[postgis-devel] 4D points

[Jorge Arévalo]

On Wed, Jan 20, 2010 at 2:44 PM, Nicklas Avén
<nicklas.aven at jordogskog.no> wrote:
> Hallo Jorge
>
> Doxygen can be very helpful sometimes to see how functions are calling each
> other and so on
> http://postgis.refractions.net/documentation/postgis-doxygen/
>
> /Nicklas
>

Really useful! Many thanks Nicklas

Best regards,
Jorge


> 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
>>>> >
>>>> _______________________________________________
>>>> postgis-devel mailing list
>>>> postgis-devel at postgis.refractions.net
>>>> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>>>
>>>
>>>
>>> --
>>> ************************************
>>> David William Bitner
>>>
>>> _______________________________________________
>>> postgis-devel mailing list
>>> postgis-devel at postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>>>
>>>
>>_______________________________________________
>>postgis-devel mailing list
>>postgis-devel at postgis.refractions.net
>>http://postgis.refractions.net/mailman/listinfo/postgis-devel
>>
>>
> _______________________________________________
> postgis-devel mailing list
> postgis-devel at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>
>