grid points within range of the optical level have been measured, it is
necessary to move the optical level to a new location.
There is a chance that,
in its new location, the optical level will be higher or lower than it was in
its old location.
it is necessary to:
a measurement (fore sight) from a
point before the optical level is moved.
a measurement (back sight) from the
same point once the optical level is set up in its new position.
- Calculate the difference in elevation between
the old and new location of the optical level.