Double rodding is a form of differential leveling in which a continuous check is maintained on the accuracy of the leveling procedure. It is called double rodding because it can be done most conveniently by two rodmen. However, it is possible to carry out the procedure using only one rodman.

In double rodding, you determine the HI at each setup point by backlights taken on two different TPs. If no mistake or large error has been made, the result will be two HIs that differ slightly from each other. Elevations computed this way will also differ slightly. In each case, the average is taken as the elevation. 

Figure 14-21 shows double-rodded level notes for a run from one BM to another by way of three

Figure 14-21.-Sample field notes from double-rodded levels.

intermediate TPs. In each case, a "higher" TP (as TP1) and a "lower" TP (as TP~~) was used, resulting in two different HIs for each. Computed by way of the higher HIs, the elevation of BM2 came to 851.98 ft. Computed by way of the lower HIs, it came to 852.00 ft. The mean (average) of 851.99 ft was taken as the correct elevation.

Indirect methods of leveling encompass both trigonometric and barometric leveling. TRIGONOMETRIC LEVELING uses vertical angles and a horizontal distance to compute the difference in elevation, BAROMETRIC LEVELING uses the difference in atmospheric pressures that are observed by a barometer or an altimeter to determine the elevation differences. Indirect methods of leveling will be discussed at the EA2 level.