[postgis-users] Does ST_AsText truncate or round

[Obe, Regina]

Thanks Andy and Greg for the clarification,

Sorry for being so dense.

So I take it sprintf would never ever do any rounding (they merely print
% significant digits).  So g and f always truncate but g can flip to
scientific if < .001

http://www.cplusplus.com/reference/clibrary/cstdio/sprintf.html

Thanks,
Regina


 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Andy
Anderson
Sent: Friday, May 16, 2008 4:36 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Does ST_AsText truncate or round

f & g are output formatting codes, f is the normal double-precision  
format, while g produces scientific notation (mmmE+xx) if the number  
is < 0.0001. The 8 and 15 represent the number of significant digits  
to print (for g).

-- Andy

On May 16, 2008, at 3:33 PM, Obe, Regina wrote:
> 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?
>
> -----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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