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
_______________________________________________
postgis-users mailing list
[email protected]
http://postgis.refractions.net/mailman/listinfo/postgis-users
