Thanks for the clarification. It would be useful to include something
like your
explanation in the online documentation to avoid confusion. Its hard to
tell
from just the name how it behaves.
It turns out that what I need is trivial now in version 2.0.0. Its simply:
ST_MakeLine(G1,G2) -- appends two LINESTRINGs into a single LINESTRING
Martin Davis wrote:
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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Dr. Andrew Ukrainec
Principal Engineer e-mail: aukrai...@accipiterradar.com
Accipiter Radar Technologies Inc. http://www.accipiterradar.com
Niagara, Ontario Tel: (905) 228-6888 Fax: (905)892-2249
Orchard Park, New York Tel: (716) 508-4432 Fax: (888)393-6421
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