[postgis-users] Does ST_AsText truncate or round

[Gregory Williamson]

Regina --

I think the lwgi is testing for whether the installation has the "light-weight" postGIS stuff -- fewer decimal places resulting (%f and %g are formatting options in a print statememnt for decimal representation -- so 8 or 15 places respectively.

(I think -- never crawled into the guts of postGIS' C code).

(Sorry for top posting and the disclaimer -- only one per thread, I promise!)

Greg Williamson
Senior DBA
DigitalGlobe

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-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net on behalf of Obe, Regina
Sent: Fri 5/16/2008 12:33 PM
To: PostGIS Users Discussion
Subject: RE: [postgis-users] Does ST_AsText truncate or round
 
Paul,

I think you mistook me for a C programmer if you actually thought I
could figure out my answer from this snippet of code :)

First I can't remember what g means.  I remember seeing fs and gs
somewhere before so I can only assume that f means round and g means
truncate?

Secondly what does lwgi stand-for - I can only guess it means
light-weight geom or something along that  - so does that mean that as
far as I'm concerned - .8g is the loop I'm interested in?  Do we even
use non-light weight anymore? 

Thanks,
Regina

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Paul
Ramsey
Sent: Friday, May 16, 2008 10:39 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Does ST_AsText truncate or round

It all falls down into this function:

write_double(double val){
        ensure(32);
        if (lwgi)
                sprintf(out_pos,"%.8g",val);
        else
                sprintf(out_pos,"%.15g",val);
        to_end();
}



On Fri, May 16, 2008 at 3:23 AM, Paragon Corporation <lr at pcorp.us>
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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