You correct a compass bearing to a magnetic bearing by applying the local attraction. You convert a magnetic bearing to a true bearing by applying the local declination.

At some time, you may be given a magnetic bearing and have to figure the corresponding compass bearing by using both local attraction and local declination.

The terms used to describe these calculations are, for the want of any better expressions, UNCORRECTING and UNCONVERTING. All YOU need to remember is that, when you are uncorrecting or unconverting, you apply local attraction and local declination in the REVERSE of the directions in which you apply them if you were correcting or converting.

For example, with a compass affected by a 10W local attraction, you want to lay off a line bearing S28W magnetic by compass. If you were correcting, you would subtract a westerly attraction in the southwest quadrant. However, for uncorrecting you ADD a westerly attraction in that quadrant. Therefore, to lay off a line bearing S28W, you would lay off S38W by the compass.

The same rule applies to azimuths. Suppose you have an azimuth-reading (measured from north) compass set up where local attraction is 10W and declination is 25E, and you want to lay off a line with true azimuth 256. The algebraic sum of these is 15E. For correcting or converting azimuths, you ADD easterly and SUBTRACT westerly corrections; therefore, if you were correcting or converting, you would add the 15 to 256. Because you are uncorrecting or unconverting, however, you subtract; and, to lay off a line with true azimuth 256, you read 241 by the compass.

Some transit compasses and most surveyors and foresters field compasses are equipped for offsetting local attraction, local declination, and/or the algebraic sum of the two. In figure 13-7, the upper view shows a compass bearing of N40W on a compass presumed to be affected by a local attraction of 10E. In this quadrant, you subtract easterly attraction; therefore, the magnetic bearing should read N30W.

In the lower view, the same compass has been oriented for an error of 10E by simply rotating the compass card 10E clockwise. On most orienting compasses, the card can be released for rotating by backing off a small screw on the face of the card. Note that you now read the correct magnetic bearing of N30W.