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two peg test inspecting line of sight

In new levels, the com- pensator has been adjusted at room temperature, so that the line of sight is hori- zontal even if the instru- ment is tilted slightly. This situation changes if the temperature fluctuates by more than ten or fifteen degrees, after a long jour- ney, or if the instrument is subjected to strong vibra- tion. 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 metres 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 from both staffs and calculate the height difference (illustration above). Staff reading A = 1.549 Staff reading B = 1.404 ∆H = A – B = 0.145 4. Set up the instrument about one metre 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 - ∆H = 0.145 Required reading B = 1.351 6. Take the staff reading B. If it differs from the required reading by more than 3mm, adjust the line of sight (refer to instruction manual).
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