Leveling Procedure
A typical procedure is to set up the instrument within 100 meters (100 yards) of a known or assumed reference elevation point. A rod or staff is held on the point and the instrument is used either manually or automatically to read the rod scale as a backsight. This determines the height of the instrument above the point and allows the height of the instrument (H.I.) above the datum to be computed.
The rod is then held on an unknown point and a foresight reading is taken in the same manner, allowing the elevation of the new point to be computed. The procedure is repeated until the destination point is reached. It is usual practice to perform either a complete loop back to the starting point or else close the traverse on a second point whose elevation is already known. The closure check guards against blunders in the operation, and allows residual error to be distributed in the most likely manner among the stations.
Some instruments provide three crosshairs which allow stadia measurement of the foresight and backsight distances. These also allow use of the average of the three readings (3-wire leveling) as a check against blunders and for averaging out the error of interpolation between marks on the rod scale.
The two main types of levelling are single-levelling as already described, and double-levelling (Double-rodding). In double-levelling, a surveyor takes two foresights and two backsights and makes sure the difference between the foresights and the difference between the backsights are equal, thereby reducing the amount of error. Double-levelling costs twice as much as single-levelling.
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