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

[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