I confirm that apparently some inconsistencies arise between the various Area tools. I will investigate ASAP.
giovanni 2012/7/11 Harish <harry.om2...@gmail.com> > 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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