A tacheometer is a theodolite, which is used to measure angles as well as distances. Polar coordinates and differences in altitude are determined from the instrument position, thereby obtaining all the data necessary for mapping in a single operation.
When recording an area topographically, the actual surveying must be preceded by a detailed inspection of the terrain in order to distinguish the archaeological objects from other structures. A prism (reflector) is held up on the chosen points in the terrain and aimed at with the tacheometer (cf. fig. left). (Modern instruments allow measuring without a reflector. The point is displayed by a laser dot.)
During the surveying process, all measured points are sketched in a so-called draft map. The true to scale depiction of the terrain in a map is carried out later. Besides ground plans of trackways and walls etc., contour lines serve to illustrate the shape of the terrain.
using a tacheometer  tacheometer
Fig. left: Surveying points using a tacheometer (Photograph: E.Laufer, Archäologie-AG Usingen)
Fig. right: Geodolite made by Geotronics in Sweden.

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