[postgis-devel] 4D points

[Jorge Arévalo]

On Tue, Jan 19, 2010 at 7:17 PM, David William Bitner
<david.bitner at gmail.com> wrote:
> '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.
>>

Ok, I understand. Is a general-purpose attribute related with each
point. It can store any additional information, like 'mile posts'
information or 'time-of-observation' information in vehicle tracks.
Reasonable and useful :-)

BTW, May I found a kind of "official" documentation of lwgeom library?
Apart from README file and comments on source files.

Many thanks!

Best regards,
Jorge


>> 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
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>> >
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>
>
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> David William Bitner
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