Making minor adjustments and minor repairs to surveying instruments are among the responsibilities of EA personnel. Minor adjustments and minor repairs are those that can generally be done in the field using simple tools. Major adjustments and major repairs are those generally done in the factory. If the defect in the instrument cannot be corrected by minor adjustment or minor repair, do not attempt to disassemble it; instead, make necessary arrangements for sending the instrument to the manufacturer. Most surveying instruments are precision instruments for which major adjustments and recalibration require special skills and tools that can be provided only by the instrument company or its subsidiaries.

As stated previously, adjustment, as used in this chapter, means the process of bringing the various parts of an instrument into proper relationship with one another. The ability to make these adjustments is an important qualification of any surveyor. To make proper adjustments, the surveyor should have the following knowledge:

3. They must know the procedure for making adjustments and the correct sequence by which adjustments must be made.

4. They must be able to tell what effect the adjustment of one part will have on other parts of the instrument.

5. They must understand the effect of each adjustment upon the instrument when it is actually used for measurement.

Generally, instrument adjustments involve the level tubes, the telescope, and the reticle; for example, if one or both of the plate-level bubbles of an engineers transit are centered when the plate is, in fact, not level, the instrument is out of adjustment. An optical instrument equipped with vertical and horizontal cross hairs is out of adjustment if the point of intersection between the cross hairs does not coincide with the optical axis. If the reflected bubble on a Locke or Abney level is centered when the optical axis is other than horizontal, the instrument is out of adjustment.

The process of adjustment chiefly involves the steps that are necessary to bring a bubble to center when it should be at center or to bring a cross-hair point of intersection into coincidence with the optical axis. Instrument manufacturers publish handbooks containing recommended adjustment procedures. These are usually small pamphlets, obtainable free of charge. The following discussion is intended to give you an idea of general instrument adjustment procedures. For adjusting your particular instruments, however, you should follow the appropriate manufacturers instructions.

Instruments should be carefully checked periodically to determine whether or not they need adjustment. There is an adage that an instrument should be checked frequently but adjusted rarely. The basis for this adage is the fact that modern quality instruments get out of adjustment much less frequently than is generally believed; consequently, a need for adjustment is frequently caused by a previous improper adjustment that was not really required but resulted from errors in checking.

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.

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 engineers level and the transit. While the principles of performing the adjustments are nearly the same for one manufacturers 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 operators 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.