[Qgis-user] Difference in Area/Perimeter calculations

Wed Jul 11 07:50:23 PDT 2012

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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