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Figure 10.1. The lines of sight in angle BAC

Horizontal angles are usually expressed in degrees. A full circle is

divided into 360 degrees, abbreviated as 360°:
a 90° angle, called a right angle, is made of two perpendicular
lines. The corners of a square are all right angles;
a 180° angle is made by prolonging a line. In fact, it is the same

as a line.
Each degree is divided into smaller units:
 1 degree = 60 minutes (60');

 1 minute = 60 seconds (60"). These smaller units, however, can
only be measured with high-precision instruments.

Figure 10.2. Horizontal angle BAC

Nowadays the mostly used are the next horizontal angle
measurement methods:

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