Author

Topic: Inland Coastal Navigation Problem#26

Ted

posted December 06, 2007 05:26 PM
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
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
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 counterclockwise (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.


