[postgis-users] Postgis Area size problem

[Paulius J]

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
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> 

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