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
Figure 14-13.-Differersthd leveling.
Figure 14-14-Sample field notes and profile of a differential-level circuit.