True or Compas

[Stu Jackson]

Originally Posted by Ex-Calif

If you are plotting a course long enough to change variation zones plot true.

Otherwise plot magnetic and save a step.

Simple, useful and helpful. Actually saves two steps: there and back again. Coastal sailing makes no sense to plot true. On charts that cover a lot of ground it's dangerous to use magnetic. I keep forgetting large vs. small scale charts.

The other thing is that it's all relative. You can't steer when sailing in as fine a line as you draw on a chart, and no one yet has mentioned leeway and currents. Plus, one should update regularly anyway.

[bill352]

FWIW, the ASA Coastal Navigation Course (105) says to use magnetic.

[FamilyVan]

Originally Posted by Stu Jackson

Simple, useful and helpful. Actually saves two steps: there and back again. Coastal sailing makes no sense to plot true. On charts that cover a lot of ground it's dangerous to use magnetic. I keep forgetting large vs. small scale charts.

The other thing is that it's all relative. You can't steer when sailing in as fine a line as you draw on a chart, and no one yet has mentioned leeway and currents. Plus, one should update regularly anyway.

So, if it's this do that if it's that do this. What's the dividing line. What's the threshold when you switch, who determines which you're using. One watch plots one way and watch two plots another way?

You're sinking and making a bee line for the nearest refuge, you quote your heading to CG- they're professional mariners and therefor every last one of them is trained to and does think in True, because its the standard. A course of 151 degrees to them means 151 degrees True, not 151 magnetic / 176 True.

If you calculate in magnetic and you join a crew of professionals for whatever reason- they will insist on you using True or take your chart privileges away. So why mess around? Stick with the standard methodology that works with algebraically correct figures and not eyeball estimates?

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[Stu Jackson]

Originally Posted by bill352

FWIW, the ASA Coastal Navigation Course (105) says to use magnetic.

Note COASTAL. Within the same variation. Coastal = relatively short distances, with day-hops. NOT across oceans. Most likley recreational sailors.

Across oceans = true

Originally Posted by FamilyVan

So, if it's this do that if it's that do this. What's the dividing line. What's the threshold when you switch, who determines which you're using. One watch plots one way and watch two plots another way?

You're sinking and making a bee line for the nearest refuge, you quote your heading to CG- they're professional mariners and therefor every last one of them is trained to and does think in True, because its the standard. A course of 151 degrees to them means 151 degrees True, not 151 magnetic / 176 True.

If you calculate in magnetic and you join a crew of professionals for whatever reason- they will insist on you using True or take your chart privileges away. So why mess around? Stick with the standard methodology that works with algebraically correct figures and not eyeball estimates?

You get on the boat, discuss it, decide what is proper for the CIRCUMSTANCES.

I go to Drakes Bay from SF = magnetic. 34 miles. Big deal.

I go to Hawaii from SF = true.

What's so hard?

Communication is the key to resolving what is not a dilemma.

Besides, every navigation text I've ever read says to note T or M on the course line, so even if someone gets mixed up it should be recorded.

The ONLY difference is which of the compass rose scales one uses.

They all should get to the same position on the chart, right?

This either/or discussion is like herding cats.

[FamilyVan]

Originally Posted by Stu Jackson

Note COASTAL. Within the same variation. Coastal = relatively short distances, with day-hops. NOT across oceans. Most likley recreational sailors.

Across oceans = true




You get on the boat, discuss it, decide what is proper for the CIRCUMSTANCES.

I go to Drakes Bay from SF = magnetic. 34 miles. Big deal.

I go to Hawaii from SF = true.

What's so hard?

Communication is the key to resolving what is not a dilemma.

Besides, every navigation text I've ever read says to note T or M on the course line, so even if someone gets mixed up it should be recorded.

The ONLY difference is which of the compass rose scales one uses.

They all should get to the same position on the chart, right?

This either/or discussion is like herding cats.

Or you could eliminate the need for discussion and use the Standard? I'll bet there are more incidents caused by errors in communication then by errors in navigation.

The same reason there are codified methods for operating your VHF, or the reason we all follow the same ColRegs, you dont just make up meeting rules to suit you at the time.

There are enough hazards to navigation without every one using a different standard for chartwork.

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[Ex-Calif]

Originally Posted by FamilyVan

You're sinking and making a bee line for the nearest refuge, you quote your heading to CG- they're professional mariners and therefor every last one of them is trained to and does think in True, because its the standard. A course of 151 degrees to them means 151 degrees True, not 151 magnetic / 176 True.

I have not traveled to the far corners of the universe but where are you that you have 25 degrees of westerly variation?

Also when stating headings Ino one in my opinion is quoting true. Everyone has to be quoting magnetic because the instrument they are quoting is a compass.

Target range 5 km, bearing 094 degrees.

Is that true or magnetic skipper?

Get of my bridge idiot

[FamilyVan]

Originally Posted by Ex-Calif

I have not traveled to the far corners of the universe but where are you that you have 25 degrees of westerly variation?

Also when stating headings Ino one in my opinion is quoting true. Everyone has to be quoting magnetic because the instrument they are quoting is a compass.

Target range 5 km, bearing 094 degrees.

Is that true or magnetic skipper?

Get of my bridge idiot

The entire province of Newfoundland is over 20 degrees variation- that's considered a boating centre to some.

If you quoted "target range 5 km, bearing 094 degrees" to me I most certainly would not assume magnetic. I would assume that was a relative bearing- just abaft the starboard beam.

Why would they have to be quoting a magnetic compas? Why couldn't it be a FOG, a gyro, a GPS stabilised RADAR or ECS?


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[Ex-Calif]

Originally Posted by FamilyVan

The entire province of Newfoundland is over 20 degrees variation- that's considered a boating centre to some.

If you quoted "target range 5 km, bearing 094 degrees" to me I most certainly would not assume magnetic. I would assume that was a relative bearing- just abaft the starboard beam.

Why would they have to be quoting a magnetic compas? Why couldn't it be a FOG, a gyro, a GPS stabilised RADAR or ECS?


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Fair point but relative to the boat (as a bearing is), is still not true or magnetic, it's relative.

And if you are sailing only around newfoundland, and not changing variation zones you can still plan your passage using magnetic and the rose.

It's all about communication. When 3rd watch comes on deck and you say steer 275 true I think that is unrealistic.

[FamilyVan]

Originally Posted by Ex-Calif

Fair point but relative to the boat (as a bearing is), is still not true or magnetic, it's relative.

And if you are sailing only around newfoundland, and not changing variation zones you can still plan your passage using magnetic and the rose.

It's all about communication. When 3rd watch comes on deck and you say steer 275 true I think that is unrealistic.

But we're not talking about course to steer, we're talking about calculations made on the chart. I'm not sure how you're boat is laid at, but my chart table is down below. I make my calculations on a true north oriented chart, convert those calculations and then either go upstairs and say steer 055 (obviously compas if that's all that's in front of them) or shout it out. The OP asked about plotting, which os done at the chart table, you're talking about steering- which is done at the helm station.

When I'm setting sail, I don't need to remember to check the oil, because although both motoring and sailing get you to where you're going, the nitwits who built my boat forgot to put a dipstick on my Genoa.

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[Stu Jackson]

We're all saying the same thing, if you think about it.

[FamilyVan]

Originally Posted by Stu Jackson

We're all saying the same thing, if you think about it.

I'm beginning to think you might be correct. It's so difficult to plot a magnetic course on a True oriented chart, it may as well be impossible, and so difficult to steer a True course, on a magnetic compass, it may as well be impossible. I have a feeling we are arguing semantics and not recognising it. Ex-Cals reference to steering clued me in that we were talking about entirely different things- internets great- isn't it?

[Ex-Calif]

Originally Posted by FamilyVan

But we're not talking about course to steer, we're talking about calculations made on the chart. I'm not sure how you're boat is laid at, but my chart table is down below. I make my calculations on a true north oriented chart, convert those calculations and then either go upstairs and say steer 055 (obviously compas if that's all that's in front of them) or shout it out. The OP asked about plotting, which os done at the chart table, you're talking about steering- which is done at the helm station.

I am arguing none of this. "All" charts are true north oriented. The only spot you and I differ is that in plotting a course in a single variation zone you can save a step by deriving the mag course from the rose. If you wanna do one extra add and subtract - go ahead.

BTW - Why do you think they put a compass rose on the chart?

BTW - I have stated that to pas the tests and to demonstrate knowledge you do it all. But I am sailing along and I see a rocky little island. I look at the chart, I eyeball the compass rose and go, "That's about M037." I don't think, "That's T12 + W25Var, +003Dev, -leeway +current."

You might and peace to you brother...

to CG- they're professional mariners and therefor every last one of them is trained to and does think in True, because its the standard. A course of 151 degrees to them means 151 degrees True, not 151 magnetic / 176 True.

I am talking about this (When you changed the topic)

If I radioed the CG in distress - I would give position, heading and speed (among other things). Heading would be in magnetic not true because I would be reading it off my compass.

At 3 knots (boat speed) either way I am sure they would find me - LOL...

[barnakiel]

Well.

When you plot Mag then your reference is the N portion of the rose.

When you plot Tru then your reference is the N portion of any long/lat intersection.

As it happens, these are the same line.

Plotting Mag to the reference of the N part of long/lat intersection is an error.

And you are free to make your own errors in your own boat.

b.

[FamilyVan]

Originally Posted by Ex-Calif

I am arguing none of this. "All" charts are true north oriented. The only spot you and I differ is that in plotting a course in a single variation zone you can save a step by deriving the mag course from the rose. If you wanna do one extra add and subtract - go ahead.

BTW - Why do you think they put a compass rose on the chart?



I am talking about this (When you changed the topic)

If I radioed the CG in distress - I would give position, heading and speed. Haading would be in magnetic not true because I would be reading it off my compass.

At 3 knots either way I am sure they would find me - LOL...

I think the extra rose is to assist you in the conversion so you can complete your chartwork, and then relay the compass course to the helmsperson.

I'm pretty sure that- yes, you're correct they'll find you at 3 knots regardless, but I can guarantee they weren't looking for your compass course or your magnetic- because they would have had no way of knowing your datum. The only constant is true, they have no way of knowing if you corrected for deviation, if you remember to add the degree for your annual change in variation. They are pro-s they only work in true, their boats are equipped with Fibre Optic Gyros and DGPS- if they have a magnetic compass on board, they have to look up some ridiculous periscope to see it. They don't use that stuff in 2015.

They want you're actual position and your actual course, they don't care what house rules you've made up for your boat.

[Ex-Calif]

Originally Posted by FamilyVan

I'm beginning to think you might be correct. It's so difficult to plot a magnetic course on a True oriented chart, it may as well be impossible, and so difficult to steer a True course, on a magnetic compass, it may as well be impossible. I have a feeling we are arguing semantics and not recognising it. Ex-Cals reference to steering clued me in that we were talking about entirely different things- internets great- isn't it?

I am sure we would get along fine on a boat. Too many of these discussions become a challenge of "right" and "wrong" and you are right that they drift and pretty soone we forget what we are talking about.

You clearly know how to plot a course. So do I. Our conversation is one of clarification and really (partly) is so the lurkers can see that this can be complex.

It also demonstrates the importance of communication, specificity and clarity.

A lot of new folks ask why things on a boat are called something "ambiguous" like a halyard is not a rope. Well it's not ambiguous. A Halyard is a halyard. Pretty specific.

True is True. Magnetic is magnetic. Heading, Course, Bearing. Not ambiguous but specific. So it needs to be learned.