Chart Plotter/Auto Pilot Heading Errors

[Witzend]

Chart plotter/Auto Pilot Heading Errors
Sailors,
I recently noticed error in my heading vector displayed on my C90W.
Below is what Raymarine had to say about it.

Question:
"How do you adjust the Heading vector? It seems to be off a bit. When I point the vessel to a distant mark and than compare where the heading vector shows on the chart they are off. The further away from the mark the greater the error (of course).
"Under the "Setup Compass" menu there is a "Adjust Heading", and "Linearize Heading", soft keys, but they are grayed out, why?

Raymarine response:
"Smart Heading Sensor heading calibration and alignment is performed via the Multifunction Display (MENU->COMPASS SETUP->LINEARIZE COMPASS & MENU->COMPASS SETUP->LINEARIZE COMPASS). These options will be desensitized (i.e. grayed out) should the MFD not be interfaced via SeaTalk to a Smart Heading Sensor. If an autopilot or instrument is supplying heading data to the MFD, then linearization and alignment is performed via the autopilot control head or instrument display respectively."

This answer is confusing.
"These options will be desensitized (i.e. grayed out) should the MFD not be interfaced via SeaTalk to a Smart Heading Sensor.
And then goes on to say,
"If an autopilot or instrument is supplying heading data to the MFD, then linearization and alignment is performed via the autopilot control head or instrument display respectively."

So what is it:
- grayed out because it is Not using the Auto pilot or other instrument.
- or grayed out because it is receiving heading info from the Auto pilot or other instrument.

My guess is that it is grayed out because it is getting the heading from the S1's fluxgate's compass.

When I did the compass heading alignment for my S1 initially I used the "Auto" function to get the heading from the C90W COG, but I guess there must have been some current, I tried to do it at flood tide with no true wind, but I'm guessing I need to tweak it some more.

Also, there may be local variation from the chart displayed that is not being accounted for, placing the chart incorrectly in respect to the vessels position and heading. This can be adjusted by (MENU->SYSTEM SETUP->VARIATION SOURCE), but only has East or West in deg. increments of 1 deg. for adjustment

What are others using MFD's and Auto Pilots interfaced together experiencing regards to heading errors?

 

[Stu Jackson]

How far out are they? How much different. How much different between far away waypoints and closer ones? I guess what I'm asking is: is this really an issue for concern if it's only a few degrees? Also, what do you have interfaced?

 

[Witzend]

With the vector at ~1mile it is off ~5deg.
Raymarine C90W and their S1 Auto Pilot.

 

[Napa Ed]

are you saying your calulated heading (boat direction) is different from your gps track?

 

[Witzend]

Your heading is more often quite different from your GPS "boat track" or COG (Course over Ground) as it is generally refered to. This is due to leeway induced by wind and current and if under close haul sail can be quit significant. This is why my calibration procedure used during commissioning my S1 is to blame for my heading error. The question I submitted to Raymarine was, "why was the adjustment on the C90W MFD grayed out?". The answer is that it is getting it's compass heading from the S1's fluxgate compass, and if you want to correct the problem go to the source.

 

[Stu Jackson]

That's nice to hear

and glad you got it resolved.

This may not apply, but I like to dust it off every once in a while:

Reminds me of an example I use often: boat's goin' down a river with a lot of current. River goes N. Boat gets caught in a whirlpool, also heading N same speed as the river current, but boat turns sideways, facing east. Compass reads 90, boat heading by GPS is zero.

Your heading is more often quite different from your GPS "boat track" or COG (Course over Ground) as it is generally refered to.

It may be helpful to apply a bit more specificity to definitions, since this is not particularly true.

Heading is most cases IS COG as far as your GPS is concerned. Compass course is the direction the bow of the boat is facing. That's why I used the example above. The heading of the boat is zero even though it's going sideways.

Your heading is not any different from your GPS COG. Your compass reading may be appreciably different.

Your compass course and heading (magnetic) may be exactly the same.

 

[Napa Ed]

Your heading is more often quite different from your GPS "boat track" or COG (Course over Ground) as it is generally refered to. This is due to leeway induced by wind and current and if under close haul sail can be quit significant. This is why my calibration procedure used during commissioning my S1 is to blame for my heading error. The question I submitted to Raymarine was, "why was the adjustment on the C90W MFD grayed out?". The answer is that it is getting it's compass heading from the S1's fluxgate compass, and if you want to correct the problem go to the source.

I just wanted to understand your starting point. I expect the GPS track to be a different heading than the boat compass heading. I'm setting up a tiller pilot to a non raymarine gps via nmea 0183 and am just looking over your shoulder.

How can you tell if the difference between the two is too much? You mention a deviation from a distance. Have you tried to go to a waypoint and missed?

 

[Witzend]

Shooting WPs only from short distance, so it's fairly insignificant, but at longer tracks it would be a problem. Most of my sailing is in the upper Chessy, but planning to go south and in the lower bay it would certainly show up.

 

[BobM]

Just be sure to keep your Blackberry away from your compass or your autopilot. If I forget and put mine in the cup holder my WP-30 goes nuts.

 

[Napa Ed]

My understanding (!) is that the deviation updates as you get closer. Again, I'm still in the reading stage; have all the parts but gps not wired to the tiller yet. I thought I'd have to get out on the bay to give it a whirl to see what happens. I was disappointed that the unit will not pick up successive way-points without a manual input. Just utters an alarm and keeps on the last known heading. Some nonsense about verifying the situation and traffic. Picky picky.

 

[Witzend]

For all practicable purposes and as represented by Raymarine, "Heading" is the direction that the GPS is pointed which is effectively where the bow is pointed and it is not the same as COG. In my case it is getting it's data from the fluxgate compass. They are represented by two different vectors and are not the same on my MFD. Heading as traditionally defined is the direction a vessel is moving as Stu has indicated, but seems to have taken on a different meaning in the GPS era.

 

[Prime Time]

Confusing terminology

and glad you got it resolved.

This may not apply, but I like to dust it off every once in a while:

Reminds me of an example I use often: boat's goin' down a river with a lot of current. River goes N. Boat gets caught in a whirlpool, also heading N same speed as the river current, but boat turns sideways, facing east. Compass reads 90, boat heading by GPS is zero.

Your heading is more often quite different from your GPS "boat track" or COG (Course over Ground) as it is generally refered to.

It may be helpful to apply a bit more specificity to definitions, since this is not particularly true.

Heading is most cases IS COG as far as your GPS is concerned. Compass course is the direction the bow of the boat is facing. That's why I used the example above. The heading of the boat is zero even though it's going sideways.

Your heading is not any different from your GPS COG. Your compass reading may be appreciably different.

Your compass course and heading (magnetic) may be exactly the same.

Stu,
I recently bought a new good old boat with a very fancy electronics package. On my E 80, heading as reported by the plotter is actually a compass heading. Course over ground is actual travel of the boat. I think the heading data comes from a fluxgate compass that is part of the autopilot. There is an option on the plotter to have heading up, COG up or north up.

 

[Stu Jackson]

Definitions do count, like spelling

For all practicable purposes and as represented by Raymarine, "Heading" is the direction that the GPS is pointed which is effectively where the bow is pointed and it is not the same as COG.

Ya know guys, the DEFINITIONS of things don't change just because they invented GPS.

The heading of a boat is where the boat is going. It does not follow, logically, that that is where the BOW is pointing, which is the ONLY place the boat's compass will be pointing!

Is there a need to differentiate between fluxgate and boat compasses? I'd be interested in your opinions.

Raymarine's definitions may not be right, whether they are "practicable" or not, because what you say in the quote is just plain wrong. The bow can be pointed elsewhere, while "heading" IS and has to be COG on a GPS. Could you please explain how it can be anything else?

"Bearing" is the direction to the next waypoint, which may be what you're calling "where the GPS is pointed."

Heading and bearing are two different things.

Will everyone please reread their Duttons?!?

Another example: bow is heading north, compass reads 000, current is coming from the left, setting the boat to the right, or east. Heading (and COG) will be near 010. Bearing to waypoint say a mile north of you is constantly changing from 000 to 355, to 350 and so on. Compass course never changes, it's still 000 and you pass the waypoint somewhere east of it if you don't change course. This is Nav 101.

 

[Napa Ed]

Whatsamattah? Cast itchy? Pointy end forward, bubbles go back. That's all you really need. What is this thing you call Dutton's? I'm still working on a sewing pin and a sliver of wood in a dixie cup.

 

[Stu Jackson]

Casting...

No cast, they use space age technology these days, just like GPS compared to astrolabes.

 

[Witzend]

Stu, I was merely sharing information regarding my MFD and how it appears to interface to the S1 using the terminology Raymarine has adopted. It's hard enough communicating with them, (as you can see from their confusion I outlined from their answer) let alone changing the terminology they have chosen.
I agree with you about the definition of” Heading”, but if we want to use and share information about technology others have defined we have to be flexible and understand the context with which we are communicating.

 

[Todd Smith]

The heading of a boat is where the boat is going. It does not follow, logically, that that is where the BOW is pointing, which is the ONLY place the boat's compass will be pointing!
...

This is Nav 101.

Do you have a source for this, because I am pretty sure that:

Heading = direction the bow is pointing.
Track (or course) = direction that the boat is moving.
Bearing = direction toward something (like a waypoint to nav aid).

If I have it wrong, I'd like to get it right.

Todd

PS. I agree that when discussing a specific piece of electronics, it helps to use the manufacturor's teminology. But you might want to define those terms for people not familiar with that manufacturor's naming.

 

[Stu Jackson]

Charts & GPS

Witz, I fully understand, and apologize for somewhat hijacking your thread. Your issue is between components of your system and between the ears of the Raymarine techs, who, I sincerely hope on your behalf, are still working.

My point is that some folks are getting somewhat sloppy about basic navigational definitions and use, which is bad for the concept of navigation itself, and bad for them if they are exhibiting misunderstandings of terms necessary to "get around."

I took the time to look at the Raymarine manuals, which I'd previously never bothered with because the last Raymarine-type piece of equipment that'll be on my boat is my Autohelm ST3000, kind of a pre-Raymarine dinosaur.

Anyway, their handy-dandy flip card instruction manual for the C80 series of GPS chartplotters, applicable to ALL of their equipment, does indeed use the correct terminology. For instance,Card No. 14, monitoring a course, shows headings, bearings cross track error, all very clearly and quite correctly. That correct information is mirrored in the 238 page C80 series Users Manual.

I just don't buy the claim from someone's post on this topic that a manufacturer is going ahead and changing basic definitions. So I checked and that statement is simply incorrect.

So, if Raymarine can get the definitions and terminology correct, it just seems to me that people (not you) who are using the equipment ought to be able to do so as well. That's all, really.

Good luck in your issues with Raymarine.

 

[Stu Jackson]

Heading = direction the bow is pointing.
Track (or course) = direction that the boat is moving.
Bearing = direction toward something (like a waypoint to nav aid).

If I have it wrong, I'd like to get it right.

PS. I agree that when discussing a specific piece of electronics, it helps to use the manufacturor's teminology. But you might want to define those terms for people not familiar with that manufacturor's naming.

Todd,

Heading is NOT necessarily the direction the bow is pointing. Please read my earlier two examples of boats in a river, and think about it some more.

Track is where the boat HAS BEEN, not necessarily the direction the boat is STILL going. Therefore, track and course are two different things. You are correct about course and bearing (bearing is the magnetic or true compass course to anything else).

Re your PS sentence, my last post discusses this. Manufacturers seems to get it right, it's Joe Q. Public who seems a tad out of whack on these basic definitions.

Sources? Yes, I mentioned Dutton's, which is the "bible" for navigation & piloting. There are hundreds of other navigation books available. I'll bet wikipedia has a section on nautical terms, because I found that they do from doing research on "velocity made good" just last week.

 

[firehoser75]

Todd,
You are correct about the definition of "heading". According to the CPSS (Canadian Power and Sail Squadron), the following are definitions I quote from their materials:
Heading: The direction in which a vessel's head (bow) is pointing.
Course: The horizontal direction in which a boat is to be steered or is being steered.
Track: A boat's intended path over the Earth's surface.
Bearing: The direction in which an object lies in respect to a reference direction.
Course Made Good: The net direction of movement from one point to another.

Course Made Good (CMG) represents the actual direction a vessel moves in a straight line between any two points over a period of time. The Course Over Ground (COG) indicated by your GPS is an instantaneous measurement of the direction a vessel is moving at any particular moment. The average COG will be fairly close to the CMG.

So for Nav. 101 if there were no wind or current, the Course, Track, and maybe even the heading (for a very careful helmsperson), in theory, would be close to CMG. However, due to the afore mentioned elements of current and wind, they never match closely. Heading changes by the second due to the influences of waves, current, and wind, whereas course is the overall direction we are trying to steer the boat to compensate for wind and current so that we can obtain our objective of where we want to end up (hopefully following the track). I hope that helps with the understanding of the "terms and phrases".

Witzend, from what I read in your quotes from Raymarine, maybe the calibration buttons on your MFD are greyed out because the C90 is not reading a signal from your fluxgate?? Even if that is the case, you should still be able to calibrate it through your Autopilot controls.
Hope all this helps,

Tom