LineMerge does a topological line merge, rather than one determinedby
the order of the points. You can think of it as computing the geometry
that would result from drawing all the geometries on paper, and then
turning it back into a vector geometry. Order is not considered, and if
there are *any* discontinuities in the input lines then a
MULTILINESTRING is created.
This is quite useful in some cases, but obviously not in yours. In
theory your use case is much simpler to compute. There may be a way of
doing it by extracting the vertices of the input geometries in order,
and then converting them to a single linestring. Some systems have a
connect() aggregate function that does this, but I don't think PostGIS
does at the moment.
On 10/18/2012 3:27 PM, Andrew Ukrainec wrote:
I've been trying to append a new LINESTRING to an existing
LINESTRING in a table, and this seems to be an impossible
task. I've searched the net for solutions, and tried all
possible obvious approaches:
ST_LineMerge(ST_Collect(geom1,geom2))
ST_LineMerge(ST_Union(geom1,geom2))
...
I wrote a function to use the aggregate version of the call
ST_LineMerge(ST_Collect(geom))
I keep getting a MULTILINESTRING() at the output rather
than a single LINESTRING().
If I run this example adapted from an example on
the OSGeo site:
SELECT ST_AsText(ST_LineMerge(ST_Collect(the_geom)))
FROM (SELECT 'LINESTRING(0 0, 0 1)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(1 0, 1 1)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(0 0, 1 0)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(1 1, 0 1)'::geometry the_geom) as a;
I get a single LINESTRING that I hoped for,
LINESTRING(0 0, 1 0, 1 1, 0 1, 0 0)
but the answer is completely counter intuitive to me.
I expected the individual LINESTRINGs to be simply appended
in order. If I adjust the data in the first LINESTRING so
that '0 1' point is now '1 0':
SELECT ST_AsText(ST_LineMerge(ST_Collect(the_geom)))
FROM (SELECT 'LINESTRING(0 0, 1 0)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(1 0, 1 1)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(0 0,1 0)'::geometry the_geom
UNION ALL
SELECT 'LINESTRING(1 1, 0 1)'::geometry the_geom) as a;
then again the operation fails and I get MULTILINESTRING()
at the output.
MULTILINESTRING((1 0,1 1,0 1),(1 0,0 0,1 0))
The PostGIS documentation only contains the mysterious
statement in the Example section:
"--If can't be merged - original MULTILINESTRING is returned"
I can accept that LINEMERGE() does not work the way I imagined
it would, but then there is no explanation of how it does work
in the documentation. In the case that one wants to glue
two LINESTRING() together into a single LINESTRING() preserving
the order of the points, can anyone suggest how this can
be done efficiently?
Thanks in advance.
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