Inspecting the Line of Sight (two-peg test)

Surveying Made Easy

Preparing to Measure | 7 Inspecting the Line of Sight (two-peg test) In new levels, the compensator has been adjusted at room temperature, so that the line of sight is horizontal even if the instrument is tilted slightly. This situation changes if the temperature fluctuates by more than ten or fifteen degrees, after a long journey, or if the instrument is subjected to excessive vibration. It is then advisable to inspect the line of sight, particularly if more than one target distance is being used. 1. In flat terrain, set up two staffs not more than 30 m (95 ft) apart. 2. Set up the instrument so that it is equidistant from the two staffs (it is enough to pace out the distance) 3. Read off both staffs and calculate the height difference (illustration below). Staff reading A = 1.549 Staff reading B = 1.404 iH = A – B = 0.145 4. Set up the instrument about one meter (3 ft) in front of staff A and take the staff reading (illustration below). Staff reading A = 1.496 5. Calculate the required reading B: Staff reading A = 1.496 – iH = 0.145 Required reading B = 1.351 6. Take the staff reading B. If it differs from the required reading by more than 3 mm (1/10 in), adjust the line of sight (refer to instruction manual). 1.549 1.404 Actual 1.496 Required 1.351


Surveying Made Easy
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