[postgis-users] Issue with ST_Line_Interpolate_Point dropping m values

[Craig McIlwee]

Well I think Ive found the issue.  The function handles LINESTRING and MULTILINESTRING just fine, but gives 0 M values for LINESTRINGM and MULTILINESTRINGM.
 
SELECT ST_AsEWKT(ST_Line_Interpolate_Point(the_line, 0.5))
                FROM (SELECT ST_GeomFromEWKT('LINESTRING(1 2 3, 4 5 6, 6 7 8)') as the_line) As foo;
"POINTM(3.5 4.5 0)"
 
Is this expected behavior?  For now I think I can work around it by pulling the EWKT over to my client, doing a string replace on MULTILINESTRINGM to MULTILINESTRING, then using ST_GeomFromEWKT in a subsequent query.  It will certainly complicate my code and introduce a bit of I/O overhead, but it seems to work.
 
Craig
 
From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Craig McIlwee
Sent: Tuesday, July 12, 2011 2:07 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] Issue with ST_Line_Interpolate_Point dropping m values
 
Hello,
 
Im using PostGIS 1.5 and am having trouble with the ST_Line_Interpolate_Point function, the M value of the returned points are always 0.   An example query with output is:
 
select st_asewkt(st_line_interpolate_point(st_geometryn(the_geom, 1), 0.88364273)) from wgs_statecov_tway;
"SRID=4326;POINTM(-90.4466123484958 38.8893086051018 0)"
 
 
My geometries are MULTILINESTRINGM type, and examination with ST_AsEWKT shows proper M values as you see below.  I also use the M value for other LRS functions.
 
select st_asewkt(st_endpoint(the_geom)) from wgs_statecov_tway;
"SRID=4326;POINTM(-90.2232337557835 38.8615691007846 136.351980792317)"
 
 
The odd thing is that when I try the example query from the reference manual everything works fine.
 
SELECT ST_AsEWKT(ST_Line_Interpolate_Point(the_line, 0.5))
                FROM (SELECT ST_GeomFromEWKT('LINESTRING(1 2 3, 4 5 6, 6 7 8)') as the_line) As foo;
"POINT(3.5 4.5 5.5)"
 
 
Based on this, it seems like I must be doing something but because the query is so simple I have no idea what that could be.  Any ideas?
 
Thanks,
Craig
Open Roads Consulting, Inc.
757-546-3401
http://www.openroadsconsulting.com
 
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