In order to measure something, there must always be a starting point or zero measurement. To express direction as a unit of angular measure, there must be a starting point or zero measure and a point of reference These two points designate the base or reference line. There are three base lines-true north, magnetic north, and grid north. The most commonly used are magnetic and grid.
- True North. A line from any point on the earth’s surface to the north pole. All lines of longitude are true north lines. True north is usually represented by a star (Figure 6-1, page 6-2).
- Magnetic North. The direction to the north magnetic pole, as indicated by the north-seeking needle of a magnetic instrument. The magnetic north is usually symbolized by a line ending with half of an arrowhead (Figure 6-1). Magnetic readings are obtained with magnetic instruments, such as lensatic and M2 compasses.
- Grid North. The north that is established by using the vertical grid lines on the map. Grid north may be symbolized by the letters GN or the letter “y” (Figure 6-1).
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