[postgis-users] srid / area

[Nicolas Ribot]

Hi Christo,

Setting the SRID to geometric column does not change the data in that
column, it just tells  the spatial reference system (SRS) of your
data. Your data are still in degree.

You must use the tranform() function to reproject your data in order
to compute areas in square meters instead of square degrees.
Choose a target SRS that preserve areas.

Something like:
select area(transform(the_geom, <target_srid>)) AS ar
from cdsmriverareaview
order by ar

Nicolas

On 5/16/06, Christo Du Preez <christo at mecola.com> wrote:
> Hi All,
>
> I've always got along quite fine importing my data and setting the srid
> to -1. Now that I need to use the area function I have run the
> select UpdateGeometrySRID('public', 'cdsmriverarea', 'the_geom', 4148);
>
> I'm very inexperienced when it comes to the spacial referencing systems.
>
> The data I'm using is Hartebeespoort94/wgs84 in decimal degrees. I used
> srid 4148 an others but I keep getting the same results, the units of
> measurement should be in meters, but the result is far from it.
>
> eg.
>
> select area(the_geom) AS ar
> from cdsmriverareaview
> order by ar
>
> 6.70955000170448e-08
>
> the expected result is roughly 670 sq meters.
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