[postgis-users] Does ST_AsText truncate or round

[Paragon Corporation]

Kevin,

Actually this would help me with the second phase, but for the first phase I
just need consistency which I was hoping AsText would do for me.  It doesn't
even need to be that good since I'm sure the other side is using AsText in
some fashion.  So I just need consistency more than correctness.

Where I start off with 

Orginal geometry X 

I think I need to guarantee that 

X =  GeomFromText(AsText(X))

Seemed like the easiest way to assure that was to just force all my Xs to X
= ST_GeomFromText(AsText(X), somesrid)    which I presume should be the same
as

ST_SetSRID(AsText(X), somesrid)   

The back end editor I'm assuming is just taking the AsText(geometry)
representation and just returning that back when no edits on a particular
point,  and so its being poisoned by its own database query.  I'm not
working on that part so that's just my general observation.

Thanks,
Regina

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Kevin
Neufeld
Sent: Friday, May 16, 2008 11:15 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Does ST_AsText truncate or round

Have you considered using ST_SnapToGrid() and make all geometries in you
database respect a specified tolerance?

Warning, snaptogrid may not behave as you might expect - this is not a
simple precision_reducer method (which I think would be valuable).

Consider:
select
   'POINT(0.877 0.778)'::geometry ~=
   snaptogrid('POINT(0.87689017271623 0.777984935324639)'::geometry, 0.001);

  ?column?
----------
  t
(1 row)

No problem there, but then there's this:
select
   'POINT(0.956 0.66)'::geometry ~=
   snaptogrid('POINT(0.956079571507871 0.659502930939198)'::geometry,
0.001);

  ?column?
----------
  f
(1 row)


To deal with this (I know it looks like a hack, but it works), I convert to
text and then back to a geometry.  Wrapping this in a trigger on your
geometry tables will guarantee any geometry edits are automatically reduced
to a certain precision.

select
    'POINT(0.956 0.66)'::geometry ~=
    geomfromtext(astext(snaptogrid('POINT(0.956079571507871
0.659502930939198)'::geometry, 0.001)), -1);

  ?column?
----------
  t
(1 row)


Cheers,
Kevin

Paragon Corporation wrote:
> Slight clarification about what I am trying to solve.
> 
> After edits I am coming back with lines that used to be intersecting 
> no longer intersecting and that still have the same AsText representation.
> 
> My hope is to reduce the precision of my geometry in the database so 
> the edits don't break the topology of my lines by doing something like 
> this
> 
> 
> Update sometable
> 	SET the_geom = ST_SetSRID(ST_AsText(the_geom), mysrid)
> 
> 
> But I fear that if AsText is doing a rounding rather than a 
> truncation, that I run the risk of breaking things that used to 
> intersect.  I am more concerned about under intersecting than over 
> intersecting.  Although I probably haven't thought this out enough so
maybe it's a non-issue.
> 
> Thanks,
> Regina
> 
>  
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of 
> Paragon Corporation
> Sent: Friday, May 16, 2008 12:01 AM
> To: 'PostGIS Users Discussion'
> Subject: [postgis-users] Does ST_AsText truncate or round
> 
> Hopefully this is a simple question.  I know that ST_AsText returns a 
> geometry that is not necessarily as prescise as what is actually 
> stored in the geometry field.  I have an editor that relies on WKT
representation.
> 
> Anyrate just wanted to know if the ST_AsText does a rounding of the 
> points or it does a truncation of the points.  Also would be nice to 
> know if I could control the precision of this since I will need to 
> truncate my actual geometry accordingly so that both are in agreement.
> 
> Thanks,
> Regina
> 
> 
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