Definition- a vertical angle is the angle between the inclined line of sight and the horizontal line.

If the angle is above the horizontal line it is called as angle of elevation it is always considered as +ve angle.

If the angle is below the horizontal line it is called as angle of depression it is always considered as -ve angle.

Procedure

1. to measure the vertical angle of an object P at a section O.
2. Set up the instrument over station O and level it carefully with respect to altitude bubble.
3. By mean of vertical circle clam and tangent screw, set zero of the vertical Vernier exactly zero of the vertical circle.
4. Bring the bubble of altitude level to the centre of it run by mean of the foot screw. The line of collimation is thus made horizontal while the Vernier read zero.
5. Loose the vertical circle clamp and direct the telescope in a vertical plane toward the object P and bisect exactly using the vertical circle tangent screw.
6. Read both the Vernier C and D the mean of the two reading gives the value of the required angle for this face.
7. The average of two value obtain from left observation and face right observation give exact value of required angle.