Two Peg Test is done to verify the calibration and accuracy of a dumpy level or laser level. It measures if the machine's line of sight is perfectly horizontal. If there is a discrepancy, the error is called the collimation error.
It can be done by following this simple, step-by-step process.
• Select Two Points (A and B) with 20-50 meters apart in a fairly levelled ground.
• Set the instrument at midpoint (L/2) of points A and B .
• Take staff readings at point A and B. Let the reading be a1 and b1 respectively.
• Calculate True Difference h1= a1-b1
• Move and set up the Instrument near to one peg.
• Take staff readings at point A and B. Let the reading be a2 and b2 respectively.
•Calculate Apparant Difference h2= a2-b2
•Now, Calculate Collimation Error E = h1-h2
• Calculate Precision P = Error/Distance
The permissible error of the two-peg test generally ranges from 2 mm to 5 mm and precision ranges from 1/10000 over a 30-meter to 50-meter test distance, depending on your required surveying tolerance. If the error exceeds this limit, the instrument must be calibrated or adjusted.
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