[postgis-users] out of memory when using simplify

[John Cole]

Ok, I see that collect will get me part of the ArcInfo dissolve
functionality (grouping lines by a field together), but I'm wondering how to
get the rest.  Namely, reducing the complexity of these grouped lines.

Is there a way to take all of the connected multilines and return a single
line that could be sent to the simplify function?

Thanks,

John

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net]On Behalf Of
strk at refractions.net
Sent: Tuesday, October 05, 2004 12:44 AM
To: 'PostGIS Users Discussion'
Subject: Re: [postgis-users] out of memory when using simplify


On Mon, Oct 04, 2004 at 02:49:01PM -0500, John Cole wrote:
> Thanks for the reply.  I've simplified my layer to 500m first and then did
> the geomunion and collect to compare them.  Both result in the same number
> of rows, but have different numbers of geometries (with the GeomUnion
being
> slightly simpler).  The Collect takes ~3 seconds while the GeomUnion takes
> over 5 hours.
> 
> In either case, you cannot run a simplify on the resulting table without
> getting the out of memory error.  I'd like to reduce the number points in
> each record to speed up drawing.  Is there a way to take a multilinestring
> and turn it into a single linestring?  I'm hoping that the simplify will
> work on a single linestring object instead of the large mulitlinestrings I
> have now.

Use GeometryN(geometry, N) to extract Nth geometry from a multi.

GeomUnion does actually modify the input geometries, merging them
on a spatial relation basis.
Collect just take them all.
Obviously the results are different!
Anyway. What you did is already reducing the number of points in
each record (simplify(the_geom)). The OutOfMemory condition happens
because you're trying to simplify a single huge geometry record
instead of each of them (run simplify on the resulting table).
Both Collect and GeomUnion are aggregates, they return a single 
value.
(I hope did help)
--strk;


> 
> Thanks,
> 
> John Cole
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net]On Behalf Of
> strk at refractions.net
> Sent: Wednesday, September 29, 2004 10:24 AM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] out of memory when using simplify
> 
> 
> On Wed, Sep 29, 2004 at 10:15:26AM -0500, John Cole wrote:
> > Hello,
> >   I'm trying to do some processing on a roads layer and simplify it for
> > rendering speed.  In ArcInfo, I would use a dissolve on the hwy_num
field
> > and then simplify it to 500 meters.
> >  
> >   From the documentation, PostGIS seems like it can do the same thing,
> first
> > selecting into a new table using either the GeomUnion or Collect
> aggregates
> > and then again using simplify, however using simplify returns the
> following
> > error:
> >  
> > ERROR:  out of memory
> > DETAIL:  Failed on request of size 4172712.
> 
> I suggest you first simplify and then collect.
> 
> >  
> > here is my query:
> >     select hwy_num, simplify(the_geom, 500) as the_geom from ra1bb
> >  
> > where ra1bb is created from:
> >     select hwy_num, geomunion(the_geom) as the_geom into ra1bb from ra1
> > group by hwy_num
> >  
> > Another question:  what is the difference between collect and geomunion
> > (besides speed)?  Which one would better approximate an ArcInfo dissolve
> > function?
> 
> geomunion implements a spatial union: two overlapping areas become
> a single area.
> collect just keeps the whole pointset making a GEOMETRYCOLLECTION
> (or a simpler type if applicable).
> 
> --strk;
> 
> 
>  
> >  
> > Thanks,
> >  
> > John Cole
> > 
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