Navigation question (Projecting a waypoint w/GPS)
 

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?

Navigation question (Projecting a waypoint w/GPS)
 

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.

Navigation question (Projecting a waypoint w/GPS)
 

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?

Navigation question (Projecting a waypoint w/GPS)
 

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?



It's the great circle conundrum. Charts aren't a fair representation of
the surface of the Earth.

--
Sir Gregory

Navigation question (Projecting a waypoint w/GPS)
 

On Wed, 19 Jun 2013 15:00:45 -0400, " Sir Gregory Hall, Esq·"
åke wrote:

Charts aren't a fair representation of
the surface of the Earth.

===

That's true but it does not relate to this particular issue.

Navigation question (Projecting a waypoint w/GPS)
 

On Wed, 19 Jun 2013 13:38:10 -0500, wrote:

Seems to me, whichever I use,
"due east" (or 90 degrees) should still mean never getting off the
lattitude where I started. Yes?/No?

===

No.

Due east magnetic will change your latitude unlees you are in a
location where declination equals zero (true = magnetic). Same with
due west of course.

Navigation question (Projecting a waypoint w/GPS)
 

wrote in message
...
On Wed, 19 Jun 2013 15:00:45 -0400, " Sir Gregory Hall, Esq·"
åke wrote:

Charts aren't a fair representation of
the surface of the Earth.

===

That's true but it does not relate to this particular issue.



It is too true. The lines of latitude show as parallel on
a Mercator projection chart while on a sphere (like the Earth)
these lines are NOT parallel as the surface of the Earth
is not flat but spherical.

Ask yourself this question: The longitudinal lines everybody
agrees are not parallel as they all run together at the poles and
have maximum separation at the equator, but the *parallels* of
latitude though parallel on the chart are NOT parallel on the
sphere of the Earth. Thus the divergence noted on the Garmin
plotter. A great circle route is actually a straight line across the
Earth's surface but on a Mercator projection it shows as a
curve. What makes you think the opposite isn't true?

HTH.

--
Sir Gregory

Navigation question (Projecting a waypoint w/GPS)
 

On Wed, 19 Jun 2013 16:08:19 -0400,
wrote:

On Wed, 19 Jun 2013 13:38:10 -0500, wrote:

Seems to me, whichever I use,
"due east" (or 90 degrees) should still mean never getting off the
lattitude where I started. Yes?/No?

===

No.

Due east magnetic will change your latitude unlees you are in a
location where declination equals zero (true = magnetic). Same with
due west of course.

hmmm.... Well, I had my "North Reference" originally set to "True". So
I changed it to "Magnetic" with a declination of zero and recalculated
my example. I ended up with a projected latitude that is different
than the "True" projection but still not the starting latitude of 30
degrees. This "Magnetic" projected latitude resulted in 30 degrees,
23.904 minutes North. That's dang close to the "True" projection but
not exact (rounding error maybe?) and still a far cry from the
original starting latitude of 30.

I'm slowly beginning to realize that, by following any particular
latitude all the way around the earth, you aren't actually traveling
due east or west unless you're following Latitude zero degrees (the
Equator). So maybe Garmin hasn't screwed up afterall (as I originally
suspected lol!).

Navigation question (Projecting a waypoint w/GPS)
 

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.

----------------------------------------

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

Navigation question (Projecting a waypoint w/GPS)
 

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.

----------------------------------------

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