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<body>Hallo Jorge
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<div align="left">Doxygen can be very helpful sometimes to see how functions are calling each other and so on</div>
<div align="left"><a href="http://postgis.refractions.net/documentation/postgis-doxygen/">http://postgis.refractions.net/documentation/postgis-doxygen/</a></div>
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<div align="left">/Nicklas</div>
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<br />
2010-01-20 Jorge Arévalo wrote:<br />
<br />
On Tue, Jan 19, 2010 at 7:17 PM, David William Bitner<br />
>
<david.bitner@gmail.com></david.bitner@gmail.com> wrote:<br />
>> 'm' is also very useful for adding time-of-observation information; in<br />
>> linear information (think vehicle tracks) often as an offset from a start<br />
>> time of the track.<br />
>><br />
>> On Tue, Jan 19, 2010 at 11:55 AM, Paul Ramsey
<pramsey@cleverelephant.ca></pramsey@cleverelephant.ca><br />
>> wrote:<br />
>>><br />
>>> 'm' is 'measure' an extra axis of information not associated with the<br />
>>> cartesian x/y/z space. The most common use for 'measure' is actually<br />
>>> for 'measurements', the adding of physically known measurements about<br />
>>> a feature to the abstract 'feature' represented in x/y space in the<br />
>>> GIS. For example, highway management systems often understand the<br />
>>> location of facilities in terms of 'mile posts'. So, in addition to<br />
>>> x/y coordinates, each vertex is also assigned a 'mile' measurement in<br />
>>> 'm' which allows the system to accurately place facility information<br />
>>> relative to the 'milepost' system. (Why not just use the x/y<br />
>>> coordinates and calculate distances off of them? Because they are<br />
>>> representational, the distances calculated from the x/y will not be<br />
>>> the same as the actual milepost measurements.)<br />
>>><br />
>>> P.<br />
>>><br />
><br />
>Ok, I understand. Is a general-purpose attribute related with each<br />
>point. It can store any additional information, like 'mile posts'<br />
>information or 'time-of-observation' information in vehicle tracks.<br />
>Reasonable and useful :-)<br />
><br />
>BTW, May I found a kind of "official" documentation of lwgeom library?<br />
>Apart from README file and comments on source files.<br />
><br />
>Many thanks!<br />
><br />
>Best regards,<br />
>Jorge<br />
><br />
><br />
>>> 2010/1/19 Jorge Arévalo
<jorge.arevalo@gmail.com></jorge.arevalo@gmail.com>:<br />
>>> > Hello,<br />
>>> ><br />
>>> > In lwgeom.h there is:<br />
>>> ><br />
>>> > typedef struct<br />
>>> > {<br />
>>> > double x;<br />
>>> > double y;<br />
>>> > double z;<br />
>>> > double m;<br />
>>> > }<br />
>>> > POINT4D;<br />
>>> ><br />
>>> > And in 3D, you have 2 point types:<br />
>>> ><br />
>>> > typedef struct<br />
>>> > {<br />
>>> > double x,y,z;<br />
>>> > }<br />
>>> > POINT3DZ;<br />
>>> ><br />
>>> > typedef struct<br />
>>> > {<br />
>>> > double x,y,m;<br />
>>> > }<br />
>>> > POINT3DM;<br />
>>> ><br />
>>> > So, my question: What exactly is a 4D point, in this context? If "z"<br />
>>> > is the third dimension (the elevation of a point), what is "m"? Seems<br />
>>> > to be a kind of a property (a "measure") of a point:<br />
>>> ><br />
>>> ><br />
>>> > http://www.geospatialanalyst.com/2009/08/get-xyzm-populate-x-y-z-and-m.html<br />
>>> > http://www.postgis.org/docs/ST_NDims.html<br />
>>> ><br />
>>> > Thanks in advance,<br />
>>> > Best regards<br />
>>> ><br />
>>> > Jorge<br />
>>> > _______________________________________________<br />
>>> > postgis-devel mailing list<br />
>>> > postgis-devel@postgis.refractions.net<br />
>>> > http://postgis.refractions.net/mailman/listinfo/postgis-devel<br />
>>> ><br />
>>> _______________________________________________<br />
>>> postgis-devel mailing list<br />
>>> postgis-devel@postgis.refractions.net<br />
>>> http://postgis.refractions.net/mailman/listinfo/postgis-devel<br />
>><br />
>><br />
>><br />
>> --<br />
>> ************************************<br />
>> David William Bitner<br />
>><br />
>> _______________________________________________<br />
>> postgis-devel mailing list<br />
>> postgis-devel@postgis.refractions.net<br />
>> http://postgis.refractions.net/mailman/listinfo/postgis-devel<br />
>><br />
>><br />
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><br />
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