[postgis-devel] [PostGIS] #475: ST_Simple should return false with duplicate points
Martin Davis
mbdavis at refractions.net
Wed Mar 31 09:49:13 PDT 2010
IMO the semantics of isSimple are fine the way they are. Consecutive
duplicate points aren't *materially* non-simple - they're just a
topologically irrelevant structural artifact.
The SFS states that:
2.1.5 Curve
Topologically a Curve is a one-dimensional geometric object that is the
homeomorphic image of a real,
closed, interval D = [a, b] = {x Î R ½ a <= x <= b} under a mapping
f:[a,b] ® Â2 as defined in [1],
section 3.12.7.2.
I think you can read both statements as applying only to situations
where the duplicate points occur at *different* indexes in the closed
interval [a,b]. This is *not* the case for duplicate points which occur
consecutively in the coordinate list - they are the image of the *same*
index x in the parameterization [a,b].
Martin
PostGIS wrote:
> #475: ST_Simple should return false with duplicate points
> ----------------------+-----------------------------------------------------
> Reporter: kneufeld | Owner: pramsey
> Type: defect | Status: new
> Priority: low | Milestone: PostGIS 2.0.0
> Component: postgis | Version: trunk
> Keywords: |
> ----------------------+-----------------------------------------------------
> The OGS specs state: ''"has no anomalous geometric points, such as self
> intersection or self tangency"'' and ''"is simple if it does not pass
> through the same point twice"''. (2.1.5)
>
> The SQL/MM specs state: ''"is not simple if any interior point has the
> same location as another interior point or a point on the boundary"''
> (4.1.4)
>
> So, by definition then, this should return false.
>
> {{{
> select st_issimple(
> 'LINESTRING(0 0, 1 1, 1 1)'::geometry
> );
> st_issimple
> -------------
> t
> (1 row)
> }}}
>
>
--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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