maintenance technicians (AMTs) to calibrate the compass  by creating magnetic fields inside of the compass housing.  The compensator assembly has two shafts whose ends have  screwdriver slots accessible from the front of the compass.  Each shaft rotates one or two small compensating magnets.  The end of one shaft is marked E-W, and its magnets affect  the compass when the aircraft is pointed east or west. The  other shaft is marked N-S and its magnets affect the compass  when the aircraft is pointed north or south. The magnetic compass is the simplest instrument in the panel,  but it is subject to a number of errors that must be considered. The Earth rotates about its geographic axis; maps and charts  are drawn using meridians of longitude that pass through the  geographic poles. Directions measured from the geographic  poles are called true directions. The north magnetic pole to  which the magnetic compass points is not collocated with  the geographic north pole, but is some 1,300 miles away;  directions measured from the magnetic poles are called  magnetic directions. In aerial navigation, the difference  between true and magnetic directions is called variation. This  same angular difference in surveying and land navigation is  called declination. Figure 5-17  shows the isogonic lines that identify the number  of  degrees of variation in their area. The line that passes near  Chicago is called the agonic line. Anywhere along this line  Some airports have a compass rose, which is a series of lines  marked out on a taxiway or ramp at some location where ther e  Deviation, unlike variation, is different on each heading, but  it is not affected by the geographic location. Variation error  cannot be reduced or changed, but deviation error can be  minimized when a pilot or AMT performs the maintenance  task known as “swinging the compass.” The magnets in a compass align with any magnetic field.  Local magnetic fields in an aircraft caused by electrical  current flowing in the structure, in nearby wiring or any  magnetized part of the structure, conflict with the Earth’s  magnetic field and cause a compass error called deviation. Flying in the Washington, D.C. area, for example, the  variation is 10° west. If the pilot wants to fly a true course of  south (180°), the variation must be added to this resulting in  a magnetic course to fly of 190°. Flying in the Los Angeles,  CA area, the variation is 14° east. To fly a true course of 180°  there, the pilot would have to subtract the variation and fly a  magnetic course of 166°. The variation error does not change  with the heading of the aircraft; it is the same anywhere along  the isogonic line. the two poles are aligned, and there is no variation. East of  this line, the magnetic pole is to the west of the geographic  pole and a correction must be applied to a compass indication  to get a true direction. Magnetic Compass Errors Variation  Deviation  0 Ň  15 Ň  N 30 Ň  N 45 Ň  N 60 Ň  N 70˚N 70˚N 70˚N 70˚N 15 Ň  S 30 Ň  S 45 Ň  S 60 Ň  N 0 Ň  15 Ň  N 30 Ň  N 45 Ň  N 60 Ň  N 60 Ň  N 15 Ň  S 30 Ň  S 45 Ň  S 0 Ň  15 Ň W 30 Ň W 45 Ň W 60 Ň W 90 Ň W 75 Ň W 105 Ň  W 120 Ň  W 135 Ň  W 150 Ň  W 165 Ň  W 180˚W 180˚W 180˚W 180˚W 15 Ň  E 30 Ň  E 45 Ň  E 60 Ň  E 90 Ň  E 75 Ň  E 105 Ň  E 120 Ň  E 135 Ň  E 150 Ň  E 165 Ň  E 0 Ň  15 Ň  W 30 Ň  W 45 Ň  W 60 Ň  W 90 Ň  W 75 Ň  W 105 Ň  W 120 Ň  W 135 Ň  W 150 Ň  W 165 Ň  W 15 Ň  E 30 Ň  E 45 Ň  E 60 Ň  E 90 Ň  E 75 Ň  E 105 Ň  E 120 Ň  E 135 Ň  E 150 Ň  E 165 Ň  E 6 0 5 0 40 3 0 2 0 10$^{1}$$^{0}$ 20 0 0 1 0 0 2 0 1 0 0 0 2 0 1 0 1 3 0 1 0 1 0 9 0 8 0 70 60 50 40 30 8 0 7 0 2 0 1 0 -40 -9 0 - 1 0 0 $_{-}$1 1 0 $_{-}$1 2 0 $_{-}$1 3 0 -5 0 -4 0 -30 -20 - 1 0 - 3 0 - 2 0 - 1 0 - 2 0 $_{-}$3 0 -1 0 - 1 0 $_{-}$8 0 - 7 0 - 6 0 - 2 0 -1 0 - 1 0 Main field dec lination (D) Contour interval:  degree s red contours positiv (east) blue negativ ( west) pink (agonic)  z ero line . Mercator Projection. P osition of dip poles Figure 5-17.  Isogonic lines are lines of equal variation. 5-12