surveying a concrete floor by levelling, after all 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 between the old and new location of the optical level before it is moved.
a measurement (back sight) from the
same point once the optical level is set up in its new position.
the difference in elevation between the old and new optical level setup.