[postgis-devel] Buck Rogers and the 3rd Dimension

[Paragon Corporation]

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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