Measurement of vertical angle b/w two stations at elevations

The vertical angle between two stations at different elevations can be measured as:

Measure the vertical angle of the high station as described earlier. Let it be α.
Measure the vertical angle of the lower station. Let it be β.
The required vertical angle between stations can be calculated by finding the algebraic difference between the two readings, taking angles of elevation as positive and angles of depression as negative.
27.10.8 Sources of error in theodolite operation