[postgis-users] why doesn't linemerge() work?

[Andrew Ukrainec]

Thanks Nicolas.   After some further browsing and thinking I arrived
on the solution.

First, I tried this:

ST_LineMerge( ST_Collect(ST_AddPoint(G1,ST_StartPoint(G2)),G2)) );

By appending the first point of G2 LINESTRING to the end of G1 LINESTRING,
the ST_LineMerge now works.  Except, it only works up to and including 3D. 
I'm using the 4th dimesion M to store a timestamp with my XYZ points,
and this gets stripped off by ST_LineMerge and returns 3D.

Then I tried:

ST_MakeLine(G1,G2)

and it works!  Very embarassing.  The key is this comment in
ST_MakeLine documentation:

"Availability: 2.0.0 - Support for linestring input elements was introduced"

This explains why older posts on the net didn't show this obvious solution.
Probably would be helpful to add a simple example to the docs with
two linestrings as input to ST_MakeLine to emphasize the functionality.

Nicolas Ribot wrote:
> Hi Andrew,
>
> This (long) thread may give you some hints on how to order and then
> merge lines together:
> "Closing polylines", 11 May 2012.
>
> Nicolas
>
> On 19 October 2012 00:27, Andrew Ukrainec <aukrainec at accipiterradar.com> 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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>>     
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>   


-- 
Dr. Andrew Ukrainec
Principal Engineer                  e-mail: aukrainec at accipiterradar.com

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