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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Great Circle Course via calculator & HO 208
From: Gary LaPook
Date: 2016 Oct 4, 04:51 +0000
I like your choice of location. I used to fly every day from Chicago Midway to South bend and then to Ft. Wayne and then return to Midway.

gl

To: garylapook@pacbell.net
Sent: Monday, October 3, 2016 4:13 PM
Subject: [NavList] Re: Great Circle Course via calculator & HO 208

```On 10/3/2016 6:03 AM, Gary LaPook wrote:
> I guess nobody noticed. I posted a little experiment for non-navigators
> so that they could use Google Earth to get a sense of the difference
> between a rhumb lilne and a great circle course.

I had tried an over-the-pole vs rhumb line experiment as well as other
great-circle routes in Google Earth after some frustration in the "Find
Greg" at Neptune's Net exercise. The results seemed to be in the ball
park. My frustration was initiated by drawing a line along the same
parallel and measuring the distance in degrees and then nautical miles
(or feet), and they did not agree. I would expect the distance of 1
degree in nautical miles to be 60*cos 41 in my first 2attached examples.
I did expect the difference in degrees to be 1. Instead it was approx.
1*cos 41.

While my cursor when drawing the line in the first 2 attached examples
was not spot on, I believe you will get the gist.

Doing a similar test using the diagonal from a 30-minutes-per-right
triangle legs and calculating using plane trig (it is too small to fret
great circle) the distance was what I might expect. Square root of
(30*cos 41.25)^2 plus 30^2=37.53 nm. I was close to spot on with the
start and end cursor positions and the results were within 0.03nm.

Conclusion: Google Earth does OK with distance and great circle routs,
but don't trust distances in degrees along a parallel.

```
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