[postgis-users] change of polygon area due to st_transform

Birgit Laggner birgit.laggner at vti.bund.de
Mon Mar 19 01:11:56 PDT 2012

Hi Steve,

thanks for your reply. Now, I thought about it, I better understand the 
consequences of using planar projections. My approach to minimize errors 
would therefore be to transform my polygons into a spherical system in 
order to compute their area.



Am 15.03.2012 14:45, schrieb Stephen Woodbridge:
> On 3/15/2012 7:41 AM, Birgit Laggner wrote:
>> Dear list,
>> using the function st_transform() in order to transform polygon
>> geometries from srid 31468 to 31467, the polygon area of the transformed
>> geometry is changed compared to the original polygon area . The
>> percentual change is around 0.1% but with big geometries, it results in
>> changes of several hectares, which surprises and concerns me a little 
>> bit.
>> Can anybody tell me if this is expected behaviour?
> Hi Birgit,
> Yes, this is expected. The reason that we have lots of transforms in 
> the first place is because each transforms has special properties that 
> make it unique in some way and allow it to solve this or that problem 
> better than some other transform. So for example, some transforms 
> preserve area better than others. Some are good approximation at large 
> scales and some are better at small scales. So you might want your 
> data in one transform for one operation and need to project it into 
> another for say area calculation, etc. Beyond that, I'm not sure I can 
> recommend which is best for your particular situation.
> You might want to construct a square(s) of known size in each 
> projection and measure them and then reproject and measure them to get 
> a better idea. You have two numbers, How do you know which it 
> "correct"? How do you know if either are correct? How is ground truth 
> established for your data?
> -Steve
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