is 5.5 feet. The bench mark elevation, added to the
backsight reading, gives a instrument height of 105.5
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.
Figure 15-49.Backsight reading to determine instrument height.
Figure 15-50.Foresight reading to determine elevation.