[postgis-users] Postgis Area size problem

[Paulius J]

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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