▪ Two Peg Test is a surveying operation carried out to determine whether the levelling bubble and telescope line of sight are parallel.
▪ Peg Test is a surveying operation carried out to determine if the leveling bubble (bubble axis) and telescope line-of-sight (line of collimation) are parallel.
▪ By using the leveling method, work for establishing Temporary Bench Mark (TBM)
▪ Bench Mark (BM) must be known first. BM point determined by Land Surveyor is final.
▪ Distances between backsight and foresight must be more or less equivalent to avoid collimation errors.
▪ The survey work is to ensure so that the true error is smaller than the permissible error.
▪ Bench Mark (BM) is a fixed point on the Earth surface which its level is known height of collimation is line of collimation level from datum.
▪ Temporary Bench Mark (TBM) is a nearest point as reference if the Bench Mark (BM) is far from surveying area.
▪ Backsight is the first sight taken after temporary leveling of the instrument has been carried out.
▪ Foresight is last sight taken before the instrument is transferred to another point.
▪ Level and Tripod (1 set)
▪ Staff (2 sets)
▪ Staff bubble (2 sets)
▪ Tape (1 set)
▪ Each group will be required to perform a peg test to check the instrument.
▪ Each person in the group should record the results of the Peg Test in their own field book.
▪ Set out and mark on the ground (with wooden pegs driven into the Earth, or roofing nails in tar) two point some 30 m apart.
▪ Set up instrument midway between two pegs [setup 1].
▪ Read staff on each peg (backsight and foresight), and calculate height difference.
▪ Next, move instrument about L/10 = 3 m beyond one of the pegs (normally fore staff position, [setup 2]
▪ Read staff on each peg again, and calculate height difference.
| |Distance |Reading Staff B (S1) |Reading Staff A (S2) |Height Difference = m | |First set up |30 m |1.500 |1.460 |0.040 | | |60 m |1.395 |1.457 |0.062 |
| |Distance |Reading Staff B (S1’) |Reading Staff A (S2’) |Height Difference = m’ | |Second set up |30 m |1.410 |1.498 |0.070 | | |60 m |1.576 |1.320 |0.256 |
|Difference (m - m’) |30 m |60 m | | |0.03 |0.194 |
• For the first peg test
▪ The height difference for the first set up was 0.040m while for the second set up was 0.070m
▪ Difference for the both set up was 0.030m which exceed the acceptable error 0.002m.
• For the second peg test
▪ The height difference for the first set up was 0.062m while for the second set up was 0.256m
▪ Difference for the both set up was 0.194m which exceed the acceptable error 0.003m
• There are two possible factor that cause the difference exceed the acceptable error either reading error or equipment error.
• To reduce the error in reading level, repeated reading is suggest to get an accurate and precise reading.
• The level must in good condition to...