One more note on this problem.
The way in which TPS geometries may be invalid is that they may contain
holes which lie outside the parent shell, or shells which lie inside
another shell. To force these geometries to be valid, you can simple
delete the offending shells or holes. (This should only happen with
relatively small shells/holes, so this doesn't impact the geometry too
badly).
I realize this is awkward to do in pgSQL, but I thought I'd provide this
insight in case it helps someone out.
Martin
Martin Davis wrote:
Stefan,
I tested your case in JTS, and it turns out that it is a known bug in
the TopologyPreservingSimplifier algortithm (which means that GEOS
also has this problem - as you're seeing).
I agree that it would be very nice if the TPS code always returned
valid geometry. It should be possible to create a fairly simple fix
for the class of cases which your example falls into (basically, small
MultiPolygon components intersecting a larger adjacent shell when it
is simplfied).
One thing to note is that if you are using simplification as a way of
perturbing geometry to avoid intersection robustness problems, then I
think you need to use a much smaller simplification tolerance. With
the tolerance that you give, the shape of the simplified geometry is
VERY distorted compared to the original one - so the result of an
intersection() call is pretty much meaningless. The TPS algorithm
should not exhibit the "shell-intersecting" bug for appropriately
small tolerance values. (For instance, when I ran TPS using a
tolerance of 0.00057 on the test shape, it produced a valid result).
I suppose the next question is "what tolerance value can be safely
used to run TPS on a geometry"? This would be a useful function to
have, for sure. At this moment I can't see a good algorithm for
computing this kind of value. However, you could do something like:
- simplify using a standard tolerance
- if the result is invalid, scale the tolerance down by some factor
(10? 2?) and try again
- iterate until the simplified result is valid
Stefan Zweig wrote:
Hi,
i am not sure whether i should point this issue to the postgis
developer forum or maybe even to the GEOS developers:
my postgis version is:
select postgis_full_version()
POSTGIS="1.3.3" GEOS="3.0.0-CAPI-1.4.1" PROJ="Rel. 4.6.0, 21 Dec
2007" USE_STATS
and i encounter the following problem
the new function of postgis 1.3.3 st_simplifypreservetopology does
not seem to work as specified in the postgis reference. the reference
says that "ST_SimplifyPreserveTopology(geometry, tolerance) Returns a
'simplified' version of the given geometry using the Douglas-Peuker
algorithm. Will avoid creating derived geometries (polygons in
particular) that are *invalid*". but in fact
st_simplifypreservetopology does not seem to prevent returning
invalid geometries. i have just tested it with the statement below. i
will attach the geometry source for testing purposes.
it would be nice if ST_SimplifyPreserveTopology would work in the
specified manner, because it would enable postgis user to do a
st_intersection(geometry,geometry) on simplified (and then still
valid) data without getting the annoying geos error: "NOTICE:
TopologyException: side location conflict 14.609 51.552
ERROR: GEOS Intersection() threw an error!
SQL Status:XX000"
maybe i should have tested this during the 1.3.3 release candidate
period.
any ideas?
best regards,
stefan
select st_astext(_g2870."the_geom"),
st_astext(st_simplifypreservetopology(_g2870."the_geom",0.0057)),
st_isvalid(_g2870."the_geom"),
st_isvalid(st_simplifypreservetopology(_g2870."the_geom",0.0057))
FROM _g2870
st_astext, st_astext, st_isvalid, st_isvalid
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51.047249,13.7367293 51.0470057))),
MULTIPOLYGON(((13.73095 51.024734,13.7123153 51.041449,13.7552745
51.0566237,13.7484397 51.0324582,13.73095 51.024734)),((13.7368533
51.0470386,13.7369099 51.0474154,13.7390933 51.047209,13.7390933
51.0471421,13.7368533 51.0470386)),((13.7367293 51.0470057,13.7346615
51.0466892,13.7347106 51.0471899,13.7367293 51.0470057))),
true,
false
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