[postgis-users] Extracting a line from multilinestring

[Peck, Brian]

Thank you very much.

 

________________________________

From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Rodrigo Martín LÓPEZ GREGORIO
Sent: Wednesday, August 15, 2007 5:44 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Extracting a line from multilinestring

 

Hi Brian...

I think you can do one of this things:

1. If you are sure that all your geometries have just one LINESTRING then you can do:

SELECT line_interpolate_point(GeometryN(the_geom,1),line_locate_point(GeometryN(the_geom,1),PointFromText('POINT(-74.5 40)'))) FROM roads ORDER BY Distance(the_geom,PointFromText('POINT(-74.5 40)')) LIMIT 1 


2. If your geometries have more than one LINESTRING but this LINESTRINGS are continuous (i.e. there are no gaps between the LINESTRINGS in a MULTILINESTRING) you can use:

SELECT line_interpolate_point(LineMerge(the_geom),line_locate_point(LineMerge(the_geom),PointFromText('POINT(-74.5 40)'))) FROM roads ORDER BY Distance(the_geom,PointFromText('POINT(- 74.5 40)')) LIMIT 1


If you want to verify if all your geometries have just one LINESTRING you can try:

SELECT count(*) FROM roads WHERE NumGeometries(the_geom) > 1


This will return the number of MULTILINESTRINGS that have more than one LINESTRING 

Rodrigo



On 8/15/07, Peck, Brian <brian.peck at lmco.com> wrote:

Hey all,

 

Does anyone if there is a way to extract a line from a MultiLineString?

 

I am trying to make the call

 

SELECT line_interpolate_point(the_geom,line_locate_point(the_geom,PointFromText('POINT(-74.5 40)'))) FROM roads ORDER BY Distance(the_geom,PointFromText('POINT(-74.5 40)')) LIMIT 1

 

But I get the error

 

ERROR:  line_locate_point: 1st arg isn't a line

 

My 'the_geom' field is filed with MultiLineString data (though only LineString in each one).

 

Ex: 

 

select AsText(the_geom) from roads limit 1;

 

                                          astext

------------------------------------------------------------------------------------------

 MULTILINESTRING((-74.4595680236816 40.0301723480225,-74.4591522216797 40.0302429199219))

(1 row)

 

- Brian Peck


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