BoatBanter.com - View Single Post - Navigation question (Projecting a waypoint w/GPS)

JohnF

wrote:
Using a Garmin's Waypoint Projection feature, and projecting a
waypoint at 90 degrees, shouldn't my projected waypoint end up on the
same latitude as the original? And if not, why not?

For example, beginning at 30 degrees N latitude and 89 degrees W
longitude, I project a new waypoint at a distance of 10 miles with a
bearing of 90 degrees. As expected, my projected longitude changes to
a point further east. But for some reason, the projected latitude
ends up at a higher latitude than 30 degrees. In my example, the
projected latitude ends up as 30 degrees, 23.866 Minutes North.

I've tried using the same example on 3 different Garmins with the same
result. What am I missing?

Don't know, and, sorry, didn't read entire thread,
but could you try plotting several additional waypoints,
all bearing 90 but each with different distances,
say 10, 20, 30, etc miles? You get a "straight line",
recognizable great circle, or what?
Maybe that additional info would suggest some clue.
Some simple (but not simple enough for me to do
off the top of my head) spherical trigonometry would
tell you if half an extra degree north at 30 north
(all true) would keep you on a great circle 10 miles out.
Maybe plotting a few points will let you see the answer
without "doing the math".
--
John Forkosh ( mailto: where j=john and f=forkosh )