Map, Compass & GPS

Map, Compass & GPS
Wild flowers along Fall Creek on the way to the Green Lakes - Oregon

Wednesday, April 18, 2012

Working With Degrees

Keeping my navigation simple is my goal. Backcountry navigators have the option to set their compass to work in degrees magnetic (in relation to magnetic north) or degrees true (in relation to the North Pole.)  I find that working in degrees true is simple and straight forward.

I spent many long hours on the bridge of destroyers while serving at sea in the Navy.  At the start of a bridge watch I carefully checked the navigation picture with a solid review of the chart (a mariners map), the compass, celestial observations reported and electronic navigation systems.

Compass and directional data presented to me was in degrees true.

It was natural then, to apply degrees true to my backcountry navigation.  It’s just so simple.

A hiker’s magnetic compass provides direction in terms of degrees measured from the earth’s magnetic north pole.  The red magnetic needles swings and points to magnetic north. 

Declination must be accounted for with any magnetic compass.  Declination is the distortion of the earth’s magnetic field caused by the movement of molten metals deep inside the earth.  (For more information, read my post on declination HERE.)

When working with map and compass the angular measurement of declination will either be added or subtracted to arrive at a true bearing.  Calculation is necessary because a topographic map is oriented to degrees true; compass and map must match.   

I found this process to be cumbersome.

Long ago I swapped out my basic compass for one that can be adjusted for declination.  The compass needle still points to magnetic north but the dial on the rotating compass housing provides data in degrees true; it has been adjusted. I can pre-set the declination adjustment at home before starting my trip.  I can find the declination information at the bottom of my USGS map, or, if the map is old, I’ll visit to get the current value.

Now my compass is matched to the map. 

The next step is to align the Global Positioning receiver (GPS) to match the map and compass.  I’ll due this via the “set up” menu.

Now all my navigation equipment matches. 

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