layer so that my cursor should not deviate. The area comes as 13.099 Sqkm which is equal to 13099000 sqm. (layer was in UTM and tool used ellipsoid as per default in Measure tool).
Now we look at area calculated by 'Export/Add geometry tool' previously using ellipsoid and layer CRS (UTM) for the same polygon which comes 13108140.7 sqm and 12988046.4 sqm. respectively. The difference between ellipsoidal measurement comes to 9140.7 (about 1 ha.) meter which may be rounding error, and error is around .076 percent. The area difference between ellipsoidal and projected measurement for same polygon comes to 116094.30 sqm around 16 hectare. so error is 1.23 percent which may rise when area will increase. The same can be repeated with the original data I have attached previously. So if I am farmer or government surveyor which method I should use? Regards Harish On Wed, Jul 11, 2012 at 7:30 PM, G. Allegri <gioha...@gmail.com> wrote: > 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> >> > > -- 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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