This document discusses trigonometric levelling, which is a method of determining elevation differences between stations using vertical angles and known distances. It presents three cases for determining the elevation of a point using a theodolite: 1) when the base of the object is accessible, 2) when the base is inaccessible and instrument stations are in the same vertical plane, and 3) when the base is inaccessible and instrument stations are not in the same vertical plane. Equations for calculating relative heights are provided for each case using trigonometric functions of the vertical angles and distances between points. Corrections may be needed for long distances to account for earth's curvature and refraction.