[Qgis-user] Difference in Area/Perimeter calculations

[G. Allegri]

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.om2000 at 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 <giohappy at 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.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
>>>>>>>> >
>>>>>>>> >
>>>>>>>> >
>>>>>>>>
>>>>>>>
>>>>>
>>>
>>>
>>
>
>
> --
> Harish Kumar Solanki, Assistant Professor
> National Institute of Rural Development (NIRD),
> Ministry of Rural Development, Government of India
> Jaipur, Rajasthan, India- Pin-302006
>
> Email- hksolanki at nird.gov.in, harry.om2000 at 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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