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.
Regards,
Birgit.
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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