Surveying - Carpenter's and Builder's Math, Plans, and Specifications - page 200

4

LEVEL

5

4 2

3 F T

3 1

4

5 F T

STATION
B

2

3 6 F T

2 1

STATION
A

1

B.M.

Figure 6-1 The terms backsight and foresight do not neces-
sarily mean backward and forward. Readings taken on a point
of known elevation (such as a benchmark or a turning point)
are called backsights, whereas readings taken on a point of un-
known elevation are termed foresights. In the illustration, the
backsight (to the benchmark) is + 6feet 0 inches, and the fore-
sight to station A is − 5feet 0 inches. The difference in the two
elevations is 6 feet − 5feet = 1foot. If the reading at station A
had been greater than the 6-foot HI (height of instrument), the
calculation would have shown a negative result, thereby indicat-
ing that station A was lower than the benchmark. The elevation
at station B is calculated in the same manner: 6 − 3 = 3feet.
Therefore, the elevations from any number of points can be
obtained in the same way, if they can be seen from the same po-
sition of the instrument. If they cannot, a new HI must be used.

Figure 6-2 A typical builder's level. (Courtesy David White Instruments, Division

of Realist, Inc.)