[postgis-users] How can I split a polyline?

[Michael Fuhr]

On Tue, Mar 07, 2006 at 11:47:37AM -0500, Fay Du wrote:
> I still have a questions. What is a true multi-line?

MULTILINESTRING as opposed to a LINESTRING.  The MULTI and
GEOMETRYCOLLECTION types represent collections of other objects;
a MULTILINESTRING is a collection of LINESTRINGs.

> select NumGeometries(GeomFromText('MULTILINESTRING((1837220.875
> 330343.625,1836865.75 330348.53125,1836449.25 330363.03125,1836208.375
> 330362.59375,1836013.375 330371.375,1835626.625 330381.46875,1834513.625
> 330397.3125))',-1));
> return 1. Should it return number of points?

No, because this MULTILINESTRING contains one LINESTRING.

SELECT NumGeometries('MULTILINESTRING((0 0,1 1))');
 numgeometries 
---------------
             1
(1 row)

SELECT NumGeometries('MULTILINESTRING((0 0,1 1),(2 2,3 3))');
 numgeometries 
---------------
             2
(1 row)

> select astext(geometryn(GeomFromText('MULTILINESTRING((1837220.875
> 330343.625,1836865.75 330348.53125,1836449.25 330363.03125,1836208.375
> 330362.59375,1836013.375 330371.375,1835626.625 330381.46875,1834513.625
> 330397.3125))',-1),1));
> Return same MULTILINESTRING. 

It should return a LINESTRING, not a MULTILINESTRING.

SELECT AsText(GeometryN('MULTILINESTRING((0 0,1 1),(2 2,3 3))', 1));
       astext        
---------------------
 LINESTRING(0 0,1 1)
(1 row)

SELECT AsText(GeometryN('MULTILINESTRING((0 0,1 1),(2 2,3 3))', 2));
       astext        
---------------------
 LINESTRING(2 2,3 3)
(1 row)

> select astext(geometryn(GeomFromText('MULTILINESTRING((1837220.875
> 330343.625,1836865.75 330348.53125,1836449.25 330363.03125,1836208.375
> 330362.59375,1836013.375 330371.375,1835626.625 330381.46875,1834513.625
> 330397.3125))',-1),2));
> Return null.

You've requested the second geometry but this MULTILINESTRING has
only one geometry.

SELECT AsText(GeometryN('MULTILINESTRING((0 0,1 1),(2 2,3 3))', 3));
 astext 
--------
 
(1 row)

-- 
Michael Fuhr