Vernier theodolite replaces compass for angular measurements. It can also be used for the measurement of vertical angles. To be able to use the Vernier theodolite, it is important to know how it works. In this article, we will discuss how to set up the Vernier theodolites and use them for measuring horizontal and vertical angles.
Working of Vernier Theodolite
Working of a Vernier theodolite starts with setting up and adjustment of theodolite for the measurement of angles.
Temporary Adjustment of Theodolite
Following are the adjustment required for theodolite before using it:
- Setting
- Centering
- Leveling
- Focusing
Setting of Theodolite
Setting means attaching theodolite to the tripod stand. The tripod is unfolded and placed on the ground with all legs at the same level. In this operation, we level the tripod by just eye estimation. From the bottom of theodolite, a plumb bob is suspended to set the instrument over the station mark.
Centering
Centering is the process in which the vertical axis of the instrument coincides with the plumb line. The legs of the tripod stand should be of equal height on the leveled ground and they have cone shape metal shoes that can easily hold the ground without any movement.
Leveling of Theodolite
Leveling of theodolite is completed by making the vertical axis of the instrument truly vertical. It is done by centering the bubble in the level tube by rotating foot screws provided. The bubbling should be done in all directions of the telescope and the centered bubble should not move while taking a reading.
Focusing
Focusing is the adjustment of the telescope to obtain clear visibility of the image through the eyepiece and objective lens. This process is done by removing parallax error by proper focusing of eyepiece and objective lens.
Measurement of Horizontal Angle using Theodolite
After the temporary adjustment of theodolite, the telescope is located at a point from which we must focus on the other point to find a horizontal angle between them. Let us say A, B, and C are three points on the same ground level with some distances as shown in the figure below. Now the angle ABC can be determined as below described procedure.
Procedure for Measurement of Horizontal Angle
- The instrument is located at point B with all temporary adjustments and ranging rods are located at points A and C.
- Now, set the Vernier A reading to zero and Vernier Breading to 180 by rotating the telescope. This should be done by releasing the upper clamp screw and tightening of lower clamp screw.
- After setting the Vernier, tighten the upper clamp screw and check the reading without any error with the help of the upper tangent screw.
- Whenever the upper clamp is tightly fixed, loosen the lower clamp screw it will allow the telescope to turn without any change in reading. Now focus the ranging rod at location A and bisect it.
- After bisecting A, release the upper clamp screw and rotate the telescope in a clockwise direction towards the ranging rod at C and bisect it. Now observe the Vernier reading which will be the horizontal angle between A and C or angle ABC.
- In the above-explained case, the vertical circle is the present left side of the telescope, then repeat the same procedure with the vertical circle right side of the telescope. The average value of angles obtained in the face left and face right conditions give a more accurate value of the horizontal angle.
Measurement of Vertical Angle using Theodolite
- After the temporary adjustment of theodolite, set Vernier reading of vertical circle to 0 using clamp screws and focus it to C.
- Center the bubble using foot screws without any collimation error. Then raise the telescope slowly towards point A as shown in fig. bisect point A and note down the Vernier reading angle AOC.
- Similarly, bisect the point B from C after which will give angle BOC. Now we can compute the angle AOB.
- Conduct the procedure in both faces left and face right conditions and the average value is considered as the accurate vertical angle.