The vertical angle is defined as the angle subtended by the inclined line of sight and the horizontal line of sight in the vertical plane. If the observed point is above the horizontal axis of the theodolite, the vertical angle is known as the angle of elevation, and if it is below it is called the angle of incidence.To measure a vertical angle subtended by the station B at the instrument station A, the following steps are involved:
Set the zero of the vertical vernier exactly in coincidence with the zero of the vertical scale using the vertical clamp and vertical tangent screw. Check whether the bubble of the altitude level is in the centre of its run. If not, bring it to the centre by means of the clip screw. In this position the line of collimation of the telescope is horizontal and the verniers read zero
Set up the theodolite over the ground station mark A and level it.Loosen the vertical circle clamp and move the telescope in the vertical plane until the station B is brought in the field of view. Use the vertical circle tangent screw for accurate bisection.
Read both the verniers of the vertical circle. The mean of the two vernier readings gives the value of the vertical angle.
Change the face of the instrument and make observations in a similar way.
The average of the two values is the required value of the vertical angle.
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