A rod reading is taken on the existing benchmark, this is called a backsight, and on the new benchmark, the foresight.
The elevation of turning points is calculated as if they were benchmarks .A backsight is recorded for the benchmark, and a foresight is recorded for the turning point.
The instrument is leveled and a backsight of 8.47 feet is recorded at station A, as shown in Figure 5-14.
For the example, the backsight rod reading of 8.47 feet on station A in Figure 5-14 is placed in the row A and column BS.
The rod readings are recorded in the columns labeled BS, backsight, and FS, foresight.
If the instrument height is not changed, the backsight for the closure will be the same rod reading as the foresight used to determine the elevation of benchmark two.
The backsight on station A is not a problem, but the foresight to station B contacts the ground before reaching station B.
The first rod reading is a backsight on station A (see Figure 5-19).
This rod reading (8.19 feet) is recorded in the station A row of the backsight column and is used to determine the height of the instrument (108.19).
Each time the instrument is moved, it must be leveled and a backsight recorded to reestablish the height of the instrument (see Figure 5-21).
The reading of 10.97 feet is recorded in the TP1 row of the backsight column and is added to the elevation of TP1, 101.66 feet, to establish the height of the instrument (112.63 feet) (see Table 5-6).