TRANSITS

elites, clinometers, levels, alignment sights, tram bars and blocks, benchmarks, and dials. TRANSITS A transit is an optical-surveying instrument that is used for measuring angles. Essentially, the transit pro- vides an optical line of sight (LOS) that is perpen- dicular to, and supported on, a horizontal axis. The horizontal axis is perpendicular to a vertical axis about which it can rotate. Spirit levels are used to make the vertical axis coincide with the direction of gravity. Graduated circles with verniers are used to read the angles. A typical transit is shown in figure 3-7. THEODOLITES Theodolites are similar to transits, but they are normally more-precise instruments. Theodolites use micrometer microscopes that are especially designed for rapid and accurate readings and are used to read the graduated vertical (for elevation) and horizontal (for train) circles. Micrometer microscopes create such precision that the accuracy of the optical reading device is governed by the circle and not by the way the circle is read. Different types of theodolites can be read directly to 10 inch, 1 inch, or 0.1 inch of arc. Figure 3-8 is an example of a typical theodolite. Figure 3-7.—Typical transit. Figure 3-8.—Typical theodolite. 3-6