[Qgis-user] Difference in Area/Perimeter calculations

Wed Jul 11 08:42:42 PDT 2012

I confirm that apparently some  inconsistencies arise between the various
Area tools.
I will investigate ASAP.

giovanni

2012/7/11 Harish <harry.om2000 at gmail.com>

> Now let us go deeper.
>
> I tried one polygon of the same layer having area of around 1300 hectares.
> I keep layer in Projected form as 32643 and used area calculate tool from
> Attribute tool bar. I keep snapping of 20 pixels for the layer so that my
> cursor should not deviate. The area comes as 13.099 Sqkm which is equal to
> 13099000 sqm. (layer was in UTM and tool used ellipsoid as per default in
> Measure tool).
>
> Now we look at area calculated by 'Export/Add geometry tool' previously
> using ellipsoid and layer CRS (UTM) for the same polygon which comes
> 13108140.7 sqm and 12988046.4 sqm. respectively. The difference between
> ellipsoidal measurement comes to 9140.7 (about 1 ha.) meter which may be
> rounding error, and error is around .076 percent. The area difference
> between ellipsoidal and projected measurement for same polygon comes to
> 116094.30 sqm around 16 hectare. so error is  1.23 percent which may rise
> when area will increase.
>
> The same can be repeated with the original data I have attached previously.
>
> So if I am farmer or government surveyor which method I should use?
>
> Regards
>
> Harish
>
> On Wed, Jul 11, 2012 at 7:30 PM, G. Allegri <giohappy at gmail.com> wrote:
>
>> 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>
>>>
>>
>>
>
>
> --
> 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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