[postgis-users] st_simplifypreservetopology

Mon Apr 21 12:52:03 PDT 2008

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
> MULTIPOLYGON(((13.73095 51.024734,13.7309323 51.0247668,13.7306959 51.0247959,13.7292724 51.0249742,13.7280216 51.0251252,13.7266598 51.0252998,13.7259617 51.0254072,13.7258854 51.0254201,13.7253253 51.0255144,13.725276 51.025492,13.724538 51.025631,13.7230288 51.0259021,13.7223529 51.0260273,13.7223299 51.0260863,13.7222292 51.026391,13.7220002 51.0273366,13.7217875 51.0282094,13.721746 51.028243,13.7217693 51.0282803,13.7215512 51.0291967,13.721513 51.029222,13.7215203 51.0292567,13.7212713 51.0295967,13.7222258 51.0299532,13.722234 51.03,13.7222931 51.0299823,13.7232514 51.0303187,13.7242514 51.0306715,13.724263 51.030714,13.7243024 51.0306951,13.7249934 51.0309315,13.7265097 51.0314552,13.7266116 51.0313952,13.7267988 51.0313334,13.7269952 51.0313243,13.72703 51.0314107,13.7271637 51.0313254,13.7272524 51.0313839,13.72739 51.031449,13.7276768 51.0313074,13.7283793 51.0309944,13.7296654 51.0304157,13.7297572 51.0303637,13.729845 51.0303139,13.7299557 51.0301763,13.7300964 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51.0354359,13.7492829 51.034867,13.7492723 51.0348311,13.7492455 51.0347398,13.7493034 51.0346612,13.7491987 51.0346142,13.748866 51.034723,13.748791 51.034201,13.748335 51.034159,13.748294 51.034034,13.748205 51.033764,13.7488691 51.0333037,13.748962 51.033245,13.7486777 51.0332252,13.7483008 51.032683,13.7484397 51.0324582,13.7469913 51.0327817,13.7466998 51.0326205,13.7459997 51.0314852,13.7460996 51.0313569,13.745967 51.0314864,13.7449355 51.0317377,13.7447301 51.0316513,13.7446705 51.0318463,13.7420262 51.0323659,13.7419131 51.0322884,13.7418636 51.0322552,13.7416501 51.0321425,13.7415567 51.0317708,13.7414972 51.0314666,13.741484 51.0311492,13.741923 51.031003,13.7418649 51.030884,13.74209 51.0304134,13.7422077 51.0300143,13.7421975 51.0299222,13.742286 51.029835,13.7421463 51.0297533,13.7420951 51.0296254,13.7415933 51.0288452,13.7414906 51.0286855,13.7414437 51.0286127,13.7413482 51.0284642,13.7410545 51.0280777,13.7407158 51.0277229,13.7401513 51.0273842,13.7392803 51.0270293,13.7382744 51.0267844,13.737321 51.0267454,13.7365929 51.0267541,13.736556 51.026812,13.7364715 51.026754,13.7357088 51.0268017,13.7353967 51.02678,13.73534 51.02685,13.7352667 51.0267757,13.734907 51.0267324,13.734824 51.02679,13.7347684 51.0267064,13.7342093 51.0266674,13.73409 51.026725,13.7340359 51.0266283,13.7335072 51.0265633,13.733407 51.02663,13.7333208 51.0265373,13.7317087 51.0263813,13.7317173 51.0263119,13.73167 51.026241,13.7317563 51.0261602,13.7318473 51.0258395,13.7318647 51.0254971,13.73183 51.0253281,13.7317736 51.0252414,13.731663 51.025181,13.7316826 51.0251114,13.7310803 51.0247604,13.73095 51.024734)),((13.7368533 51.0470386,13.7368426 51.0471226,13.7368067 51.0472669,13.7368255 51.0473828,13.7369099 51.0474154,13.7376695 51.0474677,13.7382756 51.0474245,13.738513 51.0474297,13.7386105 51.0474065,13.738705 51.0473737,13.7385856 51.0473757,13.7385618 51.0473751,13.7385263 51.0473743,13.7384706 51.0473744,13.7383071 51.0473734,13.7383822 51.0473564,13.7390821 51.047287,13.7390933 51.047209,13.7390933 51.0471421,13.7368533 51.0470386)),((13.7367293 51.0470057,13.7346615 51.0466892,13.7347551 51.0468411,13.7347754 51.0470359,13.7347106 51.0471899,13.7356421 51.0472919,13.7366963 51.0474074,13.736705 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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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022