Navigation question (Projecting a waypoint w/GPS)

[Phant...@gmail.com]

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?

[waynebatr...@hotmail.com]

===

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.

[Phant...@gmail.com]

On Wed, 19 Jun 2013 14:16:49 -0400, waynebatr...@hotmail.com
wrote:

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?

[Sir Gregory Hall, Esq�]

<Phant...@gmail.com> wrote in message
news:cak3s85p8h27i3u7f...@4ax.com...

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

--
Sir Gregory

[waynebatr...@hotmail.com]

On Wed, 19 Jun 2013 15:00:45 -0400, " Sir Gregory Hall, Esq·"
<greghall@home.fåke> 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.

[waynebatr...@hotmail.com]

On Wed, 19 Jun 2013 13:38:10 -0500, Phant...@gmail.com 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.

[Sir Gregory Hall, Esq�]

<waynebatr...@hotmail.com> wrote in message
news:np34s8p80rvlvv2u3...@4ax.com...

> On Wed, 19 Jun 2013 15:00:45 -0400, " Sir Gregory Hall, Esq�"
> <greghall@home.f�ke> 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

[Phant...@gmail.com]

On Wed, 19 Jun 2013 16:08:19 -0400, waynebatr...@hotmail.com
wrote:

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!).

[Eisboch]

wrote in message news:jks3s89e9ngg392vb...@4ax.com...

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

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

[Hank©]

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

[Phant...@gmail.com]

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.

[Sir Gregory Hall, Esq�]

<Phant...@gmail.com> wrote in message
news:cak3s85p8h27i3u7f...@4ax.com...

I suspect it's a matter of choosing the right Datum.

http://geography.about.com/od/geographyintern/a/datums.htm

The above should explain it well enough.

Your direction is only as accurate as the datum chosen.

--
Sir Gregory

[Phant...@gmail.com]

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.

[JustWaitAFrekinMinute]

Not following too closely but if you actually made the trip, does the
"target" destination come more into line as you get closer? If you know
what I mean...

[Hank©]

On 6/19/2013 10:29 PM, Phant...@gmail.com wrote:

If you correct for deviation your course heading will be 090 true.

[Hank©]

The heading to destination will change as you get closer due to drift,
gps error, and compass error. The closer you get the more course error
will be introduced.

[Phant...@gmail.com]

On Wed, 19 Jun 2013 22:46:35 -0400, Hank� <Hank@save_rec.boats.com>
wrote:

Hmmm..... I think I may have made this thread sound more complicated
than it actually is. This isn't about actual navigation. It's only
about a calculation that doesn't need GPS input. I don't know what
formula Garmin uses but I'm sure it can be done with a calculator or
computer. Maybe even a slide rule or an abacus :-)

[JustWaitAFrekinMinute]

Is there a static equation for that?

[Richard]

On 6/19/2013 10:50 PM, JustWaitAFrekinMinute wrote:
> On 6/19/2013 10:39 PM, Hank© wrote:
>> On 6/19/2013 10:29 PM, Phant...@gmail.com wrote:

Yeah.

East is least, west is best.

[Hank©]

On 6/19/2013 11:38 PM, Phant...@gmail.com wrote:
> On Wed, 19 Jun 2013 22:46:35 -0400, Hank© <Hank@save_rec.boats.com>
> wrote:
>
>> On 6/19/2013 10:31 PM, JustWaitAFrekinMinute wrote:
>>> On 6/19/2013 10:29 PM, Phant...@gmail.com wrote:

If you turn off deviation compensation in your gps the results of a
waypoint projection should theoretically be as you expected unless
Garmin's algorithms are wrong.
If using the feature doesn't produce the desired result for you I
suggest you don't use that feature. If you need to know why that feature
behaves the way it does, you need to ask Garmin's software developers.

[Hank©]

On 6/19/2013 11:50 PM, JustWaitAFrekinMinute wrote:
> On 6/19/2013 10:39 PM, Hank© wrote:
>> On 6/19/2013 10:29 PM, Phant...@gmail.com wrote:

No. But the link I provided will tell you the exact magnetic deviation
for any position at any given time.

[Hank©]

On 6/20/2013 1:50 AM, Richard wrote:
> On 6/19/2013 10:50 PM, JustWaitAFrekinMinute wrote:
>> On 6/19/2013 10:39 PM, Hank� wrote:
>>> On 6/19/2013 10:29 PM, Phant...@gmail.com wrote:

Let me guess. Are you a valley girl/boy? (ref. Calif)

[iBoaterer]

In article <kpttsu$6mh$1...@dont-email.me>, justwaitaf...@gmail.com
says...

>
> On 6/19/2013 10:39 PM, Hank© wrote:
> > On 6/19/2013 10:29 PM, Phant...@gmail.com wrote:

No, because it's not static.

[Richard]

On 6/20/2013 5:03 AM, Hank© wrote:
> On 6/20/2013 1:50 AM, Richard wrote:
>> On 6/19/2013 10:50 PM, JustWaitAFrekinMinute wrote:
>>> On 6/19/2013 10:39 PM, Hank© wrote:
>>>> On 6/19/2013 10:29 PM, Phant...@gmail.com wrote:

Another of Wilbur's socks?

[Sir Gregory Hall, Esq·]

<Phant...@gmail.com> wrote in message
news:8to4s8l21o8pv8vdj...@4ax.com...

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.

--
Sir Gregory

[Phant...@gmail.com]

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

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!

[Anonymous Remailer (austria)]

Criminal, pedo, scumbag:

Making sure everyone knows you have a little limp dick and a puke yellow boat that smells like shit, not to mention your lack of education.

Capt...@gmail.com

Neal D. Warren/Wilbur Hubbard/Gregory Hall
PO Box 1015
Tavernier, FL 33070
305 304-7546

[JohnF]

Phant...@gmail.com 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?

Don't know, and, sorry, didn't read entire thread,
but could you try plotting several additional waypoints,
all bearing 90 but each with different distances,
say 10, 20, 30, etc miles? You get a "straight line",
recognizable great circle, or what?
Maybe that additional info would suggest some clue.
Some simple (but not simple enough for me to do
off the top of my head) spherical trigonometry would
tell you if half an extra degree north at 30 north
(all true) would keep you on a great circle 10 miles out.
Maybe plotting a few points will let you see the answer
without "doing the math".
--
John Forkosh ( mailto: j...@f.com where j=john and f=forkosh )

[JohnF]

JohnF <jo...@please.see.sig.for.email.com> wrote:

> Phant...@gmail.com wrote:
>> 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.

Excuse me for following myself up, but either I'm totally confused
(a common occurrence) or that example's totally messed up...
You say your waypoint's distance is 10nm, but its latitude
changes by 23.866'. Heck, even if you headed exactly due north
for 10nm, latitude would only change by exactly 10'.

[Phant...@gmail.com]

No, you're not confused. I'm just getting projected waypoints that
make no sense. Fortunately, so far, my unit's GPS capability is spot
on, so I can still use it to navigate (with backup). I just can't
trust the "Projection" feature for accurate results. Never used it
much anyway.
By the way, I tried projecting 10 miles due north and it changed my
latitude by 33' minutes. I'm done fiddling with it at this point.
Thanks for the come back tho.

[Sir Gregory Hall, Esq�]

<Phant...@gmail.com> wrote in message
news:fiubs8l68f19ema59...@4ax.com...

Then you've got it set to magnetic north or it's stuck on magnetic
north no matter how you set it. Set it to true north and try it again.

--
Sir Gregory

[Phant...@gmail.com]

It's been set on True for much of my experimenting. I think a bug is
stuck in there somewhere. lol! But I've now worked out the original
navigation problem that set me off on this whole brain frazzler in the
first place by using this calculator:

http://williams.best.vwh.net/gccalc.htm

It projected my waypoints within a few feet of where they are supposed
to be. I'll fiddle with my GPS calculator later. I'm overdue for a
new GPS anyway.

Thanks for the input.

[Flying Pig]

I think this issue that has your point not ending up at the same north position could be resolved by plotting a waypoint at the same latitude but however far out you want it.

Then plot locations to reach that point on the line to the waypoint.

I'd bet you found that it worked slowly north to a point, then curved back south, to the waypoint. The further north you do this exercise, the more deviation from the starting and ending latitudes you'll find.

My two cents, anyway :{))

L8R

Skip

[Snidely Whiplash�]

"Flying Pig" <skipgu...@gmail.com> wrote in message
news:a6c5a31f-7cf7-4cab...@googlegroups.com...

Great circle and all that good stuff.. But, these Rubes just won't listen to
reason. No, I guess that's too much to expect.

--
SW
"Curses, foiled again!"

[Phant...@gmail.com]

I agree with what you're saying here. But I wasn't trying to find the
shortest distance between two points (the "great circle route"). My
problem had more to do with following a particular latitude (the
"great scenic route" :-)
Got it sorted out now though, thanks.