Now that you have learned how to setup and level the engineers level and how to read the leveling rod, let us take a look at an example that will explain the basic procedure of determining elevations during a leveling operation.

In figure 14-11, there is a BM at Point A with a known elevation of 365.01 ft. You wish to determine the elevation of a point on the ground at Point B. To do so, you first set up and level your engineers level approximately half-way between Points A and B. When the instrument is leveled properly, you will have a perfectly level line of sight that can be rotated all around the horizon.

The next thing to do is to determine the elevation of this line of sight. This elevation is called the HEIGHT OF INSTRUMENT, familiarly known as the HI. To obtain this elevation, the instrumentman takes a backsight (BS) on a leveling rod held on the BM and, in this example, obtains a rod reading of 11.65 ft. The HI, then, is the BM elevation PLUS the rod reading, or 365.01 + 11.56, which equals 376.57 ft. This means that no matter to which direction the telescope is trained, any point around the horizon that is intercepted by the horizontal cross hair has an elevation of 376.57 ft.

To determine the ground elevation at Point B, the instrumentman now takes a foresight (FS) on a rod held at Point B. This time, a rod reading of 1.42 ft is read. Since the elevation of the line of sight (HI) is 376.57 ft, obviously the ground elevation at Point B is the HI MINUS the rod reading, or 376.57 1.42, which equals 375.15 ft.

The balancing of the FS and BS distances is important in leveling. The effeet of curvature and refraction may be eliminated by a balanced BS and FS distance; however, instrumental error is a far more important reason for careful balancing.

Figure 14-11 Procedure for direct leveling

Figure 14-12.-Turning points.

"Balancing shots" means equalizing as much possible BS and FS distances by selecting turning points that are approximately an equal distance from both the BS and FS points. No matter how carefully you level a level telescope, it is likely to be still slightly out of the horizontal. The error this causes increases with the length of the sight taken. If the BS distance differs from the FS distance, the BS and FS errors will also differ. If the distances are the same, the errors will be the same. Balancing shots therefore eliminates the effect of instrumental error and also of curvature and refraction, other errors that increase with distance.

To balance distances for a setup, you will find that using the same number of paces for BS as for FS is helpful. In general, BS and FS distances should be kept under 300 ft except when necessary to pass or cross an obstacle.