[postgis-devel] Testing ST_LineMerge issue

[Kevin Neufeld]

Yeah, I wasn't sure if we should copy the relevant sections of ST_IsSimple and ST_IsValid to their appropriate sections 
in reference_new.xml ... might save someone from overlooking the link.  But as you say, perhaps the link is good enough.

-- Kevin


Obe, Regina wrote:
> Disregard my last comment about putting the image on the ST_ISSimple.  I 
> see you put a link to it which is good enough
> 
> 
> -----Original Message-----
> From: postgis-devel-bounces at postgis.refractions.net on behalf of Obe, Regina
> Sent: Fri 12/19/2008 2:17 PM
> To: PostGIS Development Discussion; PostGIS Development Discussion
> Subject: RE: [postgis-devel] Testing ST_LineMerge issue
> 
> 
> 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.
>  >
> _______________________________________________
> postgis-devel mailing list
> postgis-devel at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-devel
> 
> 
> 
> 
> -----------------------------------------
> The substance of this message, including any attachments, may be
> confidential, legally privileged and/or exempt from disclosure
> pursuant to Massachusetts law. It is intended
> solely for the addressee. If you received this in error, please
> contact the sender and delete the material from any computer.
> 
> ------------------------------------------------------------------------
> 
> * The substance of this message, including any attachments, may be 
> confidential, legally privileged and/or exempt from disclosure pursuant 
> to Massachusetts law. It is intended solely for the addressee. If you 
> received this in error, please contact the sender and delete the 
> material from any computer. *
> 
> ------------------------------------------------------------------------
> 
> * Help make the earth a greener place. If at all possible resist 
> printing this email and join us in saving paper. *
> 
> * *
> 
> * *
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> postgis-devel mailing list
> postgis-devel at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-devel