4D points

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4D points

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
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Re: 4D points

Paul Ramsey-3
'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
>
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Re: 4D points

bitner
'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.

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
>
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--
************************************
David William Bitner

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Re: 4D points

Jorge Arévalo
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
>
>
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Re: 4D points

Nicklas Avén
In reply to this post by Jorge Arévalo
Hallo Jorge
 
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
>
>

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Re: 4D points

Jorge Arévalo
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
>>
>>
> _______________________________________________
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> [hidden email]
> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>
>
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Re: 4D points

Mateusz Loskot
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
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