Learning my new Garmin 76

[Gary M]

Yesterday I was motoring around the harbor and noticed that the course on my bulk head mounted compass and the course on my GPS compass ring disagreed. And disagreed big.

I then spent time checking North - South and East - West and there was always a deviation.

Then, while once again checking a due south course according to the GPS, I realized I was headed straight for the due South range installed in the marina. My GPS was set for True not Magnetic.

Later I figured out how to change the setting in the GPS and it automatically set the correct deviation for this area.

I thought there may be a chance some one else had a new GPS and didn't realize that the compass ring comes from Garmin with the True setting.

Gary

[Neil Gordon]

Mine is set to true because I find it easier to refer to paper charts that way. I make the adjustment in my head from the true course to the compass course. (If I'm planning a trip, though, I'll create a table with both true and mag courses for each leg.)

I'm wondering what the rest of us here do.

[Parfait's Provider]

I run Parfait's chartplotter in magnetic so I can correlate it with the compass without any mental gynmastics. It is quite surprising how often they agree. Gives me a warm feeling about the compass and a good check on stray fields that might affect it.

[John Vigor]

Gary, a word in your ear:

Your GPS can't compensate for deviation. It can only compensate for variation. I'm sure you know the difference, but there might be others who could be misled.

Cheers,

John V.
_____________________

From Vigor's Rules for Life:
Langsam's Ornithological Axiom: "It's difficult to soar with the eagles when you work with turkeys."

[Neil Gordon]

I'm sure you know the difference, ...

So there's a difference between deviating from reality vs. varying from reality?

If anyone is actually confused, variation is the narrow "V" that you get on the compass rose if you draw lines from magnetic and true north through the center of the rose. (And no, Compass Rose is not the name of a bargirl I used to know in Subic Bay.)

[GeorgeV]

Deviation: The error in the compass heading caused by electric magnetic currents and or metal objects. The deviation error is caused by magnetic forces within your particular boat. Pieces of metal, such as an engine or an anchor, can cause magnetic forces. And also stereo and other electric equipment or wiring, if too close to the compass, introduce large errors in compass heading.

Variation: The angle between the magnetic north pole and the geographic north pole.

[Joe Myerson]

FWIW, I'm with Ken on this one.

My GPS is set for magnetic and, like Ken, I'm pleased with how closely it seems to match the heading on my boat's compass.

--Joe

[iropicc]

neil.
Compass Rose was a bargirl in subic bay. she insisted that the following should be remembered by all Pacific ocean sailors.
TRUE VIRGINS MAKE DULL COMPANIONS; ADD WISKEY

[tartansailor]

Oh my Lord.
Course is plotted in TRUE
Log entry's are recorded in TRUE
When one embarks on an extended cruise, it is certain that deviation will change and if the cruise is of any distance, variation could easily change also.
See thats how mistakes occur.

OK, so I'm an old sea dog from a bygone era, but if you are in a bad situation, and the Coast Guard checks your log, following the above directive will save you much embarrassment.

Dick

[Neil Gordon]

... if the cruise is of any distance, variation could easily change also.

Veriation will also change if you sail in the same place for long enough.

[Ben Thomas]

Here is an interesting article. I wonder, does this mean the south pole is moving as well?

http://www.livescience.com/forcesofnatu ... shift.html

[Joe Myerson]

Dick is, of course (pun intended), correct.

When I first learned navigation and piloting, 40 years ago (I'm not THAT old, I was in college at the time), the instructor insisted we use true courses on our charts and in the logbook. It made the chartwork easier, and she (the late F.W. Wright, author of the celestial navigation text used by 90-day wonders in WWII) insisted it was the only proper way to do it.

However, when I made my first (and so far only) bluewater cruise, helping a friend sail from Point Judith, RI, to Yarmouth, NS, I was astonished to see that the skipper recorded magnetic courses and "psc" courses on his charts and in his log.

It was kind of a revelation to me--it was so much easier, too.

And I'm still doing it--even though I know it isn't really the approved way. It's not exactly like driving without auto insurance--I do keep a log, but more for my own benefit than for the USCG--and if I were to ever start taking long cruises, I'd go back to the USCG-approved method of piloting.

--Joe

[Bill Cochrane]

I submit that the advent of electronic chart plotters and computer-based charting software is going to make referring to courses and bearings in degrees True a bit of arcane history. As Joe and Ken point out, it's much easier to use Magnetic...and not prone to conversion error as long as all the automatic gadgetry is set to present courses and bearings that way.

Some other points before I don my asbestos underwear...

- The line on the chart is agnostic. As long as it stretches from point A to point B, it doesn't care if you measure it in degrees True or Mag.

- Labels and log entries need to indicate T or M. Or C, for the purists who trust the deviation table that most don't even bother with these days. IF, and only IF, there is a mixture. If practice on your recreational vessel is to record everything in Magnetic, and the crew knows that, I doubt seriously that the USCG will give a dern.

- There is nothing requiring a recreational vessel to keep a deck log, let alone specifying how courses and bearings are to be recorded. One could use Mils, so long as everyone aboard knew what was being done.

I happen to believe that the safest practice is to do the least possible manual conversions; if my chartplotter set for Magnetic says to steer 085, and that's what I see on the mag compass, I'm in good shape. I can also confirm that with a line on the paper chart and a parallel rule.

If there's anyone I haven't offended yet, let me know and I'll try again.

[mahalocd36]

If there's anyone I haven't offended yet, let me know and I'll try again.

Well you didn't offend me And I'm probably about to offend more.

We were taught in our ASA navigation course that it didn't matter as long as you specific T, C, M after the direction. We were taught all, but in practice we used Magnetic.

You are right, the CG won't care. Listening to many CG calls, the first thing they ask (after telling everyone to put on their lifejackets) is, do you have a GPS on board? They want exact position. I think if you gave them a course, "we are drifting 175 Magnetic" they are not going to ask you to convert it to True for them.

That said, obviously do whatever works for you, and what you are comfortable with. Just be clear if it's True or Magnetic to everyone aboard.

[Neil Gordon]

Listening to many CG calls, the first thing they ask (after telling everyone to put on their lifejackets) is, do you have a GPS on board? They want exact position.

The second thing they ask is if you can confirm your position by reference to something geographic so that it's not just reading numbers. I suspect they've been on long searches otherwise.