# [Proj] website cartographic executables---Inv_Fwd

Gerald I. Evenden gerald.evenden at verizon.net
Tue Dec 20 13:20:17 EST 2005

```On Tuesday 20 December 2005 12:39 am, Oscar van Vlijmen wrote:
> > From: "Gerald I. Evenden" <gerald.evenden-verizon.net>
> >
> > Just an added note here: check the NGS site for full precision online
> > geodesic computations:
> > http://www.ngs.noaa.gov/TOOLS/Inv_Fwd/Inv_Fwd.html
>
> Highly recommendable!
> I use it since version 1. Here & there are still version 1 implementations
> alive, but I strongly recommend to update to the latest version. Better
> accuracy, especially around antipodal point.
> One pitfall silly ol' me fell into, long ago: I thought the 3d version gave
> the true arc length going from height 1 in point 1 to height 2 in point 2.
> But the functions only work with ellipsoidal distances as if the heights
> were zero!
I do not follow.  But, I have never used the 3D version nor really thought
> The accuracy is according to Vincenty only 0.01 mm for meridional arcs, not
> full machine precision.

What bothers me is I cannot conceptualize the 3D version.  What does the
geodesic look like as it changes elevation?  Is it going up or down a "slope"
as one travels along the geodesic?  When I think about changing elevations
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.

The near antipod problem is nasty.  The antipod must yield twice the equator-
pole distance with the azimuths 90.  The geodesic has many enigmatic
properties.

One thing I need to look into in more detail is the NS inverse geodesic
computation.  The loop in my older version only iterates once, thus
computations are quite simple---much more simple than the traditional
series evaluation used for meridian distance.  I compared the values
with my near machine precision meridianal procedure and got results
comparible to microns.

--
Jerry and the low riders:Daisy May and Joshua
"The being cannot be termed rational or virtuous,
who obeys any authority, but that of reason."
---Mary Wollstonecraft 1792
```