[postgis-devel] 4D points

[Paul Ramsey]

'm' is 'measure' an extra axis of information not associated with the
cartesian x/y/z space. The most common use for 'measure' is actually
for 'measurements', the adding of physically known measurements about
a feature to the abstract 'feature' represented in x/y space in the
GIS. For example, highway management systems often understand the
location of facilities in terms of 'mile posts'. So, in addition to
x/y coordinates, each vertex is also assigned a 'mile' measurement in
'm' which allows the system to accurately place facility information
relative to the 'milepost' system. (Why not just use the x/y
coordinates and calculate distances off of them? Because they are
representational, the distances calculated from the x/y will not be
the same as the actual milepost measurements.)

P.

2010/1/19 Jorge Arévalo <jorge.arevalo at gmail.com>:
> 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
> _______________________________________________
> postgis-devel mailing list
> postgis-devel at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>