1212321 lab2

1 1. STUDY OF THEODOLITE MEASUREMENT AND HORIZONTAL AND VERTICAL ANGELS AIM: To study about a theodolite and to measure horizontal angles and vertical angles. INSTRUMENTS: Transit theodolite, two ranging rods, one tripod. THEODOLITE: Transit or theodolite is an instrument used to measure horizontal and vertical angles. It was invented by Roemer, a Danish astronomer in 1690. About a century later it was modified to suit the surveying requirements and even to horizontal angles. In 1 830, further additions were made to this basic instrument to make possible the measurement of several things apart from the measurement of vertical and horizontal angels. Because of its variety of uses such as the measurement of horizontal and vertical angles, prolonging a straight line, Measurement of bearing and distances: TEMPORARY ADJUSTMENTS: These are the adjustments which are done before making the observations at every setup of the instrument. It consists of setting up the theodolite, leveling it and focusing the eyepiece and objective. Before setting up the t heodolite it should be ensured that 1) The leveling screws are at the Centre of their run. 2) The shifting head of the theodolite is at its center so that equal movement is possible in all directions. 3) The swing nuts on the tripod legs are tight enough so that when raised the tripod legs do not fall under their own weight PERMANENT ADJUSTMENTS: The fundamental line of a theodolite are the vertical axis the axis of plate level, the line of collimation, t he horizontal axis and the bubble line of altitude. The following adjustments are desirable. 1) The axis of the plate level must be perpendicular to the vertical axis 2) The line of collimation must be at right angles to the horizontal axis 3) The horizontal axis must be perpendicular to vertical axis 4) The axis of the telescope level must be parallel to the line of co llimation. READING A THEODOLITE:A theodolite has two verniers A and B placed on the opposite sides of the plate i.e. they are placed at a difference of 180°. For ordinary work usually the vernier A is read, whereas for precise works both the verniers A and B are read and the mean of the two reading is used. This practice minimizes the error due to eccentricity and imperfection in subdivisions which might exist in the circular scale. The main scale and vernier of a typical theodolite as graduated are shown in fig. the main scale is graduated from 0 to 360 in degrees and minutes. Each degree part is tested and divided into three equal parts. Hence the minimum reading that can be read from the main scale is 20’. The vernier scale is graduated into minutes and seconds. Each mi nute division is divided into three equal parts. Hence, the least reading that can be read from the vernier scale is 20 ”.To read an observation, first determine the value of the scale reading in degree and minutes up to the last scale division passed by the vernier zero (index). Add to this the vernier reading. A vernier reading is obtained by locating the vernier line, which coincides with the main scale line. The number of that vernier line then multiplied by the least count of the vernier to get the reading. MEASUREMENT OF HORIZONTAL ANGLE: Horizontal angles are measured on the horizontal circle of a theodolite by operating the upper clamp. The lower clamp and the upper and lower tangent screws often a beginner gets confused about the usage of the screws. It should be remembered that with both clamps set. The upper plate, the lower plate and the telescope are immobile with respect to l eveling head and tripod. PROCEDURE: 1. Set up t he in str ument over B and level it.

1212321 lab2

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