Seven-inch Repeating Theodolite

This theodolite had a seven-inch circle. It was a repeating theodolite, meaning that observed angles were repeated from different settings on the horizontal circle. The final angle was determined by dividing the total arc passed through during the observation of the angle by the number of observations. This differs from a direction theodolite, which has a fixed circle and reads directions.

Seven-Inch Repeating Theodolite

The Coast Survey used a variety of makes and models of theodolites on its projects through the years. The seven-inch repeating theodolite shown in this image was read by two verniers to 10 seconds and was in use when the Special Publication 247, Manual of Geodetic Triangulation, was issued in 1950.

According to the Manual of Geodetic Triangulation:

There are two general types of theodolites, namely, repeating theodolites and direction theodolites. A repeating theodolite is so designed that successive measures of an angle may be accumulated on the graduated circle. The reading of the accumulated sum is divided by the number of repetitions to obtain the observed angle. With a direction theodolite the circle remains fixed while the telescope is pointed on a number of signals in succession with the circle being read for reach direction...
... Direction theodolites of the best workmanship are the preferred instrument for first-order triangulation. Although a repeating instrument gives good results, its mechanical operation is not readily adaptable to an efficient observing program. Use of a repeating theodolite on first-order triangulation is confined to stations where there is not room for a direction instrument or where the support is unstable.

Historical Reference

Hassler's Great Theodolite, shown earlier in this collection, was designed as a repeating theodolite but built as a direction theodolite.

  • Instrument Shown: Repeating theodolite with seven-inch circle
  • Location: Unknown, possibly Coast Survey office in Washington, DC
  • Manufacture Date: Unknown
  • Dates of Use: Early to mid-1900s
  • Photo Date: Unknown

Works Consulted

Dracup, J. F. Geodetic Surveys in the United States 1807-1940. Retrieved May 2006, from http://www.ngs.noaa.gov/PUBS_LIB/geodetic_survey_1807.html

Mitchell, Hugh C. (1948). Definition of Terms Used in Geodetic and Other Surveys. Special Publication No. 242. Washington, DC: GPO.

Gossett, Cmd. F. R. (1950). Manual of Geodetic Triangulation. Special Publication No. 247. Washington, DC: GPO.


Related Web Sites:

National Geodetic Survey