Two Peg Test Report

TITLE: TO PERFORM A TWO PEG TEST AND DETERMINE COLLIMATION ERROR

OBJECTIVES:

• To check if the line of sight of a leveling instrument is parallel to the horizontal line of collimation.
• To calculate collimation error and apply corrections if necessary.

EQUIPMENT USED:

Leveling instrument, staff, pegs/marker, tape

THEORY:

The two-peg test identifies collimation error in a leveling instrument. If the line of sight is not horizontal, readings will be erroneous. By comparing the true and apparent height differences between two fixed pegs, the error is quantified and corrected.

PROCEDURE:

1. Drive pegs A and B at a known distance D (e.g., 30m).
2. Set up the level at midpoint P (D/2 from A and B).
3. Record staff readings a (at A) and b (at B).
4. Move the level to point P₁ (distance d = D/10 from A).
5. Record new readings a₁ (at A) and b₁ (at B).
6. Compare true difference (a – b) and apparent difference (a₁ – b₁).
7. Calculate collimation error and corrections if needed.

OBSERVATION:

Parameter	Value (m)
Distance D (A to B)
Distance d (P₁ to A)
Staff reading at A from P (a)
Staff reading at B from P (b)
Staff reading at A from P₁ (a₁)
Staff reading at B from P₁ (b₁)

CALCULATION:

1. True difference (ΔH) = a – b = ______  (+ve if Rise & -ve if Fall from A to B)
2. Apparent difference (ΔH') = a₁ – b₁ = ______
3. If true difference = apparent difference, the line of collimation is in adjustment. If not, it is inclined.
4. Error = |ΔH – ΔH'| = ______
5. Expected reading at B (e) = a₁ ± ΔH = ______
   • Use +ΔH if true slope is a fall from A to B.
   • Use −ΔH if true slope is a rise from A to B.
6. Collimation error direction:
   • If b₁ > e: Line of collimation is inclined upward.
   • If b₁ < e: Line of collimation is inclined downward.
7. Collimation corrections:
   • C₁ = (d/D) × (b₁ – e) = ______ (correction for A).
   • C₂ = [(d + D)/D] × (b₁ – e) = ______ (correction for B).
8. Corrected staff readings: (+ve for downward, -ve for upward)
• At A: a₁ ± C₁ = ______
• At B: b₁ ± C₂ = ______

RESULT:

CONCLUSION:

This test confirmed the instrument's collimation error. Corrections were applied to ensure accurate leveling measurements.