A foresight (FS) is a rod reading taken on any point of unknown elevation. In differential leveling, there is only one foresight for each instrument setup, whereas profile leveling may have several foresights per instrument setup.
The FS is subtracted from the HI to find the elevation of an unknown point. In Figure 15.3, the foresight rod reading, 3.21 ft, is subtracted from the instrument height, 105.01 ft to determine the elevation at the unknown point, 101.8 ft.
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