[postgis-users] 3d Points

[strk at refractions.net]

On Mon, Aug 01, 2005 at 07:47:38PM +1000, Christian Heine wrote:
...
> Is it advisable to use EWKT or is it better to create an extra z- 
> value column and keep the z-value separated from the lon/lat?

You don't loose anything using the extra Z-value column, you
can always extract 2d-version or OGC-standard representation from
a 3d object.

> Also,  
> what is the difference between POINTM and POINT? I couldn't find any  
> information explaining the "M".

"M" stand for "Measured". It's an additional coordinate you can use
for time or rilevated distance or whatever other parameter you
can associate to a vertex of your geometry.

In PostGIS we use the M suffix to specify that the
tree dimensions given are X,Y,M instead of X,Y,Z.

Examples:

	POINT(0 0)	<---- X,Y (2D)
	POINT(0 0 0)	<---- X,Y,Z (3DZ)
	POINTM(0 0 0)	<---- X,Y,M (3DM)
	POINT(0 0 0 0)	<---- X,Y,Z,M (4D)

Distinction between 3DZ and 3DM is there to store
semantic of the third parameter. Some operations
would compute an appropriate value for the 3rd
dimension if and only if it is a Z (and not an M).

--strk;