This method is called direct levelling or differential levelling. Indirect levelling can be done using the principle that differences in elevation are proportinal to the differences in atmospheric pressure. The difference in elevation between two points can also be determined trigonometrically using vertical angles and horizontal or inclined distances. They are permanent objects of known elevation located where there is least likelihood of disturbance. They may be a metal or concrete post set close to the ground, a notch in a tree root or a peg or spike in a tree.
Radiation is a surveying technique often used in conjunction with a plane table.
From a fixed position directly above the corresponding location on the ground bearings are taken to various points at the boundary of the survey area. The bearing lines are drawn on the paper on the table. Distances to the points are measured and then converted to the required scale on the survey sheet. Radiation surveying can be performed in a similar way directly on the ground taking bearings and distances from a fixed position and recording them for later work. Similarly plane table surveying may be used with other techniques such as a traverse or triangulation.