Chart Plotter/Auto Pilot Heading Errors

firehoser75 · Sep 24, 2009

Track Definition

Stu,
Yes the GPS definition seems to differ from the CPSS definition. GPS shows track as "where you have been" and it can be a very convoluted line. Whereas, the track as defined by CPSS is more of a planning line, as it is the direction as drawn between two points on the chart, kind of a where you are now, and the line to follow to get where you want to go. The course is then calculated factoring in the predicted wind and current effects so that you can actually (hopefully) arrive where you intend.

Tom

Stu Jackson · Sep 24, 2009

Very confusing

fh75, because their definition of heading is wrong, because, based on my examples, the bow is not always pointing where you are heading! In addition, PDF page 88 of the 238 page C80 series Chartplotter manual discusses those very difference in great detail and draws diagrams showing that Raymarine, at least, understands basic navigational terms, but, too are subtly incorrect. They show heading as the direction of the bow to differentiate from COG which can be different because of currents. It could be that they use just the fluxgate compass which shows boat heading and not necessarily bow direction.

I will attempt to retrieve my Duttons from the basement (hard with the bum leg) and give some more feedback, 'cuz I'm very disappointed in some of those definitions from our friends to the Great White North.

Stu Jackson · Sep 24, 2009

Witzend said:
...as far as this continuing discussion about nautical term definitions, like any language it changes over time. Raymarine is using "HDG" (Heading) for the name of the vector and numerical data display that responds immediately to any change in course and is directly responding to the fluxgate compass which is in effect where the bow is pointing. This is the way it is operating, so by design this must be their definition. Not what can be inferred in some quick card instruction.

I do not think so. Heading is where the boat is going, and the BOW is NOT always where that is. Please, please, reread my river examples. If you do, I cannot understand HOW anyone can viably argue that the bow is always where the boat is going.

And it's not just the quick card thing, the quick card is copied from the manual material, same stuff copied, different format for quick reference is all. I just finished reading the full manual.

I'll go find my Duttons and start a new topic, sorry again, back to your grey scale problem...

firehoser75 · Sep 24, 2009

Sorry Stu

Witzend,
Hope you resolve your issues and recalibrating should do it.
Stu,
I have also checked several books on Navigation including the "American Navigator" also known as "Bowditch" and they agree with what I have said. You are in fact in error, which is OK, our memories are fallible.
Nothing personal, just want the facts correct.
Tom

Rick Webb · Sep 25, 2009

I've had a Bit of Training on This

Aerial navigation in pilot training, Land Nav in the Army and on the water from the Coast Guard.

What I remember is:

HEADING is where the pointy end is pointed whether it be the bow the compass needle whatever.

TRACK is where you wanted to go

COURSE OVER GROUND is where you ultimately end up going

Put the glass down and read it again.

Todd Smith · Sep 25, 2009

Stu Jackson said:
I do not think so. Heading is where the boat is going, and the BOW is NOT always where that is. Please, please, reread my river examples. If you do, I cannot understand HOW anyone can viably argue that the bow is always where the boat is going.

And it's not just the quick card thing, the quick card is copied from the manual material, same stuff copied, different format for quick reference is all. I just finished reading the full manual.

I'll go find my Duttons and start a new topic, sorry again, back to your grey scale problem...

Stu,

I know that the boat is not "heading/travelling" in the direction that it is pointing.

But, I the universal definition of "Heading" in all of the nav books that I have read, is NOT "where the boat is heading", but "where the head/bow/nose of the boat is pointing".

Please check your Dutton's. If there are published definition that are different, I would like to know.

Thanks
Todd

Rick Webb · Sep 25, 2009

From My Copy of Dutton's

I was on the way out the door to change the oil in my wife's car but waiting for the washer to finish before I left so I pulled out my Copy of Dutton's.

"Heading of a ship is the direction in which the ship actually points or heads at any particular moment. It is the instantaneous angle which the ship's keel makes with the meridian. Heading is not to be confused with course. A ship is off the course but never off the heading."

I scaned it but I can't seen to add it to my post I guess you guys will just have to take my word for it.

Rick Webb · Sep 25, 2009

Worked Here I guess

Luandry is done I'm off

richk · Sep 25, 2009

amen to rick

Rick Webb said:
I was on the way out the door to change the oil in my wife's car but waiting for the washer to finish before I left so I pulled out my Copy of Dutton's.

"Heading of a ship is the direction in which the ship actually points or heads at any particular moment. It is the instantaneous angle which the ship's keel makes with the meridian. Heading is not to be confused with course. a ship is off the course but never off the heading."

I scaned it but I can't seen to add it to my post I guess you guys will just have to take my word for it.

I do. The quote is correct. Thanks, Rick.

Paul Mermelstein · Sep 25, 2009

Re: amen to rick

Three points to make re: 1) definitions, 2) fluxgate compass, and 3) answering Witzend's original question.

1. As a member, and instructor, with the US Power Squadron, I agree with the Canadian friend's statement and with other posts .... heading is where the bow is pointing, COG is the net effect considering leeway (wind) and drift (current), and track can be either the path you actually took or "intended track" is the same as your planned course.

2. That said, in practice, our magnetic compass often reads differently than my fluxgate, by 5 to 10 degrees. May need calibration again. Also, the fluxgate is in an interior locker, and any metal object like a clothes hanger can affect it. So, that might be something to check on.

3. I believe that your keys are grayed out because the heading data does not come directly from a Smart Heading Sensor, via Sea Talk. This is what the Raymarine person said, according to the quote. So, the heading data may be coming from a fluxgate other than a Smart Heading Sensor, or via NMEA instead of Sea Talk, or via the Autopilot, or some combination. Try calibration via the Autopilot control head. Should work.

Stu Jackson · Sep 25, 2009

Thanks for your input

Paul & Rick. Todd, I have my Duttons and have a few comments I'll get to later when I come back from a meeting. "Current sailing" is a section that augments the early basic definitions.

In the meantime, for those of you who have answered that heading is the bow, would you please, if you can make the time, comment on my earlier river examples which clearly indicate that the heading (even the GPS definition) is not always the bow. Thanks, see ya later. Happy Friday!

firehoser75 · Sep 25, 2009

Stu,
You are in fact correct, that the boat may not be moving in the same direction that the boat is actually "pointing" as per your river example. The problem with your argument is your incorrect use of the term "heading". Heading is in fact the direction that your bow is pointing (at any given moment). It can (and often does) change by the second due to waves, current, wind, and even whirlpools as you suggest. In your example the heading has absolutely nothing to do with the COG as read from your GPS. Also remember that COG as read from a GPS is an instantaneous calculation of your course (movement) over the earth's surface, so it too can change as conditions (in this case the whirlpool) change, even though you did not alter course (by moving the rudder).
Please listen to what all of the others (Rick, Paul, Todd, and Rich) have tried to point out to you. It seems you have an understanding of navigation, you are just using some "terms" incorrectly, particularly the term "heading".
I will not try to convince you further, if you still don't get it, well so be it, but I just was trying to help you. I have read many of your posts on other topics and have a good respect for your opinions and knowledge.
Best wishes,
Tom
"Your Canadian friend"

Todd Smith · Sep 25, 2009

firehoser75 said:
Stu,
You are in fact correct, that the boat may not be moving in the same direction that the boat is actually "pointing" as per your river example. The problem with your argument is your incorrect use of the term "heading". Heading is in fact the direction that your bow is pointing
...

Exactly Stu, we understand that the boat is not travelling in the direction the bow is pointed. It's just the definition of the term heading that we are disagreeing with.

Stu Jackson · Sep 25, 2009

OK, now I understand

Thanks, lively and interesting discussion.

Let's get together and run some rivers in our sailboats!

rpwillia · Sep 26, 2009

Let's start over

I lost the whole intent of the thread. It seems that back in the day (I am thinking Columbus, John Smith, etc) they were pretty happy to generally head West until they found land and then head North or South until they got to where they were going.

That being said, I have a chartplotter/gps, compass and S1 Autopilot with fluxgate. On various headings they agree or disagree but usually all within 10 degrees which I believe is affected by the magnetic deviation (at least for the compass and the fluxgate). Does it really matter all that much? Usually I try and pop my head up and look at the horizon or the nearest shore to verify where I am. Just my $0.02.

Rick Webb · Sep 26, 2009

Ten Degrees is A Huge Deviation

Have you swung your compass? Is your compass large enough to really read? I forget what the variation is up there three to five degrees I imagine so it may not be out of the question to have a ten degree difference between your mag compass and your flux gate compass. A quick check might be to chart out a known course with a couple landmarks in your local areaand them run that course and compare the reading on your mag and fluxgate compass. Your chart plotter would display your course according to your GPS.

On the other hand if you don't get lost and you make it home each time you go out don't worry about it.

RichH · Sep 26, 2009

Witzend-

The area of the northern Chesapeake is quite famous for 'strange magnetic anomalies' that can differ from 'charts' by many degrees. The area near the mouth of the Sassfrass River, Still Pond and Elk River, Ive found over time by active racing and also by sailing up and down those shipping channels at night, can vary from the 'calculated' magnetic flux by 6-7+ degrees .... pass the same area a few days later and its back to 'normal'. Many of the older paper charts have specific 'magnetic anomaly warnings' for your sailing area of the Upper Chesapeake. It could simply be your AP's fluxgate compass is responding to these transient anomalies common to the upper Chesapeake. The whole worlds 'magnetic flux' is currently in an 'agitated' state ... and this may be the cause of your noticed problem. A few degrees error is not going to make much difference (on an hourly/daily basis).

When was the last time you 'swung' the fluxgate compass of your autopilot? Most APs have routines to 'swing' (adjust/align) the fluxgate.

Chart Plotter/Auto Pilot Heading Errors

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