Surveyors use the transit to sight a location and read the angle of the point on a scale on the instrument. Early models of the transit measured horizontal angles only, but later models included a vertically adjustable telescope and scale for measuring vertical angles. The surveyor's transit combines the function of a compass with a telescope to provide greater accuracy in measuring angles than possible using a compass alone.
The telescope on the transit is reversible, allowing the surveyor to sight angles forward and backward along the traversed path. While originally less accurate than the English theodolite, the transit is more rugged, making it an ideal instrument for use in surveying the American frontier, according to the Smithsonian National Museum of American History.
For a century, surveyors depended on the transit for measuring angles. While electronic equipment eventually provided an easier to use instrument, it did not improve the accuracy of the transit, some of which measured to 0.5 seconds of an arc.
The transit played an important role in many transportation projects in the United States. Its use included laying out railroads, building the major roadways across the United States -including the interstate highway system - and the construction of the space shuttle.