Azimuth of a line is its horizontal angle measured clockwise from geographic or true meridian.

For field observation, the most stable and retraceable reference is geographic north. Geographic north is based on the direction of gravity (vertical) and axis of rotation of the earth. A direction determined from celestial observations results in astronomic (Geographic) north reference meridian and is known as geographic or true meridian.

The azimuth of a line is determined from the azimuth of a celestial body. For this, the horizontal angle between the line and the line of sight to the celestial body is required to be observed during astronomic observation along with other celestial observation.

Let AB be the line whose azimuth (A_AB) is required to be determined (Figure 26.1). Let O be the station for celestial observations. Let S be the celestial body whose azimuth (A_s) is determined from the astronomical observation taken at O. The horizontal angle from the line AB to the line of sight to celestial body (at the station O) is observed to be q° clockwise. The azimuth of the line, AB can be computed from

A_AB = A_S - q° (clockwise).

If A_AB computes to be negative, 360° is added to normalize the azimuth.

In order to compute the azimuth of a line with proper sign, it is better to draw the known parameters. The diagram itself provides the azimuth of the line with proper sign. For example, in Figure 26.2, first a line of sight to celestial body, OS is drawn. Then, the azimuth of the celestial body, A_S is considered in counter-clockwise from the line OS and obtained the true north direction i.e, the line ON. Similarly, the horizontal angle q° is represented in counter clockwise (since the angle from the line to the celestial body is measured clockwise) direction from OS to obtain the relative position of the line. The angle NOB represents the azimuth of the line AB.

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