How to Use Theodolite Survey: Step-by-Step Guide
1. Required Instruments
- Theodolite
- Tripod stand
- Plumb bob or optical plummet
- Ranging rods
- Measuring tape
- Pegs and hammer
- Field book
- Pencil
2. Select Survey Station
Choose a firm and clear point on the ground.
Example:
Station O is the point where the theodolite will be set.
Mark it with a peg.
3. Set Tripod
Open the tripod legs and place it over the station point.
Keep tripod head roughly level.
Press tripod legs firmly into the ground.
4. Fix Theodolite on Tripod
Place the theodolite on the tripod head.
Tighten the fixing screw properly.
Do not over-tighten.
5. Centering
Centering means placing the instrument exactly above the station point.
Use:
- Plumb bob
- Optical plummet
Adjust tripod legs until the plumb bob is exactly over the station mark.
6. Levelling
Levelling means making the instrument horizontal.
Procedure:
- Keep plate bubble parallel to two foot screws.
- Turn both screws inward or outward together.
- Bring bubble to the center.
- Rotate instrument 90°.
- Adjust third foot screw.
- Repeat until bubble remains centered in all directions.
7. Focusing
Eyepiece Focusing
Look through the telescope toward the sky or white paper.
Adjust eyepiece until cross hairs are clear.
Object Focusing
Sight the ranging rod.
Turn focusing screw until target is clear.
8. Set Initial Reading
Clamp the lower plate.
Set horizontal circle reading to 0°00′00″ if possible.
Sight the first point A.
Tighten clamp and use tangent screw for exact bisection.
9. Measure Horizontal Angle
Suppose angle AOB is required.
- Set theodolite at station O.
- Sight point A.
- Set reading to zero.
- Release upper clamp.
- Rotate telescope toward point B.
- Bisect point B exactly.
- Read horizontal angle.
This reading is angle AOB.
10. Measure Vertical Angle
- Set and level the theodolite.
- Sight the top or bottom point of object.
- Read vertical circle.
- Note whether angle is elevation or depression.
Use this for:
- Height of building
- Tower height
- Slope measurement
- Level difference
11. Face Left and Face Right Observation
For better accuracy, take readings in both positions.
Face Left
Vertical circle is on the left side of the observer.
Face Right
Transit the telescope and rotate the instrument.
Take the same reading again.
Average reading gives more accurate result.
12. Repetition Method for Horizontal Angle
Used for accurate angle measurement.
- Set reading to 0° on point A.
- Turn to point B and read angle.
- Without resetting, sight A again.
- Again turn to B.
- Repeat 3 to 6 times.
- Final reading ÷ number of repetitions = true angle.
13. Field Booking Format
| Station | Face | Sighted Point | Horizontal Reading | Vertical Reading | Remarks |
|---|---|---|---|---|---|
| O | Left | A | 0°00′00″ | — | Back sight |
| O | Left | B | 65°30′20″ | — | Fore sight |
| O | Right | A | 180°00′00″ | — | Back sight |
| O | Right | B | 245°30′10″ | — | Check |
14. Common Uses on Site
- Road alignment
- Building layout
- Column line marking
- Bridge alignment
- Railway curve setting
- Canal alignment
- Measuring angles
- Finding height of objects
15. Important Precautions
- Tripod must be stable.
- Instrument must be exactly centered.
- Bubble must remain centered.
- Avoid windy conditions.
- Do not touch tripod while reading.
- Take Face Left and Face Right readings.
- Record readings immediately.
- Check zero error before work.
Conclusion
Theodolite survey is used to measure accurate horizontal and vertical angles. Correct steps are setting, centering, levelling, focusing, sighting, angle reading, and field booking. For high accuracy, always take readings on both face left and face right.
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