[postgis-users] Does ST_AsText truncate or round

[Havard Tveite]

I checked the OGC Simple Feature Access specifications.
<URL: http://www.opengeospatial.org/standards/sfa >, and
could not find anything that limits the precision of WKT.

The SFA 1.1.0  spec
<Point> := <x> <y>
<x> := double precision literal
<y> := double precision literal

Double precision literal does not seem to be defined further,
but I would assume that this does not make it impossible to
represent the PostGIS coordinate precision.

The SFA 1.2.0 spec:
<point> ::= <x> <y>
<x> ::= <signed numeric literal>
<y> ::= <signed numeric literal>
<signed numeric literal> ::=   {<sign>}<unsigned numeric literal>
<unsigned numeric literal> ::= <exact numeric literal>
                                     |<approximate numeric literal>
<approximate numeric literal> ::=          <mantissa>E<exponent>
<mantissa> ::=                 <exact numeric literal>
<exponent> ::=                 <signed integer>
<exact numeric literal> ::=    <unsigned integer>
                                   {<decimal point>{<unsigned integer>}}
                                   |<decimal point><unsigned integer>
<signed integer> ::=           {<sign>}<unsigned integer>
<unsigned integer> ::=         (<digit>)*

This BNF gives no limit to the number of digits used before or after
the decimal point.  I have not found anything that limits the number
of digits.


Håvard

Obe, Regina wrote:
> Havard,
>  
> I think as Mark mentioned  if it did it would violate the spec.  I think 
> somewhere in the spec is some specification of guaranteed precision of 
> WKT and if there isn't - text can never represent floating to complete 
> accuracy.  The AsBinary I think on the other hand is exact fidelity.  So 
> simply trying to make them equal would probably irritate people who need 
> precision higher than what the WKT spec offers.
>  
> In my particular case of course this would be ideal - but I can't be 
> selfish  :)
>  
> So I can live with the general rule that 
>  
> ST_GeomFromText(ST_AsText(geom), srid here) != geom
>  
> As long as I can override it by  updating my millions of geometries to 
> so and 300 someodd tables with
>  
> SET the_geom = ST_GeomFromText(ST_AsText(geom))
>  
> without too many weird side effects.  I may just try it and see if it 
> causes other problems elsewhere.
>  
> testing with the badly edited records  it seems to at least make my 
> other geometries just as inprecise as the edited ones so I get 
> intersection where I should again.
>  
> Thanks,
> Regina
>  
>  
> 
> 
> ------------------------------------------------------------------------
> *From:* postgis-users-bounces at postgis.refractions.net on behalf of 
> Havard Tveite
> *Sent:* Fri 5/16/2008 2:41 PM
> *To:* PostGIS Users Discussion
> *Subject:* Re: [postgis-users] Does ST_AsText truncate or round
> 
> Sorry, for not making much sense here.  What I wanted to
> say was that ST_AsText(geom) should be equivalent to geom,
> meaning that if you go from the database/binary
> representation of the geometry to the text representation
> of the geometry (using ST_AsText) and then back to the
> database/binary representation (using ST_GeomFromText),
> you should end up with the same geometry that you
> started out with.  This is important for applications
> that use when WKT when talking to PostGIS.
> 
> Håvard
> 
> 
> Mark Cave-Ayland wrote:
>  > Håvard Tveite wrote:
>  >> ST_GeomFromText(ST_AsText(geom)) != geom
>  >>
>  >> sounds like a bug to me (ignoring the SRS).
>  >>
>  >> Håvard
>  >
>  > No, since the SFS 1.1 specification supports only 2 dimensions with no
>  > SRS (and ST_AsText() is spec compliant). If you need the SRS then you
>  > should use the PostGIS-enhanced ST_EWKT() function which will output the
>  > extra information, but is not spec-compliant.
>  >
>  >
>  > ATB,
>  >
>  > Mark.
>  >
>  > --
>  > Mark Cave-Ayland
>  > Sirius Corporation - The Open Source Experts
>  > http://www.siriusit.co.uk <http://www.siriusit.co.uk/>
>  > T: +44 870 608 0063
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