observer is determining the magnetic bearing of
the dotted line labeled Line of Sight. First, the
observer mounts the compass on a steady support,
levels it, and waits for the needle to stop
oscillating. Then, the observer carefully rotates
the compass until the north-south line on the card
lies exactly along the line whose bearing is being
taken.
The bearing is now indicated by the needle-
point. The needlepoint indicates a numerical value
of 40°. The card indicates the northeast quadrant.
The magnetic bearing is, therefore, N40°E.
Correcting for Local Magnetic Attraction
Figure 13-4 shows the compass needle lying
along the magnetic meridian. This means either
that the compass is in an area free of “local
magnetic attraction” or that the effect of local
attraction has been eliminated by adjusting the
compass card as described later. “Local magnetic
attraction” means the deflection of the compass
needle by a local magnetic force, such as that
created by nearby electrical equipment or by a
mass of metal, such as a bulldozer. When local
attraction exists and is not compensated for, the
bearing you get is a COMPASS bearing. A
compass bearing does not become a magnetic
bearing until it has been corrected for local
attraction. Suppose, for example, you read a
compass bearing of N37°E. Suppose the effect of
the magnetic attraction of a nearby pole
transformer is enough to deflect the compass
needle 4° to the west of the magnetic meridian.
In the absence of this local attraction, the
compass would read N33°E, not N37°E.
Therefore, the correct magnetic bearing is N33°E.
To correct a compass bearing for local
attraction,
you determine the amount and
direction (east or west) of the local attraction.
First, set up the compass where you propose to
take the bearing. Then, select a distant object that
you may presume to be outside the range of any
local attraction. Take the bearing of this object.
If you read a bearing of S60°W, shift the
compass to the immediate vicinity of the object
you sighted on; and take, from there, the bearing
of the original setup point. In the absence of any
local attraction at the original setup point, you
would read the back bearing of the original
bearing or N60°E. Suppose instead you read
N48°E. The back bearing of this is S48°W.
Therefore, the bearing as indicated by the
compass under local attraction is S60°W; but
as indicated by the compass not under local
attraction, it is S48°W. The amount and direction
of local attraction are, therefore, 12°W.
The question of whether you add the local
attraction to, or subtract it from, the compass
bearing to get the magnetic bearing depends on
(1) the direction of the local attraction and (2) the
quadrant the bearing is in.
As a rule, for a bearing in the northeast
quadrant, you add an easterly attraction to the
compass bearing to get the magnetic bearing and
subtract a westerly attraction from the compass
bearing to get the magnetic bearing.
Now, consider the compass shown in figure
13-5. This compass indicates a bearing of S40°W.
Suppose the local attraction is 12°W. The
needle, then, is 12°W of where it would be without
local attraction. You can see that, in the southwest
quadrant, you would subtract westerly attraction
and add easterly attraction.
From a study of the paragraphs above, it
becomes obvious that the procedure is the
opposite for bearings in the northwest or southeast
Figure 13-5.-Compass bearing affected by local magnetic
attraction.
13-4
