[postgis-tickets] [PostGIS] #3425: Clarify PostGIS handling of repeated points wrt validity in docs

[PostGIS]

#3425: Clarify PostGIS handling of repeated points wrt validity in docs
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 Reporter:  dbaston      |      Owner:  pramsey
     Type:  enhancement  |     Status:  new
 Priority:  low          |  Milestone:  PostGIS 2.3.0
Component:  postgis      |    Version:  trunk
 Keywords:               |
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 PostGIS considers polygons to be valid if they have consecutive repeated
 points within a ring.  I came across an interpretation of the OGC spec in
 which this is not considered valid
 (https://github.com/elastic/elasticsearch/issues/12644#issuecomment-155888645).

 Setting aside the question of whether there is any practical value in
 considering these polygons invalid, the interpretation goes like this:

 - a LineString is a Curve that uses a linear interpolation between points
 (6.1.6.1)
 - a Curve is simple if it does not pass through the same point twice, with
 the possible exception of the two endpoints (6.1.6.1)
 - a Curve that is simple and closed is a Ring (6.1.6.1)
 - the boundary of a Polygon consists of a set of LinearRings that make up
 its exterior and interior boundaries (6.1.11.1b)

 It seems that by this reasoning, if you've got consecutive repeated
 points, you're not a simple curve, and if you're not a simple curve,
 you're not a LinearRing, and so you're not a part of a valid polygon.  A
 counterargument to this is described here:

 https://lists.osgeo.org/pipermail/postgis-users/2010-March/026265.html

 Given that there is some confusion or disagreement on this, it seems
 worthwhile for the PostGIS docs (Sec. 4.3.5) to clarify that these
 polygons are indeed considered valid in PostGIS.

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Ticket URL: <https://trac.osgeo.org/postgis/ticket/3425>
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