[postgis-devel] 4D points

[Jorge Arévalo]

Hello,

In lwgeom.h there is:

typedef struct
{
	double x;
	double y;
	double z;
	double m;
}
POINT4D;

And in 3D, you have 2 point types:

typedef struct
{
	double	x,y,z;
}
POINT3DZ;

typedef struct
{
	double	x,y,m;
}
POINT3DM;

So, my question: What exactly is a 4D point, in this context? If "z"
is the third dimension (the elevation of a point), what is "m"? Seems
to be a kind of a property (a "measure") of a point:

http://www.geospatialanalyst.com/2009/08/get-xyzm-populate-x-y-z-and-m.html
http://www.postgis.org/docs/ST_NDims.html

Thanks in advance,
Best regards

Jorge