Understanding Magnetic Declination
By Blake Miller
Declination is a term that causes “brain cramps” for many of my students in my map and compass classes. When I mention Magnetic Declination eyes roll.
The web site www.magnetic-declination.com has an excellent discussion of what declination is and what causes it:
“Magnetic declination varies both from place to place, and with the passage of time. As a traveler cruises the east coast of the United States, for example, the declination varies from 20 degrees west (in Maine) to zero (in Florida), to 10 degrees east (in Texas), meaning a compass adjusted at the beginning of the journey would have a true north error of over 30 degrees if not adjusted for the changing declination. The magnetic declination in a given area will change slowly over time, possibly as much as 2-25 degrees every hundred years or so, depending upon how far from the magnetic poles it is. Complex fluid motion in the outer core of the Earth (the molten metallic region that lies from 2800 to 5000 km below the Earth's surface) causes the magnetic field to change slowly with time. This change is known as secular variation. Because of secular variation, declination values shown on old topographic, marine and aeronautical charts need to be updated if they are to be used without large errors. Unfortunately, the annual change corrections given on most of these maps cannot be applied reliably if the maps are more than a few years old since the secular variation also changes with time in an unpredictable manner.”
Much of land navigation is based on the relationship to the North Pole; also known as “true north. The measure of degrees of direction in relation to true north is called “degrees true.” Maps are laid out in degrees true. Land features (buttes, mountains, streams) on a topographic map are in reference to degrees true. By that I mean the bearing from one mountain peak to another will be referenced in degrees true. The map below illustrates that point.
Magnetic compasses do not point to true north (the North Pole); the magnetic needle points to an area that could be considered the magnetic North Pole.
As illustrated below, declination data can be found in the diagram at the bottom of a USGS topographic map, (on some commercially produced maps it can be hard to find.)
Because declination changes over time, I recommend that map declination information be verified at www.magnetic-declination.com. This is essential in the Pacific Northwest where maps are notoriously out of date in terms of road, city and some trail data.
So, how do we make this simple? How do we convert magnetic to degrees true?
I could do the math. In Oregon, where I live, the magnetic declination is 15.6° East declination:
· Before heading out, I verify the declination of the trailhead at www.magnetic-declination.com.
· To go from true (from the map) to a magnetic heading (the direction of travel) – subtract declination from the true direction. For example, the direction of travel from Bachelor Butte to Limpy Mountain (map above) is 185°T (185° degrees true.) My magnetic heading will be:
o 185° - 15.6° (east declination) = 169.4°M (169.4 degrees magnetic)
o The catch phrase to remember is: declination east, compass least; a reminder to subtract easterly declination.
· To go from a magnetic heading to determine true compass course add the declination.
My recommendation: make it simple and have the compass do the work so that there is no confusion with the math.
To do this, I need to choose a compass that can be adjusted for declination. Some examples are the Silva Ranger, the Suunto M3 or the Brunton 8010G. The Brunton is the most affordable and is priced $15-$18.
With one of these compasses, the compass dial or housing is adjusted and rotated manually. Both the Suunto and Silva Ranger come with a small, flat adjusting tool. The Brunton requires no tool and is adjusted by rotating the outer dial around the cylinder that houses the magnetic needle.
Consult with owner’s manual that came with the compass.
Caution: Don’t be surprised if the store clerk doesn’t have a clue about what an adjustable compass is. Some compasses will have a declination grid or information but this is not the same thing.
To compensate for declination I’ll either manually or mechanically (e.g., the Silva Ranger with the adjusting tool) adjust the compass:
· If declination is Easterly (Western U.S.) I will rotate the dial causing the baseplate’s orienting arrow to move in a clockwise direction.
· If declination is Westerly (Eastern U.S.) I will rotate the dial causing the baseplate’s orienting arrow to move in a counter-clockwise direction.
The picture below shows a compass adjusted for 16° Easterly declination.
Notice that that the red orienting arrow (engraved on the baseplate) has been adjusted to 16° Degrees to the east; clockwise.
Now, adjust the dial and align the red magnetic needle on top of the orienting arrow (the red arrow engraved on the baseplate) the compass will provide directions in degrees true.
For more information check the following references: