# Magnetic compass to true course

M sure it’s something I’m just overlooking but how do you convert a psc to true if you aren’t given the deviation ?

You can’t.

Unless you just pick a number. But then that would be False, not True.

So when your doing a chart plot and the test says your steering 272 mag do you just plot it out from the center of the rose ?

you must be missing something, they have to give you a bit more info, it would be easier to understand if you posted the entire question
C D M V T

[QUOTE=xavierts29;56269]So when your doing a chart plot and the test says your steering 272 mag do you just plot it out from the center of the rose ?[/QUOTE]

Yes, you can either use the magnetic part of the compass rose or you can use the variation and convert to true. You only need the MVT part of the CDMVT to convert between mag and true. (or just stay in mag)

You don’t need deviation unless you’re looking for compass course you don’t need that just to lay down a track-line.

You are asking two seperate questions. Write the question down word for word. Psc and Mag are two different things.

This is the tough part of a USCG exam. Actually reading and understanding what they are asking for. When I started I used to hear about all the ‘trick’ questions on the exams. This is not true. If you practice and know the material it is not tricky or mysterious. The only drawback is that time let’s you forget some of the stuff.

I like it the other way around, because true virgins make dull companions

[QUOTE=Kennebec Captain;56273]Yes, you can either use the magnetic part of the compass rose or you can use the variation and convert to true. You only need the MVT part of the CDMVT to convert between mag and true. (or just stay in mag)

You don’t need deviation unless you’re looking for compass course you don’t need that just to lay down a track-line.[/QUOTE]

No offense. That would just be a guess and not right. What he is missing is in the first part of the plot. I am sure there is a deviation table in the question, he needs to learn how to go there and determine the deviation for his heading. It also depends on where he is getting his questions. Some software inadvertently leaves the deviation table off. To go from PSC to True or vice versa you need deviation and variation.

[QUOTE=Diesel;56281]No offense. That would just be a guess and not right. What he is missing is in the first part of the plot. I am sure there is a deviation table in the question, he needs to learn how to go there and determine the deviation for his heading. It also depends on where he is getting his questions. Some software inadvertently leaves the deviation table off. To go from PSC to True or vice versa you need deviation and variation.[/QUOTE]

Yes, could be, the question is “steering a magnetic course”, is that a magnetic heading or a compass course? If it is a compass course he needs a deviation, if it is a magnetic heading he doesn’t. I assumed that if deviation is not given he must mean magnetic heading.

K.C.

Not True! You could use the sun, the almanac, and bowditch, but that opens a whole other can of worms…

Posted just to be a smart ass.

The fact that you are looking at the question and going WTF is a good sign, you know something’s not right.

I see your point. I am trying to figure out his question and sounds like he is doing a plot and needs to go from true to psc or vice versa, as in he needs course to steer, either gyro or psc.

[QUOTE=Kennebec Captain;56285]Yes, could be, the question is “steering a magnetic course”, is that a magnetic heading or a compass course? If it is a compass course he needs a deviation, if it is a magnetic heading he doesn’t. I assumed that if deviation is not given he must mean magnetic heading.

K.C.[/QUOTE]

Correct Jem…but aren’t those just ways of determining deviation! Posted just to be a smartass!

[QUOTE=Jemplayer;56286]Not True! You could use the sun, the almanac, and bowditch, but that opens a whole other can of worms…

Posted just to be a smart ass.

The fact that you are looking at the question and going WTF is a good sign, you know something’s not right.[/QUOTE]

Except he used two different terms and he hasn’t replied to clarify. At first he said psc but then he said magnetic. If the question days you are steering 253° magnetic there is no need for deviation. It gave you your magnetic course.

Deviation & Variation +/- Magnet Course = True course.
Been sailing with magnets only for years… One thing all you need to remember ; city = CTE

“C” is compared compass(magnetic), and that is left side of True ; smaller number than True Course(degree) ; Compass error is easterly.

Compass and magnetic are two different things. Magnetic is your compass corrected for deviation. If the question gives you magnetic you do not need deviation.

Example:

1. “you are steering 227 psc” you need deviation because that is a compass heading.

2. “you are on course 169 magnetic” you do not need deviation because they already gave you the magnetic course.

He said he didn’t have deviation and those will find it.

You really solve for true as your giving the bearing in PSC, but all the CG questions have you find deviation from it.

Heres The Whole Question And the heading at the top of the test.

#1 - USCG 16306
The following questions are based on chart 12354TR, Long Island Sound - Eastern Part, and the supporting publications.
Your vessel has a draft of 10 feet (3.1 meters). Your height of eye is 35 feet (10.6 meters). Use 14° W variation where required.
The deviation table is:
HDG. MAG. DEV. HDG. MAG. DEV. HDG. MAG. DEV.
000° 0° 120° 2° W 240° 3° E
030° 1° W 150° 1° W 270° 3° E
060° 2° W 180° 1° E 300° 2° E
090° 4° W 210° 2° E 330° 1° E

#10 - USCG 16315
At 0820, you change course to 301° psc and reduce speed to 7.5 knots. At 0900, you take the following visual bearings:
Branford Reef Light 023° psc
New Haven Light 293° psc
Tweed Airport Aerobeacon 332° psc
See Diagram: D16315NP
A. LAT 41° 11.9’ N, LONG 72° 50.6’ W
B. LAT 41° 11.9’ N, LONG 72° 49.5’ W
C. LAT 41° 12.1’ N, LONG 72° 48.6’ W
D. LAT 41° 12.5’ N, LONG 72° 44.3’ W

Now Heres the thing I would like to know do you need to convert that to true? If so How? If not What is the way you plot it out as is?

You can convert it to true or magnetic depending on preference. I find true easier to plot because I use the scale on the triangle against a longitude line.

It gives you deviation and variation so you can easily convert to true if you want. You cannot just plot the psc course, you need to convert to something.

The answer key was probably made by converting to true based on 14° var instead of plotting by mens of the magnetic compass rose so I would use true.

True, but at one point he says the course given was magnetic, in which case you do not need deviation. My point was that it depends on how the question was worded and he did not give us enough information. He has rectified that deficiency and it turns out he is given the deviation.