A two-peg test is used to measure the calibration of which instrument?

The two-peg test is specifically used to measure the calibration of an automatic level. This test involves setting up a level instrument over a benchmark and taking readings on two leveling rods placed at different distances, typically at known points. By doing so, the surveyor can determine if the instrument is level and whether its vertical alignment is accurate across different distances. The results of these readings will indicate if any adjustments need to be made to the automatic level.

In contrast, the other instruments listed—such as a theodolite, GPS unit, and total station—require different methods for calibration and adjustment. For instance, a theodolite would typically be tested for its horizontal and vertical angular calibration through different means, while a GPS unit relies on satellite signals for accuracy and would have its calibration checked through different procedures unrelated to the simple two-peg test. A total station, which combines the functions of a theodolite and an electronic distance measuring device, also has distinct processes for calibration that do not involve the two-peg test.