[postgis-users] ST_LineToCurve for less than 4 points

[Denis Rouzaud]

I partially agree.

If you look in the doc 
http://postgis.refractions.net/documentation/manual-svn/ST_LineToCurve.html
It is written that it should convert to circular strings or curved polygon.

In case of circular strings, you need at least 3 (and not 4) points to 
determine the circle, as explained in Stephen's link 
http://paulbourke.net/geometry/circlefrom3/

On 02/28/2012 04:32 PM, Paul Ramsey wrote:
> No. This is a misunderstanding of purposes.
>
> ST_LineToCurve is the inverse of ST_CurveToLine. CurveToLine takes in
> a circular object and linearizes it, turning curves into regular small
> sections of linear arcs to approximate the input shape.
> LineToCurve analyzes an input linestring, looking for the tell-tale
> regularity of linearized arcs and converts them back to real arcs. The
> process can only work with linestrings that have for or more vertices
> to analyze.
>
> What Denis is looking for is probably something more like
> http://trac.osgeo.org/postgis/ticket/1291
>
> P.
>
> On Tue, Feb 28, 2012 at 12:38 AM, Sandro Santilli<strk at keybit.net>  wrote:
>> On Tue, Feb 28, 2012 at 09:36:34AM +0100, Denis Rouzaud wrote:
>>> Hi,
>>>
>>> Can someone tells me why ST_LineToCurve needs at least a linestring
>>> of 4 points to be run?
>>> If I have three points, I should be able to create a circularstring, right?
>>>
>>> Here the error I have:
>>> SELECT ST_LineToCurve(ST_GeomFromText('LINESTRING(554803.177682475
>>> 145390.853708235,554796.469135702 145401.404724093,554769.634948609
>>> 145404.042478058)'));
>>> ERROR:  pta_desegmentize needs implementation for npoints<  4
>> If I read the message correctly it answers exactly your questions.
>> It "needs implementation". A patch is welcome.
>>
>> --strk;
>>
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