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. The most commonly used are magnetic and grid. A line from any point on the earth's surface to the north pole.
All lines of longitude are true north lines.
Magnetic readings are obtained with magnetic instruments, such as lensatic and M2 compasses. The north that is established by using the vertical grid lines on the map. An azimuth is defined as a horizontal angle measured clockwise from a north base line. This north base line could be true north, magnetic north, or grid north. The azimuth is the most common military method to express direction.