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 [hidden email] http://postgis.refractions.net/mailman/listinfo/postgis-devel |
'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 <[hidden email]>: > 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 > [hidden email] > http://postgis.refractions.net/mailman/listinfo/postgis-devel > postgis-devel mailing list [hidden email] http://postgis.refractions.net/mailman/listinfo/postgis-devel |
'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 <[hidden email]> wrote: 'm' is 'measure' an extra axis of information not associated with the -- ************************************ David William Bitner _______________________________________________ postgis-devel mailing list [hidden email] http://postgis.refractions.net/mailman/listinfo/postgis-devel |
On Tue, Jan 19, 2010 at 7:17 PM, David William Bitner
<[hidden email]> 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 <[hidden email]> > 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 <[hidden email]>: >> > 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 >> > [hidden email] >> > http://postgis.refractions.net/mailman/listinfo/postgis-devel >> > >> _______________________________________________ >> postgis-devel mailing list >> [hidden email] >> http://postgis.refractions.net/mailman/listinfo/postgis-devel > > > > -- > ************************************ > David William Bitner > > _______________________________________________ > postgis-devel mailing list > [hidden email] > http://postgis.refractions.net/mailman/listinfo/postgis-devel > > postgis-devel mailing list [hidden email] http://postgis.refractions.net/mailman/listinfo/postgis-devel |
In reply to this post by Jorge Arévalo
Doxygen can be very helpful sometimes to see how functions are calling each other and so on
/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 >>> > [hidden email] >>> > http://postgis.refractions.net/mailman/listinfo/postgis-devel >>> > >>> _______________________________________________ >>> postgis-devel mailing list >>> [hidden email] >>> http://postgis.refractions.net/mailman/listinfo/postgis-devel >> >> >> >> -- >> ************************************ >> David William Bitner >> >> _______________________________________________ >> postgis-devel mailing list >> [hidden email] >> http://postgis.refractions.net/mailman/listinfo/postgis-devel >> >> >_______________________________________________ >postgis-devel mailing list >[hidden email] >http://postgis.refractions.net/mailman/listinfo/postgis-devel > > _______________________________________________ postgis-devel mailing list [hidden email] http://postgis.refractions.net/mailman/listinfo/postgis-devel |
On Wed, Jan 20, 2010 at 2:44 PM, Nicklas Avén
<[hidden email]> 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 >>>> > [hidden email] >>>> > http://postgis.refractions.net/mailman/listinfo/postgis-devel >>>> > >>>> _______________________________________________ >>>> postgis-devel mailing list >>>> [hidden email] >>>> http://postgis.refractions.net/mailman/listinfo/postgis-devel >>> >>> >>> >>> -- >>> ************************************ >>> David William Bitner >>> >>> _______________________________________________ >>> postgis-devel mailing list >>> [hidden email] >>> http://postgis.refractions.net/mailman/listinfo/postgis-devel >>> >>> >>_______________________________________________ >>postgis-devel mailing list >>[hidden email] >>http://postgis.refractions.net/mailman/listinfo/postgis-devel >> >> > _______________________________________________ > postgis-devel mailing list > [hidden email] > http://postgis.refractions.net/mailman/listinfo/postgis-devel > > postgis-devel mailing list [hidden email] http://postgis.refractions.net/mailman/listinfo/postgis-devel |
In reply to this post by bitner
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. IMO, good topic to FAQ. Best regards, -- Mateusz Loskot, http://mateusz.loskot.net _______________________________________________ postgis-devel mailing list [hidden email] http://postgis.refractions.net/mailman/listinfo/postgis-devel |
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