For example, differential surveying notes should have the same number of backsights as foresights.
This equation reads, "absolute value of the sum of the backsights minus the sum of the foresights equals the absolute value of the initial elevation of the benchmark minus the closing elevation of the benchmark." The second equation is:
This equation reads, "the sum of the foresights minus the sum of the backsights equals the difference in initial elevation of the benchmark minus the closing elevation of the benchmark." The difference is in the use of the absolute values.
Note that when the instrument is set up halfway between the two stations, Figure 5-9, the error between the rod reading and horizontal is the same in the backsight as in the foresight.
In this illustration, the instrument is much closer to the rod for the backsight than for the foresight.
In common practice, a backsight would be recorded from the benchmark and the target would be set for the desired elevation of the forms.
Both profile and differential leveling use backsights, foresights, benchmarks, and turning points.
Note that the true foresights are stations that are paired with backsights. The note keeper must remember that the first backsight and the last foresight are also a pair.
It is a good idea to balance the sights for the backsights and true foresights.
Backsights are rod readings on a point of known elevation, usually either a benchmark or a turning point.
In differential leveling, each station is used as a backsight and a foresight.