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» Online Classroom   »   » Public Discussion of Inland and Coastal Navigation   » Magnetic Variation

   
Author Topic: Magnetic Variation
M. C. Rowley


 - posted August 07, 2007 06:16 AM      Profile for M. C. Rowley           Edit/Delete Post 
When determining whether or not to add or subtract magnetic variation (declination), I have always found the following memory aid useful:

For easterly variations:

Map to Compass = Subtract easterly variation
[Compass ends in "S" = "Subtract"

Compass to Map = Add easterly variation
[Map ends in both "A" = "Add" and "P" = "Plus"

For example, you want to take a 030 map bearing and convert it to a compass bearing. The map shows a 12E variation.

Using "Map to Compass" (subtract the variation)
030T - 12E = 018M

Next, take a 230 compass bearing and plot it on your map with a 12E variation.

Using "Compass to Map" (add the variation)
230M + 12E = 242T

For westerly variations:

Use the same memory aid, however, make the opposite correction. In other words, if you want to take a 030 ture map bearing and convert it to a compass magnetic bearing you would simply remember the first memory aid: "Map to Compass = subtract for easterly variations and do the just the opposite (add) for westerly variations.

This memory aid has worked for years for me with both land and aeronautical navigation and applies just as well to these navigation problems. Hope this helps...............

David Burch


 - posted August 07, 2007 09:33 AM      Profile for David Burch           Edit/Delete Post 
Thanks for that. The key is once you get something that works, stick with it.

I have always used the "Correcting add east," which i have to admit is not much of a jingle, but now i have to stick with it for that reason.

Many people have found that the longer but more general prescription of TVMDC is the best bet in the long run. (TV Makes Dull Companions... or going the other way, Can Dead Men Vote Twice) Both of these methods are in the course book.

It is a matter of personal preference. Now those new to the subject have another tested method. thanks again.

From: Starpath, Seattle, WA


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