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

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On Wed, 19 Jun 2013 14:16:49 -0400,
wrote:

On Wed, 19 Jun 2013 11:07:36 -0500, 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?

===

I'm not all that familiar with Garmin units or that feature but lets
do a little thought experiment and see if we can figure it out. If
it is going to create a new waypoint at 90 degrees, wouldn't that be
at 90 degrees to your present course-over-ground (COG)? If so, the
new waypoint would be on your present latitude only if your COG was
due north (0 degrees true) or due south (180 degrees true). Am I
missing something? Is your unit set up to report directions as "true"
or "magnetic"? If magnetic, that would account for your discrepency.

90 degrees would be due east no matter what my course over ground may
be. Or even if I'm dead in the water. Same as 270 being due west, 180
south, and zero being north.
You may be onto something with some setting or other, but I don't
think true or magnetic should matter. Seems to me, whichever I use,
"due east" (or 90 degrees) should still mean never getting off the
lattitude where I started. Yes?/No?