[postgis-users] different results for area with different programms

[Astrid Emde]

On Thu, February 23, 2012 3:22 pm, Sandro Santilli wrote:
> On Thu, Feb 23, 2012 at 02:58:49PM -0000, Astrid Emde wrote:
>
>> Hello,
>>
>>
>> I calculate the area of polygons and get different results with
>> diffrent programms.
>>
>> Here is an example polygon:
>>
>>
>> Select st_area(geometryfromtext(
>> 'POLYGON ((
>> 2569220.83 5630370.607,
>> 2569215.357 5630374.968,
>> 2569216.017 5630375.795,
>> 2569216.19 5630376.01,
>> 2569221.663 5630371.649,
>> 2569220.83 5630370.607
>> ))',31466) );
>>
>>
>> These are the results that I get:
>> * PostGIS: 9.337890625 (with "POSTGIS="1.5.1" GEOS="3.2.0-CAPI-1.6.0")
>> * AutoCAD Map 2012: 9.3362
>> * Kanal++ [1]: 9.3361645
>>
>>
>> PostGIS gives back a slightly bigger area than the other programms. Do
>> you know why and who is right?
>
> PostGIS area calculator improved in 1.5.4 (yet to be released).
> BTW, are those meters ? You're worried of 0.001 square meters precision ?
> What's the precision of your coordinates ? Are your straight lines
> in your model really straight in the phisical world ?
>
> --strk;

Hello Sandro, hello Martin,

thanks for your answers. Looks like we should get a newer PostGIS and GEOS
version.

We are handling communal data in meters and want to calculate fees for
rainwater run-off. As the area in PostGIS was always a bit bigger we
should not worry ;)

So, we were not really worried. We just noticed the difference.

Thanks for your help

Astrid