Temporary Adjustment of Theodolite and Angle Measurement Process

Temporary Adjustment of Theodolite and Angle Measurement Process

Theodolite is a precision instrument used for measuring horizontal and vertical angles in surveying. Before taking any measurement, it is essential to perform temporary adjustments to ensure accuracy. After proper setup, angles can be measured using standard procedures. For ITI Surveyor trainees, understanding these steps is very important.

Temporary Adjustment of Theodolite

Temporary adjustment refers to the process of setting up the theodolite at a station before taking observations. It includes centering, levelling, and focusing.

1. Setting up the Instrument

The theodolite is mounted on a tripod and placed approximately over the survey station. The tripod legs are adjusted to bring the instrument to a convenient height.

2. Centering

Centering is the process of placing the instrument exactly over the survey station point.

• Use a plumb bob to align the instrument with the station mark
• Adjust tripod legs for accurate positioning

3. Levelling

Levelling ensures that the instrument is perfectly horizontal.

• Adjust levelling screws
• Center the bubble in the plate level
• Repeat until bubble remains central in all directions

4. Focusing

Focusing removes parallax error and ensures a clear image.

• Focus eyepiece to see crosshairs clearly
• Focus objective lens on the target
• Ensure no relative movement between image and crosshairs

Angle Measurement Process

After completing temporary adjustments, the theodolite is ready for measuring angles. Angles can be measured in horizontal and vertical planes.

Measurement of Horizontal Angle

Procedure:

1. Set up and level the theodolite at the station
2. Set initial reading to zero or note the initial reading
3. Sight the first point (backsight) and clamp the instrument
4. Release upper clamp and rotate to sight the second point (foresight)
5. Read the angle on the horizontal circle

The difference between the two readings gives the horizontal angle.

Measurement of Vertical Angle

Procedure:

1. Level the instrument properly
2. Sight the object using the telescope
3. Read the angle on the vertical circle

The vertical angle indicates whether the object is above or below the horizontal plane.

Face Left and Face Right Observation

To improve accuracy, angles are measured in two positions:

• Face Left (FL): Vertical circle on left side
• Face Right (FR): Vertical circle on right side

The average of both readings gives a more accurate result.

Precautions During Measurement

• Ensure proper centering and levelling
• Remove parallax before observation
• Take readings carefully
• Avoid disturbance of instrument
• Repeat observations for accuracy

Common Errors

• Improper levelling
• Parallax error
• Incorrect reading of scales
• Instrument not centered properly

Importance in Surveyor Trade

Temporary adjustment and angle measurement are basic operations in theodolite surveying. Accurate execution of these steps ensures reliable results in fieldwork.

Conclusion

Temporary adjustment of theodolite is essential before taking any measurement. Proper centering, levelling, and focusing ensure accuracy. The process of angle measurement helps in determining directions and positions in surveying.

For ITI Surveyor trainees, mastering these techniques is important for accurate and professional surveying work.