[postgis-tickets] [PostGIS] #4755: ST_Transform of valid polygon can produce invalid polygon

[PostGIS]

#4755: ST_Transform of valid polygon can produce invalid polygon
--------------------------+---------------------------
 Reporter:  rouen         |      Owner:  pramsey
     Type:  defect        |     Status:  new
 Priority:  medium        |  Milestone:  PostGIS 3.1.0
Component:  postgis       |    Version:  3.0.x
 Keywords:  ST_Transform  |
--------------------------+---------------------------
 Using ST_Transform to change SRID from 3857 to 4326 can produce invalid
 (self-intersecting) polygon from valid one.

 Test case (this is simplified land polygon from OSM dataset):

 WITH sample AS
 (
   SELECT ST_GeomFromText('MULTIPOLYGON(((2382720.17390635
 9089461.95191981,2382367.6918707 9089034.6330938,2383639.33894183
 9088936.19228217,2383491.66250535 9088320.85158849,2382449.73433542
 9088378.30497631,2382055.93050479 9088845.95402014,2381571.89109492
 9088566.99620937,2382909.16100587 9087525.07043225,2383385.00730122
 9087713.76366274,2382367.6918707 9086433.9341019,2383007.61196353
 9086589.80403919,2382892.75251293 9086056.52215661,2383387.76802459
 9086009.53671596,2384109.85303357 9086307.94844515,2383500.05599495
 9084868.129924,2384106.96985876 9084899.76660559,2383959.29342227
 9083792.17446382,2384344.89300643 9083513.24218185,2383385.00730122
 9082274.42190291,2382950.18223823 9082971.7746775,2381932.87793967
 9082873.34378008,2382088.74749067 9084563.39240152,2382671.24899015
 9084481.35648623,2382310.2621454 9085268.94495686,2381481.64438374
 9084021.89443657,2381875.44821437 9082331.85126724,2382523.57255366
 9081798.56835936,2381982.09228655 9079919.82309238,2381916.46944672
 9081101.23196672,2381645.72931316 9081019.18678447,2381350.37644019
 9078295.40449812,2380538.16717147 9079558.84194849,2381055.03469917
 9081757.55388556,2380267.42703791 9082643.61171947,2380710.45634737
 9083168.68945391,2379931.06406453 9083021.01509475,2379233.7142464
 9083759.36841078,2378442.67794482 9082959.88657981,2378604.00215088
 9083424.05072713,2377563.78830111 9083562.16830069,2377341.87289622
 9083907.25370813,2377807.53345815 9084052.13288586,2376895.52626593
 9084309.0425308,2376862.720412 9084932.57788238,2377600.28996214
 9085547.16258457,2376452.50808842 9085383.80323754,2376772.47370081
 9086351.87675873,2377839.0034782 9086934.37453989,2379594.6899592
 9089526.88193449,2380136.17022631 9089494.07408576,2379758.77488863
 9090478.570831,2380341.26525615 9089937.0906658,2380251.02967691
 9090511.38319048,2379397.79917583 9090659.06515612,2379307.55246464
 9091061.06154116,2380316.65251674 9092365.52577592,2381383.19342608
 9091717.39068873,2381227.31274312 9092652.66570207,2381670.34205258
 9093128.5061353,2382991.20347059 9091815.82075834,2381965.6837936
 9091020.04296678,2382195.4026948 9090019.11564374,2380972.99223445
 9088796.72250146,2381941.08218614 9089764.79230159,2382720.17390635
 9089461.95191981)))', 3857) AS geom
 )
 SELECT ST_IsValid(sample.geom), ST_IsValid(ST_Transform(sample.geom,
 4326))
 FROM sample


 Version info:
 POSTGIS="3.0.1 ec2a9aa" [EXTENSION] PGSQL="120" GEOS="3.7.1-CAPI-1.11.1
 27a5e771" PROJ="Rel. 4.9.3, 15 August 2016" LIBXML="2.9.4"
 LIBJSON="0.12.1" LIBPROTOBUF="1.2.1" WAGYU="0.4.3 (Internal)"

-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/4755>
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