[postgis-users] st_simplifypreservetopology

Mon Apr 21 17:07:11 PDT 2008

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
>> 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 51.0300176,13.730252 51.0298919,13.7304615 
>> 51.0297932,13.730668 51.0297363,13.730743 51.029783,13.7307859 
>> 51.0298398,13.7307094 51.0301388,13.730624 51.030263,13.7306955 
>> 51.0303267,13.7301182 51.0325594,13.7300528 51.0325663,13.7301114 
>> 51.0327342,13.7301645 51.0329094,13.7300035 51.0327693,13.7299669 
>> 51.0327351,13.7299445 51.0327211,13.7298934 51.032814,13.7298539 
>> 51.0328585,13.7297737 51.0328321,13.7288526 51.0325639,13.7288201 
>> 51.0324367,13.7284426 51.0324383,13.7276461 51.032179,13.7274569 
>> 51.0321976,13.7272787 51.0322421,13.7271265 51.0322903,13.7267034 
>> 51.0322495,13.7265364 51.0322161,13.7259018 51.0324269,13.7258649 
>> 51.03242,13.725733 51.0326646,13.7251933 51.0328876,13.7247918 
>> 51.0331374,13.7244439 51.0331106,13.7242967 51.0334273,13.7239131 
>> 51.0337529,13.7237035 51.0338511,13.7235429 51.033967,13.7233375 
>> 51.0339148,13.7232064 51.0339347,13.7231786 51.0339863,13.7228848 
>> 51.0340776,13.7224481 51.0341888,13.7220471 51.0342483,13.7217493 
>> 51.0343198,13.721552 51.0343861,13.7214718 51.0344095,13.7215108 
>> 51.034534,13.7205032 51.0349932,13.7197657 51.0352983,13.7195764 
>> 51.0352291,13.7195934 51.0352797,13.7182451 51.0359157,13.7181108 
>> 51.0359003,13.7181657 51.0359571,13.717622 51.0361956,13.7159749 
>> 51.0369683,13.7159057 51.0369284,13.7158604 51.0370288,13.7157161 
>> 51.0370124,13.7157523 51.0370733,13.7153708 51.0372801,13.7150274 
>> 51.0374899,13.7144074 51.0379192,13.7138287 51.0383899,13.7137514 
>> 51.0383857,13.7137492 51.0384566,13.7134249 51.0387269,13.7130179 
>> 51.0390385,13.7125791 51.0393343,13.7120736 51.039611,13.7115839 
>> 51.0398558,13.7112945 51.0399894,13.7114637 51.0402313,13.7123153 
>> 51.041449,13.7126333 51.0417033,13.713371 51.0421453,13.7138861 
>> 51.0424061,13.7142518 51.0425683,13.7164587 51.0435668,13.7167995 
>> 51.0437957,13.7170883 51.0439897,13.7190694 51.0451663,13.7196131 
>> 51.0458277,13.7197562 51.0461521,13.7198262 51.0464192,13.7198377 
>> 51.0467389,13.7205681 51.0455573,13.7210009 51.0450379,13.7214987 
>> 51.0445401,13.7220306 51.0442859,13.7227215 51.0439558,13.7237962 
>> 51.0434514,13.723979 51.0435278,13.7241448 51.0435041,13.7241052 
>> 51.0436042,13.7247987 51.0438896,13.7250186 51.0439093,13.7250579 
>> 51.0440386,13.7257225 51.0443545,13.7259312 51.0443456,13.725955 
>> 51.0443813,13.7260235 51.0443873,13.7260682 51.0445303,13.7282191 
>> 51.0455848,13.7290532 51.045927,13.7292643 51.0458591,13.7292228 
>> 51.0459969,13.729706 51.0461854,13.7303185 51.046393,13.7309107 
>> 51.0465601,13.731546 51.0466841,13.7321939 51.0467752,13.7332896 
>> 51.0468999,13.7333733 51.0469094,13.7334778 51.0468127,13.7335706 
>> 51.0469078,13.733651 51.0470684,13.7338458 51.0471508,13.7346109 
>> 51.0472333,13.7346367 51.0471474,13.7346922 51.0470697,13.7346666 
>> 51.0470056,13.7346564 51.0468714,13.7345552 51.0467095,13.7336001 
>> 51.0465496,13.733427 51.046454,13.7335317 51.0464255,13.7347225 
>> 51.0465948,13.7348421 51.0466562,13.7349123 51.0466203,13.736811 
>> 51.0468537,13.7382043 51.0469796,13.7383487 51.0469803,13.7394909 
>> 51.0469005,13.7400899 51.0467949,13.7405051 51.0464739,13.7408331 
>> 51.0462204,13.7412027 51.0463256,13.741053 51.0466451,13.7407291 
>> 51.0469007,13.7405095 51.0469726,13.7400888 51.0470337,13.7393051 
>> 51.0471049,13.7393014 51.0472015,13.7393088 51.0473019,13.7395556 
>> 51.0473056,13.7404944 51.0472245,13.740932 51.0470192,13.7414421 
>> 51.0465652,13.7414893 51.0465576,13.7416494 51.0464916,13.7416003 
>> 51.0466074,13.7416246 51.04663,13.741668 51.0466443,13.7417272 
>> 51.0467159,13.7417503 51.0466716,13.7423587 51.0468732,13.7426958 
>> 51.0470246,13.7429143 51.0471813,13.74318 51.04726,13.7430363 
>> 51.0472995,13.7433021 51.047588,13.7434678 51.0475916,13.7433805 
>> 51.0477019,13.7436362 51.0479981,13.7446308 51.0491622,13.7447961 
>> 51.0491827,13.744722 51.0492509,13.7448536 51.0494078,13.745056 
>> 51.0494766,13.7450313 51.0496901,13.7453573 51.0500052,13.7465317 
>> 51.0512807,13.7466999 51.0513722,13.746638 51.0514149,13.7468683 
>> 51.0516781,13.7470071 51.051777,13.7469985 51.0518746,13.7470732 
>> 51.0519866,13.7471316 51.0520528,13.7472989 51.0523089,13.7472368 
>> 51.0523858,13.7473063 51.0524932,13.7473468 51.0527412,13.7473392 
>> 51.0531614,13.7472987 51.0533157,13.7473919 51.0534224,13.7472684 
>> 51.0534549,13.7472134 51.0536926,13.7472913 51.0537784,13.7473216 
>> 51.053725,13.7474649 51.0537575,13.7474492 51.053833,13.7475625 
>> 51.0537839,13.7497379 51.0544435,13.7515333 51.0551019,13.7527693 
>> 51.0555438,13.7549766 51.0564993,13.7550622 51.0565364,13.755105 
>> 51.0566612,13.7552745 51.0566237,13.7558661 51.0560648,13.7559318 
>> 51.0560101,13.755908 51.055897,13.7559252 51.0558292,13.7559566 
>> 51.0557055,13.7564494 51.0551377,13.7564124 51.0550457,13.7573213 
>> 51.0539813,13.7575007 51.0539933,13.757856 51.0540047,13.7580394 
>> 51.054028,13.7580896 51.053984,13.7580949 51.0539463,13.7579963 
>> 51.0538534,13.7581294 51.0537147,13.7582346 51.0535957,13.758354 
>> 51.053433,13.758363 51.053392,13.7583656 51.0533457,13.758359 
>> 51.0532095,13.7583338 51.0530937,13.7582902 51.0529647,13.7580365 
>> 51.0522637,13.7577683 51.051463,13.7573182 51.0501993,13.7571595 
>> 51.0497164,13.7567579 51.0490095,13.7563383 51.0482979,13.7557757 
>> 51.0473383,13.7557095 51.0472522,13.7555771 51.0471199,13.7554448 
>> 51.0470471,13.7548596 51.0462612,13.7547097 51.046054,13.7549127 
>> 51.0460086,13.7548633 51.0459174,13.7548127 51.0458413,13.7547176 
>> 51.0457237,13.7538293 51.0449222,13.7530218 51.0441346,13.7526711 
>> 51.0437838,13.752446 51.0435522,13.7522297 51.0433547,13.751704 
>> 51.042833,13.7513058 51.0424448,13.7505766 51.0417281,13.7499967 
>> 51.0411283,13.7497695 51.0408943,13.7493849 51.0405205,13.7486222 
>> 51.0397896,13.7478209 51.0390261,13.7477474 51.0389532,13.7477041 
>> 51.0389189,13.7476277 51.0388729,13.7475781 51.0388513,13.7472699 
>> 51.038726,13.747131 51.0386506,13.7469329 51.0385052,13.7468562 
>> 51.0384284,13.7466683 51.0383483,13.7467998 51.038236,13.7473841 
>> 51.0380129,13.747838 51.0378277,13.7481801 51.0376558,13.7489728 
>> 51.0370285,13.7491313 51.0368016,13.7492665 51.0363477,13.7493166 
>> 51.0359389,13.7492966 51.0358087,13.7493888 51.0356942,13.7492867 
>> 51.0357016,13.7492855 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