[postgis-devel] Testing ST_LineMerge issue

[Obe, Regina]

Very cool.  Would be nice to throw those same pictures in the IsSimple section.  I think you are right that ST_IsSimple needs some work.  I didn't really have a good concept of what is meant by Simplicity and still don't.

I was looking at what the competition was showcasing to get an idea
http://publib.boulder.ibm.com/infocenter/dzichelp/v2r2/index.jsp?topic=/com.ibm.db29.doc.spatial/rsbp4067.htm
http://webhelp.esri.com/arcgisserver/9.3/java/index.htm#geodatabases/st_issimple.htm



-----Original Message-----
From: postgis-devel-bounces at postgis.refractions.net on behalf of Kevin Neufeld
Sent: Fri 12/19/2008 2:07 PM
To: PostGIS Development Discussion
Subject: Re: [postgis-devel] Testing ST_LineMerge issue
 
I find that simplicity / validity is always a big topic, so I upgraded the section (with pictures and all) in the 
PostGIS docs regarding the OGC's definitions.

http://postgis.refractions.net/documentation/manual-svn/ch04.html#OGC_Validity

To answer your question Mark, the OGC specs say that a LINESTRING is simple if it does not pass through the same point 
twice.  To me, this means self-intersections and perhaps coincident vertices.  At the moment, PostGIS returns true for 
this when by rights it should be false:
SELECT issimple('LINESTRING(0 0, 1 1, 1 1, 2 2)');
  issimple
----------
  t
(1 row)

Something to look into?  Are coincident vertices considered to be a self-intersection?

Also, Regina, I think the examples in ST_IsSimple and ST_IsValid should be clarified.  The example in ST_Simple tests 
the simplicity on a polygon, even though this will always return true.  ST_IsValid tests the validity of a linestring, 
even though this will always return true (except for cases when the linestring is actually a linearring ... and even 
then, I believe it just tests to see if the linestring has > 2 points).

Cheers,
Kevin


Mark Cave-Ayland wrote:
> Obe, Regina wrote:
> 
>> Now there are 2 issues here.
>>
>> 1) GEOS ST_LineMerge is obviously not smart enough to realize that
>> linemerging something like that is silly.
> 
> Ooops :)
> 
>> 2) ST_MakeLine lets you get away with creating this monstrosity.  Which
>> hmm I suppose there are reasons why someone may want to, but perhaps we
>> shouldn't go there.
> 
> This reminds me actually, what is the OGC-SFS take on repeated points? I 
>  don't remember seeing any mention of this in any of the parser/unparser 
> work I did earlier, although it seems to be required for a geometry to 
> be valid. So is it also required for a geometry to be simple?
> 
> 
> ATB,
> 
> Mark.
> 
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