observer is determining the magnetic bearing ofthe dotted line labeled Line of Sight. First, theobserver mounts the compass on a steady support,levels it, and waits for the needle to stoposcillating. Then, the observer carefully rotatesthe compass until the north-south line on the cardlies exactly along the line whose bearing is beingtaken.The bearing is now indicated by the needle-point. The needlepoint indicates a numerical valueof 40°. The card indicates the northeast quadrant.The magnetic bearing is, therefore, N40°E.Correcting for Local Magnetic AttractionFigure 13-4 shows the compass needle lyingalong the magnetic meridian. This means eitherthat the compass is in an area free of “localmagnetic attraction” or that the effect of localattraction has been eliminated by adjusting thecompass card as described later. “Local magneticattraction” means the deflection of the compassneedle by a local magnetic force, such as thatcreated by nearby electrical equipment or by amass of metal, such as a bulldozer. When localattraction exists and is not compensated for, thebearing you get is a COMPASS bearing. Acompass bearing does not become a magneticbearing until it has been corrected for localattraction. Suppose, for example, you read acompass bearing of N37°E. Suppose the effect ofthe magnetic attraction of a nearby poletransformer is enough to deflect the compassneedle 4° to the west of the magnetic meridian.In the absence of this local attraction, thecompass would read N33°E, not N37°E.Therefore, the correct magnetic bearing is N33°E.To correct a compass bearing for localattraction,you determine the amount anddirection (east or west) of the local attraction.First, set up the compass where you propose totake the bearing. Then, select a distant object thatyou may presume to be outside the range of anylocal attraction. Take the bearing of this object.If you read a bearing of S60°W, shift thecompass to the immediate vicinity of the objectyou sighted on; and take, from there, the bearingof the original setup point. In the absence of anylocal attraction at the original setup point, youwould read the back bearing of the originalbearing or N60°E. Suppose instead you readN48°E. The back bearing of this is S48°W.Therefore, the bearing as indicated by thecompass under local attraction is S60°W; butas indicated by the compass not under localattraction, it is S48°W. The amount and directionof local attraction are, therefore, 12°W.The question of whether you add the localattraction to, or subtract it from, the compassbearing to get the magnetic bearing depends on(1) the direction of the local attraction and (2) thequadrant the bearing is in.As a rule, for a bearing in the northeastquadrant,youaddaneasterlyattractiontothecompassbearingtogetthemagneticbearingandsubtractawesterly attraction fromthecompassbearingtogetthemagneticbearing.Now, consider the compass shown in figure13-5. This compass indicates a bearing of S40°W.Suppose the local attraction is 12°W. Theneedle, then, is 12°W of where it would be withoutlocal attraction. You can see that, inthesouthwestquadrant,youwouldsubtractwesterlyattractionandaddeasterlyattraction.From a study of the paragraphs above, itbecomes obvious that the procedure is theopposite for bearings in the northwest or southeastFigure 13-5.-Compass bearing affected by local magneticattraction.13-4
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