Measurement of Horizontal Angle by Theodolite

Measurement of Horizontal Angle by Theodolite

Measurement of horizontal angles is one of the primary functions of a theodolite. It is widely used in surveying for traversing, triangulation, and setting out works. In ITI Draughtsman Civil, understanding the correct procedure for measuring horizontal angles is essential for accurate fieldwork and plotting.

1. Definition of Horizontal Angle

A horizontal angle is the angle between two lines measured in the horizontal plane. It is usually measured clockwise from one line to another.

2. Methods of Measuring Horizontal Angle

Horizontal angles can be measured by the following methods:

(a) Ordinary Method (Direct Method)

Procedure

1. Set up the theodolite over the station point.
2. Perform temporary adjustments (centering, leveling, focusing).
3. Set the vernier to zero.
4. Rotate the telescope and sight the first point (A).
5. Clamp the instrument and note the reading.
6. Rotate the telescope to sight the second point (B).
7. Note the reading.
8. The difference between readings gives the horizontal angle.

(b) Method of Repetition

Procedure

1. Measure the angle several times without resetting the instrument.
2. Add all measured angles.
3. Divide by the number of observations to get the average angle.

Advantages

• Improves accuracy.
• Reduces instrumental errors.

(c) Method of Reiteration

Procedure

1. Measure several angles around a point.
2. Sum of all angles should be 360°.
3. Distribute error if necessary.

Advantages

• Suitable for multiple angle measurement.
• Provides check on accuracy.

3. Important Terms

• Face Left (FL): Vertical circle on left side.
• Face Right (FR): Vertical circle on right side.
• Mean Angle: Average of FL and FR readings.

4. Precautions

• Ensure proper leveling and centering.
• Take readings in both FL and FR positions.
• Avoid parallax error.
• Handle instrument carefully.

5. Importance of Measuring Horizontal Angles

• Used in traversing and triangulation.
• Helps in preparation of maps and layouts.
• Essential for alignment of structures.

Role of Draughtsman Civil

A Draughtsman Civil uses horizontal angle data to prepare accurate plans and layouts. Proper understanding of measurement techniques ensures precise plotting and design.

Conclusion

Measurement of horizontal angles is a basic but important operation in theodolite surveying. Different methods like ordinary, repetition, and reiteration are used for accurate results.

For ITI Draughtsman Civil students, mastering these methods is essential for accurate surveying and successful engineering work.