Vernier theodolite replaces compass for angular measurements. It can also be used for 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 setup the Vernier theodolites and use it 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 of Theodolite
Setting means attaching theodolite to the tripod stand. The tripod is unfolded and placed on the ground with all legs in 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 station mark.
Centering is the process in which the vertical axis of the instrument is coincided with the plumb line. The legs of tripod stand should be of equal height on leveled ground and they have cone shape metal shoes which can easily hold the ground without any movement.
Leveling of Theodolite
Leveling of theodolite is completed by making the vertical axis of 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 direction of telescope and the centered bubble should not move while taking reading.
Focusing is the adjustment of telescope to obtain clear visibility of image through eyepiece and objective lens. This process 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 the other point to find horizontal angle between them. Let us say A, B and C are three points on same ground level with some distances as shown in 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 B reading to 180 by rotating 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 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 clockwise direction towards 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 present left side of the telescope, then repeat the same procedure with vertical circle right side of telescope. The average value of angles obtained in face left and face right conditions gives more accurate value of 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 the 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 face left and face right conditions and the average value is considered as accurate vertical angle.