2. Wire Readings and Check

Figure G.2 shows shows a rod BS with the three wire readings.

		top: 4.31 middle: 4.16 bottom: 4.02
Figure G-2 Reading All Three Wires

Because the wires are equally spaced, the center reading should be the average of all three. The average may not exactly match the center reading because all three readings are subject to random errors.

Average = (4.31+4.16+4.02) / 3 = 4.163'   which closely matches the middle reading

The BS reading is 4.163. We're carrying an additional decimal place because:

It's the result of multiple readings

It conforms with significant figures rules; the reading sum is 12.49 which has 4 sig fig.

The stadia interval, i, is the difference between the top and bottom wires: i = 4.31-4.02 = 0.29'

If instrument's stadia multiplier is 100, then the horizontal distance to the rod is 100(0.29') = 29'

Except for reading all three wires, and field note modification, the process of three-wire leveling is the same as differential leveling:

Elev Inst = Elev_A + BS_A

Elev_B = Elev Inst - FS_B

In differential leveling, however, a reading error is hard to trap since only the center wire is read. In three-wire leveling, reading errors are immediately identified and can be corrected.