[postgis-users] Purpose of mesure value in 4D geoms

[strk at refractions.net]

On Fri, Apr 22, 2005 at 11:26:45PM +1200, Jeremy Palmer wrote:
...
> Sorry if this seems to be a simple question but could someone explain the
> use and purpose of the M value in the new lwgeom geometry structure.

If you don't find an use for it you don't have to keep M values.
See force_3dz() function for this.

The thing that pushed me personally introducing M values was
to have a stricter mapping with shapefiles. Shapefiles come
in the XY, XYM and XYMZ flavors. LWGEOM is finally able to do a
lossless roundtrip between PostGIS and Shapefiles.

Usage of the M value is up to your fantasy, what I've heard
is that it's commonly used to store "Measure" (ie. distance
of a road from its origin), which is pretty inaccurate when
computed by vertex distance as compared to human relevations.

--strk;