[Qgis-user] Difference in Area/Perimeter calculations

[Harish]

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 <kidsmake6until2019 at 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" <giohappy at 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.om2000 at 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.Hood at 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.om2000 at gmail.com>
>>> > To: Qgis-user at lists.osgeo.org
>>> > Subject: [Qgis-user] Difference in Area/Perimeter calculations
>>> > Message-ID:
>>> >         <CAKCjEX8-H++RBnVFF5trsKMv=
>>> LdqOo43bbiVAPOD6jOX2nbmmA at 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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