Figure  15-49.Backsight  reading  to  determine  instrument  height.

is 5.5 feet. The bench mark elevation, added to the backsight reading, gives a instrument height of 105.5 feet. NOTE:  Since   the   backsight   (BS)   reading   is added  to  the  elevation  of  the  bench  mark  (BM)  to obtain the instrument height, it is usually called a plus (+) sight. Once  the  height  (HI)  of  the  instrument  is established, you can determine the elevation of any To determine the elevation of a point after the height (HI) has been established, place the level rod on the  point  in  question  and  take  a  reading  through  the level. This sighting is called the  foresight (FS) reading and is subtracted from the height to obtain the elevation of  the  point.  The  formula  for  determining  elevation  is as follows: HI - FS = EL For example, as shown in figure 15-50, the height (HI) of the instrument is 105.5 feet. The foresight (FS) reading is 2.3 feet. The instrument height minus the foresight reading gives that point an elevation of 103.2 point within the instrument range. feet. Figure  15-49.—Backsight  reading  to  determine  instrument  height. Figure  15-50.—Foresight  reading  to  determine  elevation. 15-29


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