[Qgis-user] Difference in Area/Perimeter calculations

[Harish]

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 <giohappy at 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.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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