Category: 2. Levelling

Incorrect Bisection of Signals: If the signal arriving at the observed station is not clearly visible, due to vegetation cover or intervening ground, the observer may incorrectly bisect the signal. This introduces an error whose magnitude varies inversely with the length of the gaze. It can be eliminated by looking clearly and always at the…

Theodolites are very delicate and sophisticated surveying instruments. Despite best efforts during manufacturing, perfect adjustment of the primary axes of the theodolite may not be possible. Instrument faults can be further subdivided as discussed below.

The sources of error in theodolite operation can be broadly divided into three categories: Equipment errorsPersonal mistakesNatural errors

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…

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…

This method is usually adopted when measuring multiple angles that have a common vertex. In this method, angles are measured successively, starting from a reference station and ending at the same station. A double observation at the starting station checks the sum of all angles around the station. The sum must always equal 360°. This…

Let ABC be the required angle between the sides BA and BC. An iterative method is generally used for accurate and precise work. In this method, the angle value is mechanically added several times and the correct angle measurement value is obtained by dividing the accumulated reading by the number of repetitions.To measure a small…

Set up, center and level the theodolite at ground point B.Loosen the top plate, set the vernier to read zero and clamp the top plate.Loosen the lower plate and swing the telescope until the left point A is visible. Tighten the lower clamp. The correct bisection of the arrow placed at station A is done…

The purpose of this adjustment is to ensure that the line of collimation remains horizontal when the bubble of the level tube mounted on the telescope is brought to the center of its run. This adjustment is necessary when the theodolite is used as a level and when vertical angles are observed.

This adjustment includes horizontal hair adjustment and vertical hair adjustment. The purpose of adjusting horizontal hairs is to bring the horizontal hairs of the eyepiece into the horizontal plane through the optical axis. If the horizontal hairs do not lie in the horizontal plane through the optical axis, the vertical angles will be distorted. This…