Basic route and concept of leveling

Leveling is a traditional method to measure height. The measuring principle is: based on a base point, measure the horizontal distance between another measured point and the base point on the map, and then measure the included angle between the measured point and the horizontal line at the base point with a level, so that the relative elevation between the measured point and the base point can be calculated by trigonometric mathematics. In this way, through a series of measurements, the absolute elevation of Mount Everest can be calculated from the sea level.