Any substantial object may be used as a bench
mark. Figure 15-48, view A, shows typical
monuments set to mark important alignment points,
but which may also be used as bench marks. Spikes
may be driven into posts or power poles, as shown in
figure 15-48, view B, or chiseled into stone or concrete
structures, as shown in figure 15-48, view C. For
clarity, the marks are shown on the wing wall; but in
practice, one mark only is usually chiseled or
spray-painted on a flat surface.
The location, elevation, and description of bench
marks are usually shown on the project drawings or in
the surveyors field notes.
Once a bench mark is established, certain
formulas are used for determining elevations. You
first must figure the height of the instrument. This is
done by taking a reading on a level rod that is placed
on a known elevation, such as a bench mark. This is
known as a backsight (BS).
To determine the height (HI) of the instrument,
add the bench mark (BM) elevation to the backsight
(BS) reading from the level rod. This formula is
written as HI = BM + BS.
For example, as shown in figure 15-49, the bench
mark elevation is 100.00 feet. The backsight reading
Figure 15-48.Temporary bench marks.