[postgis-users] [postgis-devel] Buck Rogers and the 3rdDimension

[Sufficool, Stanley]

My 2 cents.

Why do spheroid projections assume zero altitude is Z miles from core.
What about other planets? What if I want to map Mars?

Isn't the geography projection model is broken? You can't have a sphere
without a diameter and the Z measure is just a plus/minus of the given Z
constant from the core.

> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net 
> [mailto:postgis-users-bounces at postgis.refractions.net] On 
> Behalf Of Paragon Corporation
> Sent: Saturday, October 31, 2009 9:00 AM
> To: 'PostGIS Development Discussion'
> Cc: 'PostGIS Users Discussion'
> Subject: Re: [postgis-users] [postgis-devel] Buck Rogers and 
> the 3rdDimension
> 
> 
> Paul,
> I always wondered that - I guess 2 questions I would ask
> 
> 1) How do other spatial databases handle this, or do they not 
> really do anything with the z coordinate anyway especially 
> with polar coords? Seems to me SQL Server doesn't do anything 
> with it but haven't tested it enough to be absolutely sure. 
> But not sure about Oracle, Informix, IBM (or maybe for their 
> geodetic calcs they ignore it)
> 
> 2) The instruments collecting this stuff, I guess gps and 
> what-not -- how do they collect this extra coordinate or is 
> it always a separate field and what measurement is it in?  I 
> suppose if they always measure altitude in meters, then we 
> would for users have to come up with some mechanism to allow 
> them to convert it to degrees if you assume all axis are the 
> same type (polar).
> 
> I would say the whole spatial ref model falls apart anyway 
> since it seems to me it completely ignores this (questionably 
> non-polar dimension so if you think in polar its not as clear 
> cut as the planar. that Z is an unknown so the best answer is 
> to do nothing with it and take it literally . 
> 
> Thanks,
> Regina
> 
> -----Original Message-----
> From: postgis-devel-bounces at postgis.refractions.net
> [mailto:postgis-devel-bounces at postgis.refractions.net] On 
> Behalf Of Paul Ramsey
> Sent: Saturday, October 31, 2009 1:14 AM
> To: PostGIS Development Discussion
> Subject: [postgis-devel] Buck Rogers and the 3rd Dimension
> 
> While updating the old geometry spheroid functions, I noted 
> the existence of a length3d_spheroid() variant. It is 
> actually the default call for
> st_length_spheroid() as it happens.
> 
> The kinds of things you have to pass into this function to 
> get sensible results are pretty restricted, as it turns out. 
> For example,
> 
> st_length_spheroid('LINESTRING(0 0 1000, 0 0.001 1001)', 
> 'SPHEROID["WGS84",
> 6378137,298.257222101]')
> 
> Note that the units of Z are meters while the units of X and 
> Y are degrees. To get a sensible answer out, in fact, the 
> units of your altitude have to match the units of your spheroid.
> 
> Anyhow, my geography routines right now are strictly 2D so I 
> haven't renovated this particular variant, but it's put my in 
> the mind of wondering what the right thing to do is, in 
> geography. If I get a '3D' geography, do I assume the third 
> dimension is in meters? Do I calculate a "3D" length (or 
> distance?) by default? That's what our geometry routines do 
> right now, and it hasn't caused harm yet.
> 
> It seems like an interesting submerged assumption in the 
> geodetic space, that the units of your extra dimension will 
> match the units of the axes of your spheroid.
> 
> As I recall, we added this function many years back, to 
> calculate as exactly as possible the drive lengths of roads 
> in a road network (BC is a hilly place). Not sure if anyone 
> else is using it. And still not sure if a 
> "length3d_spheroid()" function is a wise proposition in 
> general, given the required assumptions.
> 
> P.
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