Differential leveling (also called direct leveling) is generally used in determining elevations of points to establish a chain or network of BMs for future use. It requires a series of instrument setups along the survey route; and for setup, a horizontal line of sight is established, using a sensitive level. The SEABEEs commonly use this type of leveling in determining elevation during construction surveys.

As shown in figure 14-13, the basic procedure used to determine elevations in a differential leveling operation is the same as previously discussed. First, you take a BS on a rod held on a point of known elevation (KE). Then add the BS reading to the known elevation to determine the HI. Next, take an FS on a rod held at the point of unknown elevation (UKE). Finally, subtract the FS reading from the HI to establish the elevation of the new point.

After you complete the FS, leave the rod on that point and move the instrument forward. Set up the instrument approximately MIDWAY between the old and new rod positions. The new sighting on the back rod becomes a BS, and you can now establish a new HI. The points other than the BMs or TBMs on which you hold the rods for the BSs and FSs are called TURNING POINTS (TPs). Other FSs made to points not along the main route are known as SIDESHOTS. You can use this procedure as many times as necessary to transfer a point of known elevation to another distant point of unknown elevation.

Figure 14-14 shows a sample differential leveling run. The rod is held on BM 35 (Elev. = 133.163). The level is set up midway between BM 35 and

TBM 16. The BS reading of +6.659 is added to the elevation of BM 35 and gives the resulting HI (139.822). The rod is moved to Peg 16 (which later becomes TBM 16). The FS reading of 4.971 is subtracted from the HI to get the elevation of Peg 16. Note that the distance (220 ft each way) is also recorded for balancing. The process continues until BM 19 is reached.