A rod is used to measure the vertical distance between the line of sight, established by the instrument, and the object on which the rod is resting. To provide an accurate reading it must be held in an upright position. Failure to hold the rod in a vertical position is a common error in surveying. The easiest way to ensure that the rod is vertical is to use a rod level. If a rod level is not available, the instrument person (person on the instrument) can determine if the rod is vertical to the left or the right by instructing the rod person to move the rod until it is aligned with the vertical cross-hair of the instrument. The fore and aft alignment must be controlled by the rod person. When a rod level is not available, the fore and aft vertical position of the rod can be determined by rocking the rod. When rocking the rod the rod person slowly rocks the rod forward and backward past center. The instrument person records the shortest reading. This is illustrated in Figure 15.5.
The shortest reading will occur when the rod is vertical. The distance D1 will always be less than distance D2.
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