(27) Enter the difference between (22) and (26), and divide this difference by 3. In this case, the
backsight readings are larger; otherwise, the value in (22) would be carried over to the right-hand page
and subtracted from (26).
(28) Enter the quotient. The quotient is a check against the difference of elevation obtained by
subtracting (29) from (23) and should agree within one or two millimeters.
(29) Enter the sum of the mean foresight readings.
(30) Enter the DE obtained by subtracting (29) from (23) with proper sign (backsight - foresight =
(31) Enter the sum of the back of the rod readings for the foresight rods.
(32) Enter the difference in elevation in feet. Obtain this figure by subtracting (31) from (24) and
multiplying by 0.3048 to get the difference in meters. This value should agree approximately with the
DE shown in (30).
(33) Enter the final sum of the foresight thread intervals. This sum must not differ from the
backsight sum (25) by more than 20 meters (60 intervals for a level with a stadia constant of 1/300 and
67 intervals for a level with a stadia constant of 1/333).
(34) Enter the length of section in kilometers. This is the sum of (33) and (25) multiplied by the
stadia constant of the instrument and divided by 1000.
(35) Enter the final foresight. This reading was taken on benchmark 2.
c. Instructions for Obstacle Crossings. When crossing a river, ravine, or other obstacle requiring a
line of sight greater than 75 meters, make simultaneous reciprocal observations from each of two points,
one on each side of the obstacle. Ensure that backsights and foresights are balanced, and establish a
TBM near the edge of the obstacle. Establish another TBM on the far side of the obstacle, at about the
same elevation. Place rods, equipped with two targets each, on each of the TBMs. Place the
instruments, one on each side of the obstacle, where they can be directed on both rods and form a
closed-circuit crossing as shown in Figure 4-9, page 4-22.