Test

Two pegs are set at some distance (of about 60 to 90 m) on a fairly level ground. A dumpy level is set up on a point which is equidistant from the pegs and preferably, in a line with the pegs. Staff readings are taken at the pegs, say the readings are a and b respectively. Then, the true difference in elevation between the points is h = (ab). Now, the instrument is set on the line joining the pegs near one of the pegs but opposite to the other peg, as shown in Figure 12.5. Let D₁ and D₂ are the distances of the near and far peg from the instrument position. Staff readings are again taken at the pegs, say the readings are c and d respectively. Then, the apparent difference in elevation between the points is h' = (cd). Now, if h' is found to be equal to h, the line of sight of the level is parallel to the axis of the bubble tube. Otherwise, an adjustment of the bubble tube is required.

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