This theodolite was designed in 1924 by Douglas L. Parkhurst, chief of the Instrument
Division of the United States Coast and Geodetic Survey. It was made by August Heim, an
instrument maker who worked with Parkhurst and contributed several ideas to its development.
It was completed in July 1927. The horizontal circle is silvered, graduated to 5 minutes, and read
by two micrometer microscopes to single seconds.
Aiming to produce an instrument suitable for first order geodetic work, Parkhurst was
concerned that there be "no appreciable change in the fit of the vertical axis bearing due to
changes in temperature," and no frictional drag between the telescope alidade and the graduated
circle. Moreover, "The entire design must be made with a view to ruggedness and speed of
manipulation and to provide for simple and easy adjustment under field conditions." By 1928 he
could boast that "field tests" had proven his theodolite "to be one of the most accurate, rapid and
durable instruments" that the Survey had ever used.
Ref: Douglas L. Parkhurst, "A New First Order Theodolite," Journal of the Franklin Institute
Douglas L. Parkhurst, "Unusual Design in New Theodolite," Engineering News-Record
101 (1928): 806-808.