Keywords: Levelling, principle, differential levelling, station, backsight, foresight, intermediate sight, change point, turning point, balancing
Let’s now move on to the basic principle of levelling. If you want to refresh the basic concepts of levelling, you can read them here.
When the levelling instrument is properly leveled, the bubble tube axis and the line of sight will be horizontal and the vertical axis of the instrument will be vertical. The bubble must be central and traverse. The line of sight will remain in a horizontal plane when the telescope is rotated. Thus if the telescope is sighted towards a staff kept on a point of known elevation, the height or elevation of the line of sight can be determined. If the telescope is now directed to staff kept on points of unknown elevation, the staff readings can be read. From which the reduced levels of the unknown points can be determined.