[postgis-users] Postgis Area size problem

Wed Mar 12 12:58:09 PDT 2008

Paulius,

I referred to Nicolas Ribot's answer to your question on
how to get the area. He suggested to do the transform to
UTM 40N (which has a SRID = 32640, see his statement). This
was likely because he thought your data was somewhere in the
United Arab Emirates, due to the swap in lat, lon.

UTM 35N has SRID = 32635

Hope this solves the puzzle,

Guido
>-- Original Message --
>Date: Wed, 12 Mar 2008 09:52:35 -0700 (PDT)
>From: Paulius J <paulius at digitalwork.lt>
>To: postgis-users at postgis.refractions.net
>Subject: Re: [postgis-users] Postgis Area size problem
>Reply-To: PostGIS Users Discussion <postgis-users at postgis.refractions.net>
>
>
>
>Hello, GL,
>
>Thanks, for your post. The first part of the problem is easy and at last
>I
>got the query working :) While I have some questions about the second:
>'- You should use UTM zone 35N, which is the one for Lithuania'. Here as
>I
>understand you are talking about SRID`s? If yes, so is it a difference
which
>SRID I am using (I use the database only to save points or group of points
>from GPS and Google Maps? In database I only want to save the lat and lon
>from GPS device or Google Maps on the global scope, so which SRID should
>I
>use, or there is no difference in my situation? The boundaries of Lithuania
>is only an example because I live here.
>
>
>P.S. sorry for such a question but I am new in this.
>
>Paulius J
>
>Guido Lemoine wrote:
>> 
>> PJ,
>> 
>> I guess your dealing with Lithuania data. Your problem is twofold:
>> 
>> - You have lat and lon in the wrong order
>> - You should use UTM zone 35N, which is the one for Lithuania
>> 
>> Hence, do the following:
>> 
>> select st_area(st_transform(geometryFromText('POLYGON((21.06079 56.05364
>>  ,22.12646 56.43213 ,24.55444 56.27386 ,24.88403 56.41998
>>  ,25.10376 56.17002 ,26.63086 55.6528 ,26.25732 54.965 ,25.75195 54.78168
>>  ,25.62012 54.31652 ,25.74097 54.14957 ,25.53223 54.18173 ,25.47729
>> 54.28447
>>  ,24.37866 53.90434 ,23.47778 53.96255 ,23.20313 54.25881 ,22.77466
>> 54.35496
>>  ,22.82959 54.78802 ,22.65381 54.95239 ,21.32446 55.26034 ,21.0498 56.0475
>>  ,21.06079 56.05364 ))', 4326), 32635));
>> 
>> --> 64039242950.5568 (sqm)
>> 
>> GL
>> 
>> Paulius J wrote:
>>> Thank you for your quick reply Nicolas,
>>>
>>> Now I get the results which are more likely to be the truth, but they
>>> differ
>>> from reality quiet a lot.
>>> Maybe it can be a problem that I got these coordinates from Google Maps?
>>> If
>>> I am correct, the polygon which I posted as example should be about
64
>>> 000
>>> square kilometers.
>>>
>>> Paulius J
>>>
>>>
>>> Nicolas Ribot wrote:
>>>   
>>>> On 11/03/2008, Paulius J <paulius at digitalwork.lt> wrote:
>>>>     
>>>>>  Hello,
>>>>>
>>>>>  I have such a problem. Here is my polygon and it looks that everthing
>>>>> is
>>>>>  allright:
>>>>>  POLYGON((56.05364 21.06079,56.43213 22.12646,56.27386
>>>>> 24.55444,56.41998
>>>>>  24.88403,56.17002 25.10376,55.6528 26.63086,54.965 26.25732,54.78168
>>>>>  25.75195,54.31652 25.62012,54.14957 25.74097,54.18173
>>>>> 25.53223,54.28447
>>>>>  25.47729,53.90434 24.37866,53.96255 23.47778,54.25881
>>>>> 23.20313,54.35496
>>>>>  22.77466,54.78802 22.82959,54.95239 22.65381,55.26034 21.32446,56.0475
>>>>>  21.0498,56.05364 21.06079))
>>>>>
>>>>>  But when i want to find the area of it using Area i get the result:
>>>>>  9.04832470074997
>>>>>  I do not know how to get the area size in square meters or something
>>>>> similar
>>>>>  to this? Maybe i should use some other function, but according the
>>>>> postgis
>>>>>  manual I use Area or ST_Area and have such problem...
>>>>>
>>>>>  P.S. i am using SRID = 4326 for the Geometry column.
>>>>>
>>>>>       
>>>> ST_area computes the area in geometry's units. Here square degrees.
>>>> Repreject your data in a metric reference system (like UTM for
>>>> instance) with st_transform(), then compute the area.
>>>>
>>>> Something like:
>>>>
>>>> select st_area(st_transform(geometryFromText('POLYGON((56.05364
>>>> 21.06079,56.43213 22.12646,56.27386 24.55444,56.41998
>>>> 24.88403,56.17002 25.10376,55.6528 26.63086,54.965 26.25732,54.78168
>>>> 25.75195,54.31652 25.62012,54.14957 25.74097,54.18173 25.53223,54.28447
>>>> 25.47729,53.90434 24.37866,53.96255 23.47778,54.25881 23.20313,54.35496
>>>> 22.77466,54.78802 22.82959,54.95239 22.65381,55.26034 21.32446,56.0475
>>>> 21.0498,56.05364 21.06079))', 4326), 32640));
>>>>
>>>> Nico
>>>> _______________________________________________
>>>> postgis-users mailing list
>>>> postgis-users at postgis.refractions.net
>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>
>>>>
>>>>     
>>>
>>>   
>> 
>> -- 
>> ----------------------------------------------------------------
>> Guido Lemoine
>> Joint Research Centre, European Commission
>> Institute for the Protection and Security of the Citizen (IPSC)
>> Support to External Security
>> Via E. Fermi, 2749 TP 267 Ispra 21027 (VA), Italy
>> Tel. +39 0332 786239 (direct line) Fax. +39 0332 785154
>> WWW: http://ses.jrc.it
>> ----------------------------------------------------------------
>> Disclaimer:
>> Views expressed are those of the individual and do not represent 
>> the views of the European Commission
>> 
>> 
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>> 
>> 
>
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