Two Peg test is also known as collimation test. This test is carried out to test whether the line of collimation is parallel to the axis of bubble tube or not. It is applied for the adjustment of the line of collimation.
Figure 11 Two peg
1.Two points A and B were chosen on a fairly leveled ground at a distance of 30m. Instrument was setat C which was exactlyat the midway of A and B.
2.Staffs were kept at points A and B and three wire readings were taken on the staff when the bubble was exactlycentered.
3.Difference in elevation was calculated between two points, i.e. A & B. The difference in two staff readings give the correct difference in elevation even if the line of sight is inclined as balancing of back sight and fore sight is well carried out.
4.The level machine was shifted to point D about 5m from A and three wire readings were observed on boththe staffskept at A &B.
5.The level was shifted to another point E about 5m from B and three wire readings were observed on boththe staffskept at A &B.
6.Again the differences in elevations were carried out. If the level difference obtained previously is equal to level difference obtained, line of collimation is
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parallel to the axis of bubble tube. In this case, the collimation error should be less than 1:10000.
7.If collimation error is greater than 1:10000, permanent adjustment of the level instrument should becarried out.
5.4COMMENTS AND CONCLUSION
The accuracy obtained was within the permissible range of 1:10000. Collimation error occursdue to the following reasons:
The ground was not well leveled.
The focusingpower of the instrument wasjust satisfactory.
There were some observational errors as the eye power of different observers is not same.
The staff graduation waspoor asitwas old.
There may be errors while measuring the distance between the two staff stations. As the observation was carried out in the morning, atmospheric conditions also playa vitalrole.