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?

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.

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?

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?

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

On Wed, 19 Jun 2013 18:56:22 -0400, "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?

No, I'm using a simple handheld Garmin 76Map maybe 10 years old. And
the projection feature doesn't use either compass or satellites to do
the projection calculation. Doesn't even matter if the unit is in
simulation mode, it can still do the calculation. It's a fairly
standard feature on Garmin handhelds as well as lower end fixed mount
Garmins. I'm not very familiar with high end Garmin units or other
manufacturers but I would guess it would be standard with them too.
You plug in a location (lat/lon), then plug in a distance and bearing.
It instantly gives you the projected Lat/Lon.

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

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.

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

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.

--
Sir Gregory