How to Use Theodolite Survey: Step-by-Step Guide

 How to Use Theodolite Survey: Step-by-Step Guide

A theodolite is a precision instrument used in land surveying to measure horizontal and vertical angles. Conducting a theodolite survey requires executing temporary adjustments (instrument calibration at the station) in strict order before taking angle readings.

Theodolite Setup & Surveying Procedure

1.Establish Station & Set Up Tripod:Prerequisite for instrument stability.

1. Mark the ground station point precisely using a surveyor's stake or nail.

2. Extend the tripod legs to eye level, spread them evenly over the station, and press the feet firmly into the ground.

3. Ensure the top mounting plate of the tripod is roughly horizontal by eye.

2.Mount & Center the Theodolite:Aligning vertical axis directly over station mark.

1. Securely attach the theodolite to the tripod head using the central mounting screw.

2. Attach a plumb bob to the hook beneath the instrument, or look through the optical/laser plummet.

3. Adjust the tripod legs or shift the instrument head until the plummet crosshair sits directly over the station nail.

3.Level the Instrument:Ensures vertical axis is truly vertical.

1. Rotate the upper plate until the plate bubble tube is parallel to any two levelling foot screws (screws A and B).

2. Turn both screws simultaneously—either both inward or both outward—until the bubble centers. (Note: The bubble moves in the direction of your left thumb).

3. Rotate the telescope $90^\circ$ so the bubble tube is perpendicular to screws A and B (directly over the third screw, C).

4. Turn screw C until the bubble centers.

5. Rotate back to the original position and repeat until the bubble remains centered in all $360^\circ$ rotations.

4.Eliminate Parallax (Focusing):Prevents sighting displacement errors.

1. Focus the Eyepiece: Point the telescope toward a clear sky or white sheet of paper. Rotate the eyepiece ring until the crosshairs appear sharp and dark.

2. Focus the Objective: Direct the telescope at your target (e.g., ranging rod or prism). Turn the side focusing screw until the target image appears crisp.

3. Move your eye slightly side-to-side; if the crosshairs appear to shift relative to the target, adjust the focusing screw until no movement (parallax) remains.

5.Measure Horizontal Angles:Determining bearings between two points.

1. Backsight Alignment: Direct the telescope toward your first target point (Station A).

2. Clamp the lower plate and set the horizontal Vernier/digital display to zero ($00^\circ 00' 00"$). Use the lower slow-motion tangent screw for fine targeting.

3. Foresight Alignment: Release the upper clamp, rotate the telescope clockwise to the second target point (Station B), and tighten the upper clamp.

4. Use the upper slow-motion tangent screw to align the vertical crosshair precisely on Station B.

5. Read the horizontal angle on the digital display or Vernier scale.

6.Measure Vertical Angles:Calculating elevation or depression angles.

1. Direct the crosshairs to the target point whose vertical angle is required.

2. Tighten the vertical clamp screw and use the vertical tangent screw to center the horizontal crosshair on the mark.

3. Read the vertical angle on the vertical circle display. Positive values indicate elevation; negative values indicate depression.

1. Required Instruments

• Theodolite
• Tripod stand
• Plumb bob or optical plummet
• Ranging rods
• Measuring tape
• Pegs and hammer
• Field book
• Pencil

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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.

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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.

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4. Fix Theodolite on Tripod

Place the theodolite on the tripod head.

Tighten the fixing screw properly.

Do not over-tighten.

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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.

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6. Levelling

Levelling means making the instrument horizontal.

Procedure:

1. Keep plate bubble parallel to two foot screws.
2. Turn both screws inward or outward together.
3. Bring bubble to the center.
4. Rotate instrument 90°.
5. Adjust third foot screw.
6. Repeat until bubble remains centered in all directions.

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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.

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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.

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9. Measure Horizontal Angle

Suppose angle AOB is required.

1. Set theodolite at station O.
2. Sight point A.
3. Set reading to zero.
4. Release upper clamp.
5. Rotate telescope toward point B.
6. Bisect point B exactly.
7. Read horizontal angle.

This reading is angle AOB.

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10. Measure Vertical Angle

1. Set and level the theodolite.
2. Sight the top or bottom point of object.
3. Read vertical circle.
4. Note whether angle is elevation or depression.

Use this for:

• Height of building
• Tower height
• Slope measurement
• Level difference

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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.

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12. Repetition Method for Horizontal Angle

Used for accurate angle measurement.

1. Set reading to 0° on point A.
2. Turn to point B and read angle.
3. Without resetting, sight A again.
4. Again turn to B.
5. Repeat 3 to 6 times.
6. Final reading ÷ number of repetitions = true angle.

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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

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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

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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.

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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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