Using Compasses with Maps Map and compass, a powerful combination Compasses work best with maps. The main tasks are to orient the map correctly with the terrain, and to transfer bearings, courses, and locations to and from the map and compass. The security of having a clear picture of where you are turns map and compass into a powerful navigational system. Maximum navigation system Magnetic Errors Magnetic errors cause inaccuracy in magnetic compass readings. Since this error is primarily a difference between true direction and magnetic direction, it is only important when mixing the two, as when transferring angular dimensions between map and field.
Declination is fairly regular, predictable, and easy to correct.
Annual Magnetic Variations Annual magnetic variations are periodic magnetic errors peculiar to geographic locales. They may result from the difference between the spin rate of the earth's crust and that of its molten core. They change fairly regularly over time. Annual magnetic variation behaves like magnetic declination and can be corrected in the same way, at the same time. As one nears a magnetic pole the lines of magnetic force become more vertical.