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

Hank©[_3_]

On 6/19/2013 6:56 PM, Eisboch wrote:

wrote in message ...

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.

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Does that Garmin navigation system have an input from a fluxgate compass
on the boat as well as receiving the satellite data?

Here's a site that calculates declination (variation).
http://www.ngdc.noaa.gov/geomag-web/#declination