[postgis-users] Merge lines

[Sufficool, Stanley]

Thanks! I guess having a zero area bounding box would be an illegal
geometry.

> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net 
> [mailto:postgis-users-bounces at postgis.refractions.net] On 
> Behalf Of Kevin Neufeld
> Sent: Wednesday, October 07, 2009 11:19 AM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] Merge lines
> 
> 
> Sufficool, Stanley wrote:
>   > How would the bounding box on a point be a problem with 
> group by? I am
> > aware of the classic example of a line 0,0-5,5 and 0,5-5,0 being 
> > treated as same when grouped. But does this really apply to points?
> 
> Unfortunately, yes.  Consider these two close points that 
> have the same bounding box:
> 
> SELECT column1::box2d
> FROM (VALUES
>    ('POINT(0.123456789 0.123456789)'::geometry),
>    ('POINT(0.123456788 0.123456788)'::geometry)) AS foo;
> 
>                                     column1
> --------------------------------------------------------------
> ----------------
>   BOX(0.123456783592701 0.123456783592701,0.123456791043282 
> 0.123456791043282)
>   BOX(0.123456783592701 0.123456783592701,0.123456791043282 
> 0.123456791043282) (2 rows)
> 
> So, using a group by on the bounding box will treat these two 
> points will group them together, just like your linear 
> example above.
> 
> If your dataset is of significant precision,  grouping by the 
> hex string of the point works well.
> 
> Cheers,
> Kevin
> 
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