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» Online Classroom   »   » Public Discussion of Inland and Coastal Navigation   » Inland Coastal Navigation Problem#2-6

   
Author Topic: Inland Coastal Navigation Problem#2-6
Ted


 - posted December 06, 2007 05:26 PM      Profile for Ted           Edit/Delete Post 
This problem asks for the true course you are steering if the compass reads(A) 335C, (B) 032C, (C) 317C, and (D) 285C.
The answer book does not give the true courses as called for, but only gives the deviations for each problem. I have no problem working out the deviations, but I am not confident that I have worked out the correct true courses using:
T=M+12 and M=C+/-dev
Please provide me with the true course answers.
It is not clear whether I use + or - in each case. I prefer to draw a 360° circle and put in each course/heading so I can visualize the entire problem from basic principles, but I am getting confused for some reason. Can you help me think it through so I can always draw in the deviation into the 360° diagram correctly...I need to see it so I can confidently know I have it right. Thank you very much for your help.

From: Yukon
David Burch


 - posted December 06, 2007 08:39 PM      Profile for David Burch           Edit/Delete Post 
Please check to see if these look right

code:
		A	B	C	D	
TRUE 348.0 061.2 323.0 286.4 Twice
Variation 12 E 12 E 12 E 12 E Vote
MAGNETIC 336.0 049.2 311.0 274.4 Men
Deviation 1.0 E 17.2E 6.0 W 10.6 W Dead
COMAPSS 335.0 032.0 317.0 285.0 Can

Plus East going up, Plus West going down

Here are all the examples:

Magnetic = Compass + dev East
Magnetic = Compass - dev West

Compass = Magentic - dev East
Compass = Magnetic + dev West

True = Magnetic + var East
True = Magnetic - var west

Magnetic = True - var East
Magnetic = True + var West

We use the rule "correcting add east" and we have to remember that what we mean by "correcting" is converting compass to magnetic or converting magentic to true.

then if you reverse anything, you reverse the sign.

with all the examples above you can figure out what works best for you. Everyone has their favorite way to solve these.

From: Starpath, Seattle, WA
HHEW


 - posted December 07, 2007 08:10 AM      Profile for HHEW           Edit/Delete Post 
In visualizing, it sometimes helps to see that E or W changes mean the magnetic compass card as being skewed or twisted by Variation or Deviation. E designates a turn of the card clockwise (i.e., toward the E on the card). So, a twist of the card clockwise will show you a lower number on the lubberline. Example, 360* (N) showing on the lubber, variation 10* East, 350* is at the lubber.

Conversely, a twist of the card counter-clockwise (Variation or Deviation W) of 10* and 010* is at the lubber.

In working the other way, from a given magnetic course (number showing at the lubber) to determine the True course, it's necessary to visualize the card as having already been turned.
Thus, steering 350* with a Variation of 10* E, you know the card is skewed 10* clockwise, so you have to mentally turn it counterclockwise 10* to see the True course is 360* (N).

Years ago someone made a device with three compass cards mounted atop one another on a common center pin. One card was True, one was for Variation and the third for deviation. This let you literally do what I've described as a mental exercise - twist the compass roses relative to one another to put in Variation and Deviation.



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