[postgis-devel] 4D points

[Nicklas =?utf-8?Q?Av=E9n?=]

Hallo Jorge Doxygen can be very helpful sometimes to see how functions are calling each other and so onhttp://postgis.refractions.net/documentation/postgis-doxygen/ /Nicklas

2010-01-20 Jorge Arévalo wrote:

On Tue, Jan 19, 2010 at 7:17 PM, David William Bitner
> 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
>> 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:
>>> > 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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>>
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