areas of mistakes. Errors cannot be completely
eliminated, but they can be minimized so that their
effect on the survey accuracy will be small and
within the tolerances permitted.
Identifying Leveling Mistakes
The leveling mistakes discussed here are not
intended to include all possibilities but will
give an idea of the more common ones. The
survey party personnel should be aware of these
possibilities and should be careful to avoid these
mistakes. Some of the common mistakes are as
1. Not setting the rod on the same point for
an FS and the following BS. Using a turning pin,
pedestal, stake, or marking the location with chalk
on hard surfaces will help you to recover the
2. Neglecting to clamp the target or the rod
when extended. Any slippage can pass unnoticed
and result in a wrong reading that may require
an entire rerun of the line to discover the mistake.
The rodman should watch the rod or target for
any movement as the clamp is tightened. The rod
extension or target should be read again after the
clamp has been set.
3. Reading the wrong mark. This is a common
mistake. The figures on a rod may be obscured
by brush or may fall in a position in the field of
view so that the instrumentman cannot see two
consecutive numbers. Under these conditions, he
may read the wrong mark or even read in the
wrong direction. This is a great possibility when
an inverting eyepiece is being used. For example,
if the figure 2 is the only number visible, the
instrumentman might read up the rod2.1,
2.2, 2.3 when actually he should be reading 1.9,
1.8, 1.7. Another possibility is miscounting the
number of divisions. There is no way to check or
discover these mistakes except to be aware of their
possibility and to read carefully.
4. Recording a reading in the wrong column.
In leveling, readings are not entered into the
notebook in a normal sequence, such as left to
right across the page. There is always a chance
that one or more values may be recorded in the
wrong column. The recorder must be alert to
avoid making this mistake.
5. Reading the wrong angle sign in trigono-
metric leveling. The instrumentman can accidentally
call out a wrong sign in reading the angle. This
type of mistake can be eliminated by the recorder
watching the telescope as a pointing is made on
the rod. If the wrong one is called out, both the
recorder and the instrumentman can resolve it
6. Recording the wrong sign. The sign varies
depending on whether the rod reading is a BS or
an FS, and whether the angle is a depression or
an elevation. Also, the difference in elevation
computation requires a sign reversal if the angle
is read for the BS, but not for the FS. These
variations can be confusing; the recorder has to
be careful to avoid mistakes. This can be done
by recording the angle and rod reading signs as
read. The sign conversion, if needed, shows up
when you compute the DE. Examining the
computations to see if all BS DEs have a sign
opposite to the angle sign is simple.
7. Subtracting the BS or adding the FS in
differential leveling. If the BS or FS is recorded
properly (see Number 4 above), you can discover
the mistake when you add the BS column and the
FS column for a computation check.
8. Using the wrong horizontal cross hairs.
This occurs on an instrument provided with stadia
Identifying Leveling Errors
Generally, errors cannot be totally eliminated,
but they can be contained within acceptable
tolerances. This requires you to use the prescribed
methods and instruments and apply corrections
established either mathematically or by experience.
Some of the conditions that produce errors are
1. Instrument not properly adjusted. A small
amount of residual error will always exist in any
adjustment. For the more accurate surveys, the
residual error can be minimized by using BS and
FS balancing and, in trigonometric leveling, by
taking direct and reverse (circle left and circle
right) readings for the angles.
2. Instrument not leveled properly. Unlike the
residual adjustment error that will affect the
readings one way consistently, this is a random
or accidental error. It may affect the line of sight
differently at each setup. This error can be
minimized only by careful leveling each time the
instrument is set up and by recentering the bubble
before each reading.
3. Telescope not focused properly. Misfocusing
and parallax in the eyepiece create accidental
errors that cannot be corrected. The only way to
avoid or minimize this error is to take care to
focus properly at each setup. The instrumentman