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<P><FONT SIZE=2>Disregard my last comment about putting the image on the ST_ISSimple. I see you put a link to it which is good enough<BR>
<BR>
<BR>
-----Original Message-----<BR>
From: postgis-devel-bounces@postgis.refractions.net on behalf of Obe, Regina<BR>
Sent: Fri 12/19/2008 2:17 PM<BR>
To: PostGIS Development Discussion; PostGIS Development Discussion<BR>
Subject: RE: [postgis-devel] Testing ST_LineMerge issue<BR>
<BR>
<BR>
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.<BR>
<BR>
I was looking at what the competition was showcasing to get an idea<BR>
<A HREF="http://publib.boulder.ibm.com/infocenter/dzichelp/v2r2/index.jsp?topic=/com.ibm.db29.doc.spatial/rsbp4067.htm">http://publib.boulder.ibm.com/infocenter/dzichelp/v2r2/index.jsp?topic=/com.ibm.db29.doc.spatial/rsbp4067.htm</A><BR>
<A HREF="http://webhelp.esri.com/arcgisserver/9.3/java/index.htm#geodatabases/st_issimple.htm">http://webhelp.esri.com/arcgisserver/9.3/java/index.htm#geodatabases/st_issimple.htm</A><BR>
<BR>
<BR>
<BR>
-----Original Message-----<BR>
From: postgis-devel-bounces@postgis.refractions.net on behalf of Kevin Neufeld<BR>
Sent: Fri 12/19/2008 2:07 PM<BR>
To: PostGIS Development Discussion<BR>
Subject: Re: [postgis-devel] Testing ST_LineMerge issue<BR>
<BR>
I find that simplicity / validity is always a big topic, so I upgraded the section (with pictures and all) in the<BR>
PostGIS docs regarding the OGC's definitions.<BR>
<BR>
<A HREF="http://postgis.refractions.net/documentation/manual-svn/ch04.html#OGC_Validity">http://postgis.refractions.net/documentation/manual-svn/ch04.html#OGC_Validity</A><BR>
<BR>
To answer your question Mark, the OGC specs say that a LINESTRING is simple if it does not pass through the same point<BR>
twice. To me, this means self-intersections and perhaps coincident vertices. At the moment, PostGIS returns true for<BR>
this when by rights it should be false:<BR>
SELECT issimple('LINESTRING(0 0, 1 1, 1 1, 2 2)');<BR>
issimple<BR>
----------<BR>
t<BR>
(1 row)<BR>
<BR>
Something to look into? Are coincident vertices considered to be a self-intersection?<BR>
<BR>
Also, Regina, I think the examples in ST_IsSimple and ST_IsValid should be clarified. The example in ST_Simple tests<BR>
the simplicity on a polygon, even though this will always return true. ST_IsValid tests the validity of a linestring,<BR>
even though this will always return true (except for cases when the linestring is actually a linearring ... and even<BR>
then, I believe it just tests to see if the linestring has > 2 points).<BR>
<BR>
Cheers,<BR>
Kevin<BR>
<BR>
<BR>
Mark Cave-Ayland wrote:<BR>
> Obe, Regina wrote:<BR>
><BR>
>> Now there are 2 issues here.<BR>
>><BR>
>> 1) GEOS ST_LineMerge is obviously not smart enough to realize that<BR>
>> linemerging something like that is silly.<BR>
><BR>
> Ooops :)<BR>
><BR>
>> 2) ST_MakeLine lets you get away with creating this monstrosity. Which<BR>
>> hmm I suppose there are reasons why someone may want to, but perhaps we<BR>
>> shouldn't go there.<BR>
><BR>
> This reminds me actually, what is the OGC-SFS take on repeated points? I<BR>
> don't remember seeing any mention of this in any of the parser/unparser<BR>
> work I did earlier, although it seems to be required for a geometry to<BR>
> be valid. So is it also required for a geometry to be simple?<BR>
><BR>
><BR>
> ATB,<BR>
><BR>
> Mark.<BR>
><BR>
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