# NavList:

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

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Re: Computing great circle and rhumb lline courses
From: Joe Schultz
Date: 2010 Feb 8, 00:48 -0800

Gary,

Your logic (lines 11-29) is pretty neat - I let my programs crash as punishment for not engaging the brain, and to save precious RAM on my pocket computer. Three buttons clear and restart the programs, so crashing isn't a big deal for me.

Lots of ways to skin a cat, including stepping back in time to the pre-radio days. "Towson's Tables" are now on Google Books (yahoo!). By John Thomas Towson, partial title is "Tables to Facilitate the Practice of Great Circle Sailing, and the determination of azimuths...." Bonus is a pretty neat way to do a compass check. Did a search on this list for "Towson" and nothing turned up. Perhaps a forgotten technique for the experts to ponder?

Purchased my pocket computer, Sharp EL-5500II, in 1984 and the CE-129P Printer and Cassette Interface (docking station) in 1985 (\$70 each). Docking station stays in the box until I need to dump/reload programs when changing batteries in the computer.

Rhumb lining: first select ellipsoid (used a few in the Pacific), then input departure & destination LAT/LON. May as well use the same ellipsoid as the chart. Computer did it the old way, finding Meridional Parts and solving the plane trigonometry.

Great Circle: done the old way, with the addition of finding the final course - makes a French curve/flexible stick plot quick and easy. Just "break" the program if you didn't want the two-for-one vx LAT/LON pairs.

Programmed this way to fit the Nav forms. Exams were, of course, sans electronic calculators, so the computer also helped in exam practice. Didn't program the mid-latitude rhumb or HO-229 great circle methods.

Changed the great circle program a bit for the Atlantic tin can. You input the number of waypoints between departure and destination, then the computer figures out that many equally spaced (longitude) LAT/LON pairs, after finding the vertex and final course. That way you don't have to engage the brain if the vertex is outside the track or off the chart (and I was beyond navigation exams by then).

My stuff doesn't do any leg summing, but it's a good idea. I should change my input/output format to XXd YY.YYYm to match today's GPS, and I suppose the purist would great-circle the ellipsoid (not me, I'm not a missle programmer).

Joe

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