declination.

A simple compass adjustment or modification is necessary to correct

for magnetic declination. In areas west of the line of zero declination, the
magnetie needle points somewhere to the east (to the right) of true north (fig. 4-
9), so these areas are said to have east declination. It works just the opposite on
the other side of the line of zero declination. Here, the magnetic needle points
somewhere to the west (left) of true north, so these areas have west declination.
Consider a mountain traveler in Colorado, where the declination is 14 degrees
east. The true bearing is a measurement of the angle between the line to true
north and the direction-of-travel line. The magnetic needle, however, is pulled
toward magnetic north, not true north. So instead it measures the angle between
the line to magnetic north and the direction-of-travel line. This "magnetic
bearing" is 14 degrees less than the true bearing. To get the true bearing, you
must add 14 degrees to the magnetic bearing. Like those in Colorado, climbers in
all areas west of zero declination must add the declination to the magnetic
bearing. In central California, for example, about 18 degrees must be added. In
Washington State, it is about 20 degrees. East ofthc zero-declination line, the
declination is subtracted from the magnetic bearing. In Maine, for example, the
magnetie bearing is 20 degrees greater than the true bearing. Subtracting the
declination of 20 degrees gives a wilderness traveler in Maine the true bearing.
This is all very simple in theory but can be confusing in practice, and the
wilderness is no place to practice mental arithmetic that can have life-and-death
consequences. A more practical way to handle the minor complication of
declination is to pay somcwhat more for your compass and get onc with an
adjustable declination arrow instead of a fixed orienting arrow. The declination
arrow can be easily set for any declination. Then the bearing you read at the
index line will automatically be the true bearing, and concern about a declination
error is one worry you can leave at home. On compasses without adjustable
declination ar- THE COMPASS • 69 10' West \ ,, , , , 40 West L: '5 z '" % 20'
<-:-...__• I / , , 140 East ',........ \; 'X 10° East,' '-. , I -_._--------- . , --- 180 East 20'
East Fig. 4-9. Relationship betweelltrue and magnetic north in the United State,\'
rows, you can get the same effect by sticking a thin strip of tape to the top or
boltom of the rotating housing to serve as a customized declination arrow. Trim
the tape to a point, with the point aimed directly at the specific declination for
the area where you will be climbing. [n Colorado, your taped declination arrow
must point at 14 degrees east (clockwise) from the 0 point (marked N for north)
on the rotating compass dial (fig. 4-IOa). In Maine, the declination arrow must
point at 20 degrees west (counterclockwise) from the 0 point on the dial (fig. 4-
IOb). In Washiogton State, the declination arrow must point at 20 degrees cast
(clockwise) from the 0 point. To take or to follow a bearing in the field, follow
exactly the same procedure used in the earlier examples from Ohio, where the
declination is zero. The only difference is that from now on, you will align the
magnetic needle with the declination arrow instead of with the orienting arrow.
From here on in this chapter. it is assumed you are using a compass with a
declination arroweither an adjustable arrOw or a taped arrow that you have
added. For all bearings in the field, you will align the needle with this declination
arrow. Unless otherwise stated, all bearings referred to are true bearings. not
magnetic. PRACTICING WITH THE COMPASS Before you count on your compass
skills in the wilderness, test them in the city. The best place to practice is a place
where you already know all the answers. like a street intersection where the
roads run north-south and east-west. Take a bearing in a direction you know to
be east. When you have lined up the declination arrow with the magnetic needle,
the number at the index line should be 90 degrees, or within a few degrees of
90. Repeat for the other cardinal directions, south, west, and north. Then do the
reverse. Pretend you don't know which way is west. Set 270 degrees (west) at
the index line and hold the compass io froot of you as you turn your entire body
until the needle is again aligned with the declination arrow. The direction-of-
travel line should now point west. Does it? Repeat for thc other cardinal
directions. This set of exercises will help develop precision and self-confidence at
compass reading and also is a way to check the accuracy of the compass. And if
you make a mistake or two, well, no hann done. Look for chances to practice in
the mountains. A good place is any known location-such as a summit or a lake
shore-from which you can see identitlable landmarks. Take bearings as time
permits, plot them on the map. and see how close the result is to your actual
location. CAUTIONS ABOUT COMPASS USE As you've gathered by now, there's a
big difference betwecn using a compass for working with a map and using a
compass for field work. When measuring and plotting bearings on a map. the
compass needle is ignored. Just align the meridian lines on the compass housing
with the north-south lines on the map. In the field, you must use the magnetic
needle. You may have heard that metal can mess up a compass reading. It's true.
Fcrrous objects-iron, steel, and other materials with magnetic properties-will
deflect the magnctic needle and produce false readings, as will a battery-
powered watch that is within 6 inches of a compass. Keep the compass away
from belt buckles, ice axes, and other metal objccts. If a compass reading
doesn't seem to make sense, see if it's being sabotaged by nearby metal. Keep
your wits about you when pointing the declination arrow and the direction-of-
travel line. If either is pointed backward-an easy thing to do--the reading will be
180 degrees off. If the bearing is north, the compass will say it's