(1) Two targets are set on each rod, one above and the other below the point on the rod

intersected by the center crosshair of the instrument when the bubble is leveled. Read the drum scale

with the telescope in three positions: first, with the middle thread bisecting the top target; second, with

the telescope level (bubble centered); and third, with the middle thread bisecting the bottom target. The

reading on the rod for a level line of sight can be determined, since the distance from the level line of

sight to the two targets will be proportional to the corresponding differences in the readings on the drum

scale.

(2) In Figure 4-10, A and B are the positions of the upper and lower targets on the rod, and C is

the point where the leveled line of sight intersects the rod. Assume that the readings of the drum scale

are 25.8 and 40.2 for A and B respectively, and the reading at C (telescope level) is 30.6. If the targets

(A to B) are 0.4000meter apart, the distance from target A to point C can be found in the following

formula:

If the setting of target A is 1.8930 meters, then the line of sight C intersects the rod at 1.7597 meters

(1.8930 - 0.1333).