[Proj] Geodesic distances away from the ellipsoid

Karl Swartz karl at kls2.com
Fri Mar 20 12:30:45 EST 2009

> At the risk of asking a dumb question, do any of the geodesic 
> algorithms allow calculation of the geodesic distance if the path is 
> not on the ellipsoid? Like in an airplane or satellite whose path 
> could be assumed to be a constant height above the ellipsoid?

It doesn't answer your real question but for airlines the altitude is
not factored in, at least not anywhere that I've seen.  The geodesic
distance is just a theoretical value since the actual path flown is
often quite different due to winds and other factors.  Time aloft is
what drives fuel burn, pilot legalities, etc., regardless of the
distance flown.  In addition, for topics like frequent flier miles,
it wouldn't go over well if my SFO-LHR flight accrued more miles than
yours because we flew at a higher altitude.

Satellites aren't my area but for most missions I'd guess they just use
the orbital ellipse, defined by apogee and perigee, without regard to
the terrain underneath.  Some missions might notice the gravitational
variation but I doubt most would.  The question goes away for satellites
in geosynchronous orbit, of course, since they stay over the equator --
one expect spot on the equator.

 -- Karl

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