[Qgis-user] Difference in Area/Perimeter calculations

[G. Allegri]

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.om2000 at 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 <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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