[postgis-users] Establish versus of a line

[strk]

You're probably looking for an ST_IsCCW() function
( is counter-clock-wise ? ).

Such a function could only work with closed and simple
lines (see ST_IsClosed and ST_IsSimple).

It's not available at the moment, but defining the interface
is half work, so shouldn't cost too much to add :)

--strk;

  ()   Free GIS & Flash consultant/developer
  /\   http://strk.keybit.net/services.html

On Sun, May 02, 2010 at 11:27:25AM +0200, Andrea Peri wrote:
> Hi,
> 
> I do some test.
> 
> I notice that the ST_OrderingEquals
> 
> return true only when two lines are perfectly identical (same sequence
> vertex too).
> 
> For example:
> 
> Select ST_OrderingEquals(
>     ST_GeomFromText('LINESTRING(2 3, 3 2, 6 1)',3003
> ),ST_GeomFromText('LINESTRING(2 3, 3 2, 6 1)',3003 )
> )
> 
> But if I add a single vertex to one of lines it return false.
> Select ST_OrderingEquals(
>     ST_GeomFromText('LINESTRING(2 3, 3 2, 6 1, 8 9)',3003
> ),ST_GeomFromText('LINESTRING(2 3, 3 2, 6 1)',3003 )
> )
> 
> So it is not useful to detect the versus of lines because
> the linestring of the line table is not sure they are perfectly identical to
> the definition of the boundary of polygons.
> The boundary of polygons are always closed lines.
> Instead the same closed arc may be require more lines of the line table.
> 
> More again:
> even supposing the lines-table elements was closed like the boundary of
> polygons the ST_OderingEqual require they are
> first vertex identical.
> 
> So this case of two lines identical, and same versus but different starting
> vertex, return false.
> 
> Select ST_OrderingEquals(
>     ST_GeomFromText('LINESTRING(2 3, 3 2, 6 1, 8 9, 2 3)',3003
> ),ST_GeomFromText('LINESTRING(3 2, 6 1, 8 9, 2 3, 3 2)',3003 )
> )
> 
> Regards,
> 
> 
> -- 
> -----------------
> Andrea Peri
> . . . . . . . . .
> qwerty àèìòù
> -----------------

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