[Qgis-user] Difference in Area/Perimeter calculations

[G. Allegri]

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.om2000 at 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 <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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