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

[Andrew Ukrainec]

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

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