Figure 13-7.-Orienting a compass for a 10° easterlyattraction.For example, with a compass affected by a10°W local attraction, you want to lay off aline bearing S28°W magnetic by compass. If youwere correcting, you would subtract a westerlyattraction in the southwest quadrant. However,for uncorrecting you ADD a westerly attractionin that quadrant. Therefore, to lay off a linebearing S28°W, you would lay off S38°W by thecompass.The same rule applies to azimuths. Supposeyou have an azimuth-reading (measured fromnorth) compass set up where local attraction is10°W and declination is 25°E, and you want tolay off a line with true azimuth 256°. Thealgebraic sum of these is 15°E. For correcting orconverting azimuths, you ADD easterly andSUBTRACT westerly corrections; therefore, ifyou were correcting or converting, you would addthe 15° to 256°. Because you are uncorrecting orunconverting, however, you subtract; and, to layoff a line with true azimuth 256°, you read 241°by the compass.Orienting a CompassSome transit compasses and most surveyor’sand forester’s field compasses are equipped foroffsetting local attraction, local declination,and/or the algebraic sum of the two. In figure13-7, the upper view shows a compass bearing ofN40°W on a compass presumed to be affected bya local attraction of 10°E. In this quadrant,you subtract easterly attraction; therefore, themagnetic bearing should read N30°W.In the lower view, the same compass has beenoriented for an error of 10°E by simply rotatingthe compass card 10°E clockwise. On mostorienting compasses, the card can be released forrotating by backing off a small screw on the faceof the card. Note that you now read the correctmagnetic bearing of N30°W.Conducting a Compass-Tape SurveyFigure 13-8 shows field notes from a compass-tape survey of a small field. The instrument usedwas a surveyor’s compass. The compass was firstset up at station A, shown in the sketch drawnon the remarks page. The first bearing taken wasthat of the line AE. This was actually the backbearing of EA, taken for the purpose of laterchecking against the forward bearing of EA.Next, the bearing of AB was taken, and thedistance from A to B was chained. The observedbearing (S62°20´E) was entered beside B in thecolumn headed “Obs. Bearing.” The chaineddistance was entered beside B in the columnheaded “Dist.”The compass was shifted to station B, and theback bearing of AB (that is, the bearing of BA)was taken as a check on the previously takenbearing of AB. The back bearing turned out tohave, as it should have, the same numerical value(62°20´) as the forward bearing. A difference inthe two would indicate either an inaccuracy inreading one bearing or the other or a differencein the strength of local attraction.13-6