The compass and New York's declination

The three norths

• True north, the geographic pole. What maps are drawn to
• Magnetic north, where the needle points. It moves year by year
• Grid north, the top of the map's grid. Close enough to true for guiding

Declination is the angle between true and magnetic north at your location. In New York it is always west, running from roughly 10 degrees along Lake Erie to about 14 degrees in the northeastern Adirondacks, and drifting slowly. The declination diagram in the map margin gives the value for the year the map was made, so check a current source for anything precise.

The conversion, both ways

The mnemonic is a common source of error because it only describes one direction of travel. Hold on to the physical fact instead: in New York the needle points west of true north, so any magnetic bearing reads higher than the true bearing of the same line. If your converted number went the wrong way, that sentence catches it.

Worked example. Map bearing 072° true, declination 13°W. Set 072 + 13 = 085° magnetic on the compass and walk it. Coming the other way: compass reads 214° magnetic, so 214 − 13 = 201° true to plot.

A compass with an adjustable declination screw removes the arithmetic entirely: set it once for the region and read true bearings directly. It also removes your ability to notice when someone else's compass is set differently, so know the arithmetic anyway.

Taking a bearing off the map

1. Lay the compass edge along the line from where you are to where you are going, direction-of-travel arrow pointing the way you want to go
2. Turn the housing until the orienting lines are parallel with the map's north-south lines, north on the housing to map north. The needle is irrelevant during this step
3. Read the bearing at the index. That is a true bearing
4. Add the west declination to get the magnetic bearing to walk

Walking it

1. Hold the compass level, at your waist, square to your body
2. Turn your whole body until the needle sits inside the orienting arrow, "red in the shed"
3. Look up along the direction-of-travel arrow and pick a feature on the line: a distinctive tree, a boulder, a snag
4. Walk to it without watching the compass, then repeat

Sighting on features rather than staring at the needle is what keeps a line straight. In dense cover or fog with no features, send a party member ahead and direct them left or right onto the line, then walk to them.

Back bearing

To reverse a bearing: under 180, add 180; over 180, subtract 180. Shooting a back bearing to where you came from is a cheap way to check you are still on line, and it is how you find your way back to a car you cannot see.