The theodolite is an instrument that measures vertical and horizontal angles precisely. With stadia lines etched on the reticule of its telescope and the appropriate stadia rod (a flat-faced rod graduated vertically, its subdivisions corresponding to one/one hundredth of the focal distance) it will measure distances with accuracy sufficient for third order triangulation. As used by Blakslee and Gierhart, it served in several ways:
- To measure the dip of almost flat-lying strata, the rodman moving laterally until the maximum number of rod divisions below the horizontal was signaled by the instrument man.
- To measure the approximate thickness of inaccessible rock units on a cliff face.
- To lay out a measured distance in kilometers in order to calibrate the odometer of the sedan. See Odometer-Altimeter Traverse.
- To tie the location and elevation of measured sections into traverse lines.
- To make measurements of the position of one or more stars listed in the ephemeris in order to determine latitude and longitude.
The precise timing necessary for observations of zenith was provided by BBC broadcast of Greenwich time signals.