Hi Jochen,
On 01/04/2013 02:50 PM, Jochen Topf wrote:
You can create a buffer (with ST_Buffer) around one geometry and then see
whether the other geometry is inside this buffer or not. Unfortunately that
is a rather expensive operation, so it might be too slow for your use.
Yeah, Kompza on IRC suggested a similar solution with ST_Buffer. I will
try that.
I came up with something else:
st_equals(
st_snaptogrid(st_segmentize($1, 0.00002), 0.0002),
st_snaptogrid(st_segmentize($2, 0.00002), 0.0002)
);
Unfortunately this is slow for long linestrings, perhaps even slower
than ST_Buffer.
If the number of nodes in a way didn't change you could optimize by just
comparing them coordinate by coordinate.
Yes, it's also easy to determine that they are NOT similar so that I can
jump out early from further processing.
I tried ST_HausdorffDistance[1] from PostGIS but for now it does not
yield useful results or maybe I am misinterpreting them.
Any ideas?
http://www.postgis.org/docs/ST_HausdorffDistance.html
The documentation says "This is the Hausdorff distance restricted to discrete
points for one of the geometries." I am not sure what the consequences of
that are, but it might be too restrictive for your use.
Exactly, the most problematic cases are linestrings with different set
of nodes. That's why I had to add the expensive ST_Segmentize in my
solution above - to make snapping work.
I also found Frechet distance[1] which looks more interesting than
Hausdorff for my use but it's not implemented in GEOS/PostGIS and it
would probably need a similar operation to segmentization.
On the other hand, it was suggested on IRC that OWL should not really
filter out such non-changes and show everything. I'm trying to filter
them out to speed up processing, whether it makes sense to do it from
user perspective I don't know... I personally don't care too much about
ways that were simplified (without topology changes) or changed only
very slightly. Other opinions are welcome...
[1] http://www.cim.mcgill.ca/~stephane/cs507/Project.html
Paweł
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