True North, Magnetic North, Compasses, and GPS

Adjustable Declination Compasses

A compass with adjustable declination allows you to rotate the orienting arrow independently of the compass dial.  If you have such a compass, you may calculate your map bearing without adding or subtracting the amount of local magnetic declination.  To calibrate your compass in this fashion, rotate the inner liquid capsule (or turn the screw with the key) until the orienting arrow deviates from the compass ring’s north indicator by the amount & direction of the local magnetic declination.  For example, if local declination is 10 degrees east of true north, rotate the inner liquid capsule (or turn the screw with the key) until the orienting arrow points to 10 degrees east. 

How Should I Set My GPS and Compass?

Let’s assume the local declination is 10 degrees east of true north, and the waypoint you wish to navigate to is directly north of your current location.

If using a compass without adjustable declination (the orienting arrow cannot be adjusted – it always points to North on the compass dial), set your GPS to magnetic north.  Your GPS will indicate a bearing of 350 degrees must be followed to reach the waypoint.  Dial 350 degrees at the index line on your compass.  With the direction-of-travel arrow pointed directly away from you, turn your body & compass in one motion until the red magnetic needle overlays the orienting arrow.  Site a landmark along that bearing, and proceed.  You will be traveling directly north toward the waypoint.

If using a compass with adjustable declination, set your GPS to true north.  Your GPS will indicate a bearing of zero degrees must be followed to reach the waypoint.  Adjust the declination on your compass so the orienting arrow points to 10 degrees east.  Dial zero degrees at the index line on your compass.  With the direction-of-travel arrow pointed directly away from you, turn your body & compass in one motion until the red magnetic needle overlays the orienting arrow.  Site a landmark along that bearing, and proceed.  You will be traveling directly north toward the waypoint

SETTING THE CORRECT DATUM AND COORDINATE SYSTEM:

GPS receivers internally store coordinates in the “Earth-Centered, Earth-Fixed (ECEF) Coordinate System”.  These coordinates never appear on your screen, but receivers use this system because it permits fast, accurate distance and direction calculation.  When automatically capturing waypoints in the field, your GPS can be set to any datum and coordinate system.  When recalling those coordinates from memory, the receiver will convert them from ECEF to the datum and coordinate system you specify.  When manually entering waypoints you’ve calculated from a map, however, make sure your GPS is setup to match the map’s datum.

TIPS FOR USING YOUR GPS RECEIVER:

• KEEP YOUR UNIT IN 3D MODE:  some models are locked into 3D and do not permit you to toggle between 3D and 2D.

• RE-INITIALIZE YOUR GPS IF YOU’VE TRAVELED SEVERAL HUNDRED MILES SINCE LAST USING IT:  this will make acquiring a position fix faster.  Some models automatically perform this function.

• ALWAYS VERIFY YOU’RE USING THE CORRECT MAP DATUM, ELEVATION MODE, NORTH INDICATOR, AND COORDINATE SYSTEM.  Some models will default to the manufacturer’s specifications when the unit is turned off.

• KEEP THE ANTENNA POINTED SKYWARD FOR BEST RECEPTION.

• ALWAYS VERIFY YOU’RE RECEIVING ENOUGH SIGNALS (via the satellite status screen) BEFORE MAKING CRITICAL NAVIGATION DECISIONS.

• ALWAYS CARRY SPARE BATTERIES.

• PRACTICE YOUR GPS SKILLS IN SAFE, FAMILIAR SURROUNDINGS BEFORE GOING INTO THE WILDERNESS.

• DON’T USE THE BACKLIGHT FUNCTION UNTIL NECESSARY – IT WILL DRAIN YOUR BATTERIES.

• YOU MAY SET YOUR GPS TO THE UTM COORDINATE SYSTEM & ALSO SET THE NAVIGATIONAL UNITS TO FEET/MILES.  This will allow you to use the metric-based UTM coordinate system, but display navigational distances in feet/miles.

• BEFORE GOING INTO THE WILDERNESS, IT’S A GOOD IDEA TO TAKE A GPS WAYPOINT AT YOUR CAR.  This way, you’ll always know the straight-line distance and direction to safety.

• GIVE YOUR GPS WAYPOINTS INTUITIVE NAMES.  For example, if hiking in Carson National Forest, name them CNF1, CNF2, CNF3, etc.

• GPS VOCABULARY & ABBREVIATIONS:

Bearing:  This is the direction your GPS wants you to follow

Heading:  this is the direction you are currently following

XTE:  cross track error; this is the receiver’s estimate of the

        distance and direction your heading is deviating from

        the intended bearing

TRUE NORTH:  bearing is adjusted for local magnetic

        declination

MAGNETIC NORTH:  bearing is NOT adjusted for local

        magnetic declination