3. Inexact setting or reading of a vernier. The
use of a small, powerful pocket magnifying glass
is helpful here. Also, when you have determined
the vernier graduation that most nearly coincides
with a limb graduation, it is a good idea to check
your selection by examining the graduations on
either side of the one selected. These should fall
in coincidence with the limb counterparts by about
the same amount.
4. Failure to line up the vertical cross hair with
the true vertical axis of the object sighted. The
effect is similar to that of not plumbing exactly
over the station, which means that the error
increases drastically as the length of the sight
5. Failure to bring the image of the cross hair
or that of the object sighted into clear focus
(parallax). A fuzzy outline makes exact alignment
Common mistakes in transit work are the
1. Turning the wrong tangent screw. For
example, by turning the lower tangent screw
AFTER taking a backsight, you will introduce an
error in the backsight reading.
2. Forgetting to tighten the clamp(s), or a
clamp slipping when it is supposed to be tight.
3. Reading in the wrong direction from the
index (zero mark) on a double vernier.
4. Reading the wrong vernier; for example,
reading the vernier opposite the one that was set.
5. Reading angles in the wrong direction; that
is, reading from the outer row rather than the
inner row, or vice versa, on the horizontal scale.
6. Failure to take a full-scale reading before
reading the vernier. For example, you may drop
20 to 30 min from the reading, erroneously
recording only the number of minutes indicated
on the vernier, such as 15°18´ instead of 15°48´.
Do not be so intent on reading the vernier that
you lose track of the full-scale reading of the
CARING FOR AND MAINTAINING
The accuracy and quality of a survey depend
upon the condition of the surveying instrument
and the experience of the surveyor. The life
expectancy and usefulness of an instrument
can be extended considerably by proper and
careful handling, stowing, and maintenance.
Undoubtedly, by simply working in your rating
conscientiously, you will become experienced in
the proper use of the instrument.
As stated earlier, every instrument is accom-
panied by an instruction manual that tells you not
only the proper operation and components of the
instrument but also procedures for its proper care
and maintenance. Study this instruction manual
thoroughly before you even attempt to use the
Carrying and Stowing
Every transit, theodolite, or level comes
equipped with a carrying box or case. The instru-
ment and its accessories can be stowed in the case
in a manner that ensures a minimum of motion
during transportation. The instrument should
ALWAYS be stowed in the carrying case when
it is not in use.
Cleaning and Lubricating
In general, all surveying instruments, equip-
ment, or tools must be cleaned thoroughly
immediately after you have used them. For
example, you dust off the transit or theodolite and
wipe it dry before placing it back in its case after
each use. Remove all dust with a clean cloth. This
applies particularly to the optical parts. Chamois
leather is suitable for this purpose, but it is
better to use a clean handkerchief than a soiled
chamois leather. Use no liquid for cleaning
neither water, petrol, nor oil. If necessary, you
can breathe on the lenses before polishing them.
When the instrument becomes wet, you should
remove its case and dry it thoroughly at room
temperature as soon as you get home. If you leave
the instrument in the closed case, the air inside
the hood will take up humidity by increasing
temperature and will in time diffuse inside the
instrument. While cooling off, the water will
condense and form a coating or tarnish that may
make any sighting with the telescope and reading
of the circles difficult.
Remove any mud or dirt that may adhere to
the tripod, range pole, level rod, and so forth,
after each use. Clean each instrument, equipment,
or tool after each use to eliminate the chance of
forgetting it. This is important, especially when
the surveying gear is made of a material that is
susceptible to rust action or decay.
When lubricating the instruments, you must
use the recommended lubricant for each part in
conjunction with the climatic condition in your
area. For instance, it is recommended that