Before assuming that adjustment is necessary, you
must positively ascertain that an apparent maladjust-
ment actually exists. The following procedures apply, in
general, to all tripod-mounted optical instruments that
you may use in surveying:
1. Check the instrument on a cloudy day, if
possible.
2. Ascertain that the tripod shoes are tight and that
the instrument is screwed all the way down on the tripod.
3. Set the tripod up on firm ground in the shade,
but in a good light, where a sight of at least 200 feet can
be taken in opposite directions.
4. Spread the tripod feet well apart and place them
so that the plate is approximatel y level. Press the shoes
in firmly, or set them in cracks or chipped depressions
if on a hardened surface. (Avoid setting up on asphalt
pavement in warm weather.)
5. After the tripod feet are set, release and then
retighten the wing nuts. The purpose of this is to release
any possible residual friction that, if not released, might
cause an eventual shift in the legs.
6. Level the instrument with particular care. After
leveling, loosen all level screws slightly (again to
release residual friction) and relevel. Tighten all screws
with equal firmness but avoid overtightening. Too much
tightness will eventually deform the centers, causing
both friction and play.
7. Carry out all checks in the order prescribed for
the instrument. Do NOT make an adjustment unless the
same check, repeated at least three times, indicates the
same amount of error every time.
8. Remember that most tests show an error that is
double the actual displacement error in the instrument.
Be especially watchful for creep; that is, a change
in position caused by settlement or by temperature
change in the instrument. To detect any possible creep,
you should allow every set bubble or setline of sight to
stand for a few seconds and ensure that no movement
occurs during the interval.
Before an adjustment is made, consider whether or
not the error discovered will have a material effect on
field results. Make adjustments in a prescribed order.
After making an adjustment, retighten the adjusting
parts firmly but not too tightly. Then repeat the original
check and readjust if necessary. After making all the
contemplated adjustments, repeat the entire round of
checks in the prescribed order. This will indicate
whether or not an adjustment has been disturbed by a
subsequent adjustment.
In the following sections, we will discuss the field
tests and adjustments that you need to know how to
perform for the engineer’s level and the transit. While
the principles of performing the adjustments are nearly
the same for one manufacturer’s level or transit as
compared to those produced by another manufacturer,
there are some differences in detail. For this reason,
when preparing to perform an adjustment to an
instrument, you should first consult the operator’s
manual for that instrument.
When a high degree of accuracy is required for
surveying results, the level or transit used must be in
perfect adjustment. In this event, you must perform the
tests described in the following sections and make any
necessary adjustments to the instrument. When results
of lower accuracy can be tolerated, however, you can
usually compensate for the maladjustment of a part until
a proper adjustment can be made. Therefore, at the end
of each of the following instrument-adjustment
discussions, a method of compensating for the
maladjustment is noted. You should keep in mind,
however, that if you frequently check your instruments
and keep them in good adjustment, these compensations
should seldom be necessary.
Engineer’s Level Adjustments
Regardless of how well an engineer’s level is
manufactured, you should perform certain checks and
field adjustment at regular intervals; for example, you
should test the instrument every day before starting
work. You also should check it for proper adjustment
anytime the level is bumped or jolted. The parts of the
level that you will check are the level tube and the cress
hairs. For the latter, be sure that parallax is removed and
that the cross hairs and objective are sharply focused.
To do this, use a well-defined object at least 250 feet
away. When parallax is present, the image is not exactly
in the plane of the cross hairs, and the objective focusing
must be refined. Since this condition can occur each
time the objective lens is focused, you must make a
parallax check each time you observe a new object.
When adjusting the engineer’s level, it is important
that you accomplish the tests and adjustments in a
prescribed sequence. The reason for this is that one
adjustment may depend upon, or alter, another
adjustment. The following paragraphs describe, in
proper sequence, the test and adjustment procedures that
you should follow when checking and adjusting the
engineer’s level.
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