A compass measures the direction by measuring the angle between the line and a reference direction, which is the magnetic meridian.
Components and their functions of Theodolite
A compass measures the direction by measuring the angle between the line and a reference direction, which is the magnetic meridian. A compass can measure angles up to an accuracy of 30¢ and by judgement up to an accuracy of 15¢. The principle of working of the compass is based on the property of the magnetic needle, which when freely suspended, takes the north–south direction. Compass measurements are thus affected by external magnetic influences and therefore a compass is unsuitable in some areas. In this here, we will discuss another method of measuring directions of lines; a theodolite is very commonly used to measure angles in survey work.
There are a variety of theodolites—vernier, optic, electronic, etc. The improve-ments (from one form to the other) have been made to ensure ease of operation, better accuracy, and speed. Electronic theodolites display and store angles at the press of a button. This data can also be transferred to a computer for further processing. We start our discussion with the simplest theodolite—the vernier theodolite.
The vernier theodolite is a simple and inexpensive instrument but very valuable in terms of measuring angles. The common vernier theodolite measures angles up to an accuracy of 20¢ in a compass, where the line of sight is simple, restricting its range, theodolites are provided with telescopes which provide for much greater range and better ac-curacy in sighting distant objects. It is, however, a delicate instrument and needs to be handled carefully. The theodolite measures the horizontal angles between lines and can also measure vertical angles. The horizontal angle measured can be the included angle, deflection angle or exterior angle in a traverse. The vertical angle is the angle in a vertical plane between the inclined line of sight of the instrument and the horizontal. In the following sections we will discuss the vernier theodolite as well as its applications in surveying.