Magnetic Compass

[NGS Surveyor]

Magnetic Compass Corrections

 

Recently there have been several discussions about compass corrections.

 

Years ago I learned a couple of rhymes that make it easier to correct for the differences between true north and the direction a compass reads. One rhyme is for converting from compass to true (#1) and the other is for converting from true to compass (#2). Here are the rhymes (read down the column):

 

RHYME#1 STANDS FOR MEANING

Can Compass Compass reading

Dead Deviation Angle between compass and magnetic field*

Men Magnetic Actual direction of the local magnetic field

Vote Variation Angle between magnetic and true**

Twice True Geographic north

At Add (as in arithmetic)

Elections? East Direction of correction

 

* Difference due to local magnetic disturbances, such as the steel hull of a ship

** Difference due to the fact that the geographic pole and the magnetic pole are not at the same location.

 

RHYME#2 STANDS FOR

Timid True

Virgins Variation

Make Magnetic

Dull Deviation

Companions Compass

At Add

Weddings West

 

On a ship the Deviation is found by calibrating the compass using a method called “swinging the ship,” because the ship is swung around and calibrated at different headings. The Variation is found on maps and charts, and at the site: http://www.ngdc.noaa.gov/geomag/declination.shtml . This www site computes the variation for a particular location when the latitude and longitude (or zip code), and date are entered. Note the Variation changes with time and with location. Magnetic variation is sometimes called magnetic declination.

 

When using a hand-held compass, you can usually consider the Deviation to be zero (as long as you stay away from cars or other iron or steel objects), and just correct for the Variation.

 

GeorgeL

NGS

[+Black Dog Trackers]

Yikes !

Excellent rhymes, George L !

 

Let me try that.....

 

		STANDS FOR   MEANING
Can		 Compass	  Compass reading
Dead		Deviation	Angle between compass and magnetic field* 
Men		 Magnetic	 Actual direction of the local magnetic field
Vote		Variation	Angle between magnetic and true**
Twice	   True		 Geographic north
At		  Add		  (as in arithmetic)
Elections?  East		 Direction of correction

 

		STANDS FOR
Timid	   True 
Virgins	 Variation 
Make		Magnetic 
Dull		Deviation
Companions  Compass
At		  Add
Weddings	West

[+GEO*Trailblazer 1]

Yikes !

Excellent rhymes, George L !

 

Let me try that.....

 

		STANDS FOR   MEANING
Can		 Compass	  Compass reading
Dead		Deviation	Angle between compass and magnetic field* 
Men		 Magnetic	 Actual direction of the local magnetic field
Vote		Variation	Angle between magnetic and true**
Twice	   True		 Geographic north
At		  Add		  (as in arithmetic)
Elections?  East		 Direction of correction

 

		STANDS FOR
Timid	   True 
Virgins	 Variation 
Make		Magnetic 
Dull		Deviation
Companions  Compass
At		  Add
Weddings	West

 

Now both make good sense.

[NGS Surveyor]

Black Dog Trackers, Thanks for putting the "tabs" back in my document.

 

GeorgeL

NGS

[Papa-Bear-NYC]

Here in the northeast (New England and New York) where the declination is West and one of the best known outdoors organizations (hiking, huts, campgrounds, etc.) is the Appalachian Mountain Club, commonly known as the AMC, we have:

 

A			 Add
M			 Map (for)
C			 Compass

Edited October 29, 2008 by Papa-Bear-NYC

[+Black Dog Trackers]

Fun game !

 

True   True
To	 to
Meg	Magnetic
All	add
The	the
While  West


Meg	Magnetic
The	to
True   True
At	 add
The	the
Feast  East

Who is Meg (Mag)?

The one you're attracted to !

[tosborn]

When going from true to magnetic (as in going from geodetic azimuths on a datasheet to a non-declination-adjustable compass) "EAST IS LEAST, WEST IS BEST." Meaning subtract Easterly declinations and add Westerly declinations. When going from magnetic to true, just do the reverse.

[+Steve838]

The "True Virgins....." and the "East is Least....." rhymes take me back to my days at USAF Navigator Training in the pre-GPS days of real navigating. Thanks for the memories.