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Re: Great Circle Course via calculator & HO 208
From: Bill B
Date: 2016 Oct 3, 19:04 -0400
From: Bill B
Date: 2016 Oct 3, 19:04 -0400
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.