On Thu, 20 Jun 2013 14:45:50 -0400, " Sir Gregory Hall, Esq·"
åke wrote:

wrote in message
.. .
On Wed, 19 Jun 2013 21:01:14 -0400, " Sir Gregory Hall, Esq·"
åke wrote:

wrote in message
...
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 suspect it's a matter of choosing the right Datum.
http://geography.about.com/od/geogra...n/a/datums.htm
The above should explain it well enough.
Your direction is only as accurate as the datum chosen.

My unit has 110 different Datums to choose from. So far, I've just
always used the Garmin default (WGS 84) for navigation and it has
always been fine for that. The waypoint projection feature is the only
thing that seems a bit squirrely. So if you have a datum
recommendation that might cure that, I'll be glad to give it a go.

I don't know where you live. But I do know most Garmins default to
WGS 84 which the article linked to stated is out of date as of 2010.

You need to choose the best available datum for the area of the
world where you live. And remember, the world isn't a perfect
sphere. Chart datum for a particular part of the globe will take this
into account and correct for it even if the charts aren't representing
it accurately.

I saw that but I'm not sure what they mean by "invalid", especially
since a chart isn't even involved in what I'm trying to do. I mean, a
"datum" is not a system that can be switched off or abandoned, like
Loran for instance.

Anyhow, I've tried switching to NAD 83. That's the datum that the
article says most units are using now as default. But I still get
results as squirrely as before.

Basically, no matter what datum I use, I should be able to project a
second waypoint. Then, starting with my projected (second) waypoint, I
should be able to project a third point, using the same distance but
reciprocal bearing, and the third waypoint should be exactly the same,
(or reasonably close given rounding errors) as the first. But it's
not. It's further off than I would think is reasonable.

By the way, I'm on the Gulf Coast between Mobile and New Orleans.
Another by the way, I went to the Garmin Suppport site and they want
$30 to answer a frickin' question. I guess I just don't want to hear
their explanation that bad lol!