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: [email protected] > [mailto:[email protected]] 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: [email protected] > [mailto:[email protected]] 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. > _______________________________________________ > postgis-devel mailing list [email protected] > http://postgis.refractions.net/mailman/listinfo/postgis-devel > > > > _______________________________________________ > postgis-users mailing list [email protected] > http://postgis.refractions.net/mailman/listinfo/postgis-users > _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
