[postgis-devel] 4D points

[David William Bitner]

'm' is also very useful for adding time-of-observation information; in
linear information (think vehicle tracks) often as an offset from a start
time of the track.

On Tue, Jan 19, 2010 at 11:55 AM, Paul Ramsey <pramsey at cleverelephant.ca>wrote:

> '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
> >
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>



-- 
************************************
David William Bitner
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