ST_LineMerge documentation should mention no support for M values

[pauldzy]

ST_LineMerge strips off M values from input geometries. It would be nice if the documentation mentioned this.

SELECT ST_AsEWKT(

ST_LineMerge(

ST_GeomFromEWKT('SRID=0;MULTILINESTRINGM((-29 -27 10,-30 -29.7 20,-36 -31 30,-45 -33 40),(-45 -33 40,-46 -32 50))')

)

);

[strk]

Any function implemented by GEOS will strip the M ordinate value, until GEOS adds support for it: ​https://github.com/libgeos/libgeos/pull/45

[robe]

strk you have that GEOS ticket closed was it ever committed or was it committed into some test branch

output is still same as of:

POSTGIS="2.4.0 r15853" PGSQL="100" GEOS="3.7.0dev-CAPI-1.11.0 8fe2ce6" PROJ="Rel. 4.9.3, 15 August 2016" GDAL="GDAL 2.2.2, released 2017/09/15" LIBXML="2.7.8" LIBJSON="0.12" LIBPROTOBUF="1.2.1" RASTER

 SELECT ST_AsEWKT(

    ST_LineMerge(

        ST_GeomFromEWKT('SRID=0;MULTILINESTRINGM((-29 -27 10,-30 -29.7 20,-36 -31 30,-45 -33 40),(-45 -33 40,-46 -32 50))')

    )

); 

                       st_asewkt
-------------------------------------------------------
 LINESTRING(-29 -27,-30 -29.7,-36 -31,-45 -33,-46 -32)
(1 row)

Do we need to change something in PostGIS to enable M output o

[strk]

Nope, branch not merged yet, it is "mvalues" and conflicts with current master.

Nobody seems to be taking care of that work anymore.

[robe]

In 16686:

Put a note that ST_LineMerge strips M dimension. Provide an example of Z-dimension linestrings
Closes #3317

[robe]

In 16687:

Fix typo in last commit
References #3317 for PostGIS 2.5.0