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 N37E. 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 N33E, not N37E. Therefore, the correct magnetic bearing is N33E.

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 S60W, 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 N60E. Suppose instead you read N48E. The back bearing of this is S48W. Therefore, the bearing as indicated by the compass under local attraction is S60W; but as indicated by the compass not under local attraction, it is S48W. The amount and direction of local attraction are, therefore, 12W.

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 S40W. Suppose the local attraction is 12W. The needle, then, is 12W 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

quadrants. In these quadrants, you add westerly attraction and subtract easterly attraction to the compass bearing to get the magnetic bearing.