What Heading Am I Really On ?

[btrayfors]

Dave,

Understand, completely. All completely valid.

But, as this discussion is based on all magnetic headings, I posted the above.

BTW, I'm an old navigator, too (and piloting/nav instructor, celestial instructor, etc.). Everything on my boat runs in magnetic; just makes it easier for my tired brain :-). I never have to consider True heading/bearing of anything.

The radar is head up. The charting program uses magnetic headings. The hand bearing compass is, of course, magnetic. The main steering compass is magnetic. The fluxgate compass for the autopilot is magnetic. The paper charts can be used with magnetic headings. The multiple GPSs are set for magnetic readout. etc., etc.

Hey, we need to get together soon. You here for awhile?

Bill

[Don1500]

Don,
I'm going to take a wild guess at this. The Mag Heading you get from your magnetic compass will include any Deviation your boat/equipment throws at it. You can balance this out as best you can but the compass will always point close to but not exactly what the charts says. Your autopilot may or may not take its heading information from your compass. It may take it from a magnotometer. If you get your mag heading from a secondary source it will give you something different from your Compass. The magnotometer should be swung the same as your compass. GPS COG can not give true or mag heading. It can only tell you which way you are going, not where your bow is pointing.

[cchesley]

Originally Posted by Auspicious

Bill,



GPS will compute COG in true. Some have an internal variation table to adjust to magnetic. So even if the user doesn't see the variation adjustment it is happening and it would be wise to understand the implications.

I run everything displayed on my boat in true except--of course--the binnacle compass. I'm a cranky old-fashioned sailor as you know *grin* and since all the old maneuvering board work was done in true I try to stay as close that as I can.

I hope we all understand at this point. Variation does indeed apply when comparing a magnetic reading to a true reading. I run everything in True mode from radar, to chartplotter. When I go 'straight up or down on my chart, I just like things to read 000 or 180' but that's me. Since I can't steer to a magnetic compass much closer than 5 degrees +/- anyway, a quick 'nearest 5 deg estimate' of variation is pretty easy to do (for my brain anyway)

[redcobra]

Originally Posted by Don Lucas

just as a follow-up ... regardless of the question I'm not an idiot!

all the electronic units are set to magnetic

wonder if the mounting of the autopilot and GPS a foot away from the comprass could effect it this much (was a little closer when the seas settled down some) both these should both be measuring the same magnetic heading

I have the same issue. I just average the three and then get a different reading with my compass bearing binocs.
The compass is correct, the GPS is correct (COG and very little lag) and the auto helm compass is located too near the engine (but on the centerline) and I don't feel like running new cable to the forward end of the boat to move it.

[redcobra]

Originally Posted by btrayfors

Magnetic Heading = Compass Heading plus or minus Deviation

Course Over Ground = Magnetic Heading plus or minus Leeway plus or minus Current

Note that since we're dealing only with magnetic headings here, Variation does not apply (it's only used to compute True headings).

The GPS computes Course Over Ground directly. However, as several have noted, it averages the course over a certain time period, usually adjustable, and may include certain errors.

To compare a Compass Heading with a GPS Course Over Ground readout, you'd need to convert the Compass Heading to Magnetic Heading then to Course Over Ground, as indicated in the first two lines above.

Ain't this fun? :-)

Bill

My head hurts. If I wanted to do math I would not have retired.

[IdoraKeeper]

Up here in the PNW a decently swung compass is a must. An error when murking about at night or in the fog does not land you in mud. Rather more often it's sharp rock. Don may be primarily concerned academicaly but I like to be able to put the boat within a few feet of where it is planned to be.

Todd

[Auspicious]

Originally Posted by btrayfors

The radar is head up. The charting program uses magnetic headings. The hand bearing compass is, of course, magnetic. The main steering compass is magnetic. The fluxgate compass for the autopilot is magnetic. The paper charts can be used with magnetic headings. The multiple GPSs are set for magnetic readout. etc., etc.

To each their own. I run radar head up also since that was the only choice when I learned. Chartplotter is true-North up since that is how paper charts are, and what I learned on. I do spatial conversion in my head. *grin*

The deviation of my fluxgate compass for the autopilot is different from my binnacle compass. Ain't we having fun now? Running true is one less mental conversion on my boat.

Originally Posted by btrayfors

Hey, we need to get together soon. You here for awhile?

In and out. Deliveries and radio work is keeping me pretty busy. I'll send you an e-mail to keep from drifting the thread.

Originally Posted by IdoraKeeper

Don may be primarily concerned academicaly but I like to be able to put the boat within a few feet of where it is planned to be.

If you can get within a few feet of plan with variation, deviation, set, and drift you are a better man than I. Once I'm out of pilotage I can't possibly stay that close. Feet? Geez.

[sailorboy1]

Originally Posted by Don1500

GPS COG can not give true or mag heading. It can only tell you which way you are going, not where your bow is pointing.

Given that I've only sailed my new boat once and still have a lot to learn about the electronics on it (all I want do is sail and get where I planned), but on my RM charplotter for COG is says it is magnetic. Guess I don't really know what the unit is using to convert to this.

[boatman61]

And folks ask why I love Paper, pencil and parallel rule....
Enjoy.....

[sailorboy1]

Originally Posted by boatman61

And folks ask why I love Paper, pencil and parallel rule....
Enjoy.....

No I know why, and I still get my charts out and do the course before leaving even though I'm going to be mainly using the chartplotter later. There's probably age where one is comfortable being completely paperless.

[btrayfors]

Originally Posted by Don Lucas

Given that I've only sailed my new boat once and still have a lot to learn about the electronics on it (all I want do is sail and get where I planned), but on my RM charplotter for COG is says it is magnetic. Guess I don't really know what the unit is using to convert to this.

Don,

The GPS and a connected or standalone chartplotter w/GPS can report bearings and courses in either True or Magnetic. Mine are set to Magnetic, and I believe many sailors prefer this (except, maybe, for Dave on Auspicious :-).

Magnetic headings are True headings plus or minus Variation. Variation depends on where in the world you are and the year, since they change slightly from year to year.

Since the GPS knows where you are, it also then can figure out the Variation in that place, and apply it to True bearings to give you Magnetic bearings, or vice-versa.

Example:

You are at the mouth of the Potomac River at Point Lookout Light. It is 2011. The variation at this place and year is 11 degrees West.

You wish to compute a Magnetic bearing (or rhumb line course) from Pt. Lookout light to the entrance channel buoy #1 on the west coast of Smith Island. It's bearing is 103 degrees True, and it is 13.2NM distant.

To convert true bearings to magnetic bearings you simply add westerly variation:

103 degrees True plus 11 degrees = 114 degrees Magnetic.

Note that this is NOT the same as compass bearing. To get that, you'd need to apply your compass deviation which is specific to your boat and your compass. And, of course, you have that on a deviation card located right near the compass don't you??? :-)

Bill

[David M]

Don, I highly recommend a navigation course so you understand the fundamentals and to make yourself more familiar with what is both a science and an art. You need to learn what TVMDC +W -E means.

A boat only has one heading...the direction it is physically pointed, which is not necessarily the path that the boat is taking. The path your boat is taking whether or not it is pointed that way is called your Course Over Ground.

What units you reference the heading to can be many things such as relative to true north or magnetic North or even a landmark if you want.

A single GPS cannot provide heading. Two GPS antennas can be integrated to provide heading which is called a satellite compass.

Example...
Product Detail - SC30

[John A]

GPS calulates where you are and compares it to where you were by utilizing two pseudo waypoint positions 0 and -1. From that calculation it can figure out the angle between the two points (cog) and the time it took to move between the two points (sog). True or magnetic are just two different options that may be used in the cog equation. All of this calculation occures in your GPS unit as all the four or five satellite do is tell your GPS unit where it is. These calulations occure many times a second. I had two handheld GPS units for backup to the main unit.

I'm with Boatman, I took the position from the GPS and put it on a paper chart to run a DR.
I started working in the computer industry in 1968 and learned early on never to completly trust someone else's program. Any computer small enough to carry was classed as a toy.

[Pelagic]

Originally Posted by nigel1

Hi Don,

Best way to check the compass is to find an accurate navigation transit line, put the boat on the line and have the bow pointing directly along the transit, then note and compare the magnetic and fluxgate compass headings. A less accurate method is to park the boat along a pier, note the run of the pier from chart, get boat parallel to pier and check heading (obviously a wooden pier )
I just recently calibrated my fluxgate compass and set it to True, having set the heading on a transit, and then swung the boat to calibrate the compass. It made a huge difference, as far as I can tell, the fluxgate gives a proper true reading on all headings, and the difference between the fluxgate and the mag. compass is equal to variation

Don…. Nigel has pretty much nailed it on the professional way to manage your heading sensors.

There are many advantages in an emergency or in fog or in troubleshooting extreme deviation to being able to provide others with your True heading.

If you set up your electronic hdg sensors to always show true and prove them with your chart as Nigel suggest, then you can also log the accuracy of your magnetic compass using TVMDC…aw and CDMVT…ae

If you were ever in a situation where GPS was out and you had to DR …Then if True Hdgs were your default piloting practice, you can navigate with more confidence knowing the errors of the MAG compass at every hdg

Also there are a great many advantages to using North up radar presentations, especially in the fog when in high traffic density and constrained waters

Your first post indicated a certain naïveté when you compared COG to compass bearings so you should understand that we needed to identify your experience first before we could offer help.

Never too late to teach an old (sea) dog new tricks!...myself included…

[bewitched]

Isn't there a significant point being missed here?

Neither the binacle compass nor the fluxgate compass (whether displaying true or magnetic) actually tell you where the boat is going. They only tell you which.way the bow is pointing. The HEADING It's fairly irrelevant information.

You.could do the TVMDC thing and then allow for current and leeway to get the COURSE

But i think most would look at the COG from the gps to figure out how the boat is moving relative to their destination.

The binacle compass to me has become little more than a backup should the fluxgate fail. The only time I look at it is for a quick and very rough reference for gybe angles, bearings etc.

I can't remember the last time I actually worked out which direction the boat was moving in using the binacle compass. Does anyone with a gps actually do this any more?