# [Proj] website cartographic executables---Inv_Fwd

Martin Vermeer martin.vermeer at hut.fi
Tue Dec 20 15:39:29 EST 2005

```On Tue, Dec 20, 2005 at 09:06:54PM +0100, Oscar van Vlijmen wrote:
> > From: "Gerald I. Evenden" <gerald.evenden-verizon.net>
>
> >>> geodesic computations:
> >>> http://www.ngs.noaa.gov/TOOLS/Inv_Fwd/Inv_Fwd.html
>
> > What bothers me is I cannot conceptualize the 3D version.  What does the
> > geodesic look like as it changes elevation?
> Now don't let it keep you awake, since invers3d.for doesn't compute it
> anyway. It would have been nice though to get a geodesic-like arc along an
> ellipsoid that inflates or deflates while going from a point at height 1 to
> another at height 2. I cannot imagine how I could fly such a route reliably
> and it's a bit difficult to find a terrain that follows such a height
> development, but dreaming doesn't hurt, innit?

I think it is meaningless. The geodesic is defined as the shortest path
_within_ the two-dimensional curved space of the ellipsoidal (or more
general) surface. If you go outside that space, it's not the geodesic
anymore.

> > I can't help of thinking as the problem as line-of-sight.  That is, when I
> > look at Mt. Everest from a plain 30 miles away I feel the problem is a
> > matter of a simple xzy hypotenuse problem.
> invers3d.for does compute that. It does chord distances, zenith distances,
> north-east-up differences, that sort of things. And of course the distance
> along the geodesic with height difference zero.

Yes... those are geodesics on three-dimensional (non-curved) space, AKA
straight lines ;-)

- Martin

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