Figure 13-6.-Magnetic declination (west).quadrants. In these quadrants, you add westerlyattraction and subtract easterly attraction to thecompass bearing to get the magnetic bearing.Determining Magnetic Declination and DipThe angle between the true meridian and themagnetic meridian is MAGNETIC DECLINA-TION. If the north end of the compass needle ispointing to the east of the true meridian, thedeclination is said to be east. If the north end ofthe compass needle is pointing to the west of thetrue meridian, the declination is said to be west.(See fig. 13-6.)The magnetic needle aligns itself with theearth’s magnetic field and points toward theearth’s magnetic pole. In horizontal projections,these lines incline downward toward the north inthe Northern Hemisphere and downward towardthe south in the Southern Hemisphere. Since thebar takes the position parallel with the lines offorce, it inclines with the horizontal. Thisphenomenon is the MAGNETIC DIP.Converting Magnetic Bearingsto True BearingsWhenfor localyou have corrected a compass bearingattraction, you have a MAGNETICBEARING. As explained previously, in mostareas of the earth, a magnetic bearing differsfrom a true bearing by the amount of the localmagnetic declination (called magnetic variationby navigators). The amount and direction of localdeclination are shown on maps or charts of thearea in a format similar to the following:“Magnetic Declination 26°45°W (1968), AnnualIncrease 11´. ” This means, if you are working in1988 (20 years later), the local declination isTo convert a magnetic bearing to a TRUEBEARING, you apply the declination to themagnetic bearing in precisely the same way thatyou apply local attraction to a compass bearing.If the declination is east, it is added tonortheast and southwest magnetic bearings, andit is subtracted from southeast and northwestmagnetic bearings. If the declination is west, itis added to southeast and northwest magneticbearings and subtracted from northeast andsouthwest magnetic bearings.When you have a compass bearing and knowboth the local attraction and the local declination,you can go from compass bearing to true bearingin a single process by applying the ALGEBRAICSUM of local attraction and local declination,Suppose that local attraction is 6°W and declina-tion, 15°E. You could correct for local attractionand convert from magnetic to true in the sameoperation by applying a correction of 9°E to thecompass bearing.Uncorrecting and UnconvertingYou correct a compass bearing to a magneticbearing by applying the local attraction. Youconvert a magnetic bearing to a true bearing byapplying the local declination.At some time, you may be given a magneticbearing and have to figure the correspondingcompass bearing by using both local attractionand local declination.The terms used to describe these calculationsare, for the want of any better expressions, UN-CORRECTING and UNCONVERTING. All YOUneed to remember is that, when you areuncorrecting or unconverting, you apply localattraction and local declination in the REVERSEof the directions in which you apply them if youwere correcting or converting.13-5
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