[postgis-users] PointM, PolyLineM, PolygonM, ...

[Paul Ramsey]

Please bear in mind that there are other uses for the higher dimensions 
than Z and M. There could also be a time dimension (think of a GPS 
track, for example). I don't like the idea that we are setting up 
implicit uses for our higher dimensions (M shall always be the 4th 
entry, Z shall always be the 3rd entry!).

I would rather anonymously fill out the higher dimensions using the 
least storage space and have any function that expects to operate on 
them sufficiently flexible to be pointed at the right coordinate space. 
So if have have a measured_point(<linstring>,<%>) function, it needs a 
third parameter, ordinate, to tell it which ordinate to look in for the 
measure. measured_point(<linestring>,<%>,<ordinate = 3|4>).

strk at refractions.net wrote:
> On Thu, Sep 23, 2004 at 12:12:28PM +0200, Pierrick Brihaye wrote:
> 
>>Hi,
>>
>>strk at refractions.net a écrit :
>>
>> > Now that postgis supports up to 4 dimensions geometry, what do you
>>
>>>think should be done ? My vision is this:
>>>
>>>Type  <-> geometry2d
>>>TypeM <-> geometry3d
>>>TypeZ <-> geometry4d
>>>
>>>What do you think ?
>>
>>How long shall we pay for ESRI's weird data model ? :-)
>>
>>IMHO, the 3rd dimension has well-known semantics : Z.
>>
>>Therefore, any other dimension should be considered as a 4th one.
>>
>>Any other thoughts ?
> 
> 
> I'm with you, but quetions are:
> 
> 1) how should we import TypeM shapefiles ?
> 	a) as x,y,Z,m // wasting Z space in postgis 
> 	b) as x,y,z   // interpreting M as Z
> 
> 2) how should we import TypeZ shapefiles ?
> 	a) as x,y,z,m // respecting shapefile meanings 
> 	b) as x,y,z,m // swapping M<->Z meaning
> 	c) as x,y,z   // discarding M
> 	d) as x,y,z   // using M, discarding Z
> 
> 3) how should we export geometry3d ?
> 	a) as TypeM // using M for Z values
> 	b) as TypeZ // wasting M space in shapefile
> 
> 3) how should we export geometry4d ?
> 	a) as TypeZ // respecting shapefile meanings (4th dim -> M)
> 	b) as TypeZ // swapping M<->Z meaning
> 
> Does anyone see the light ?
> I suppost some command line switches would do but finding an easy
> interface is not straightforward ...
> 
> --strk;
> 
> 
>>Regards,
>>
>>-- 
>>Pierrick Brihaye, informaticien
>>Service régional de l'Inventaire
>>DRAC Bretagne
>>mailto:pierrick.brihaye at culture.gouv.fr
>>+33 (0)2 99 29 67 78
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> 
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