Through normal use and wear, all leveling instruments will likely become malad justed from time to time. The need for some adjustments may be noticed during use, for example, level vials on tilting levels. Others may not be so obvious, and therefore it is important that instruments be checked periodically to determine their state of adjustment. If the tests reveal conditions that should be adjusted, depending on the particular instrument, and the knowledge and experience of its operator, some or all of the adjustments can be made immediately in the field. However, if the parts needing adjustment are not readily accessible, or if the operator is inexperienced in making the adjustments, it is best to send the instruments away for adjustment by qualified technicians.
Requirements for Testing and Adjusting Instruments
Before testing and adjusting instruments, care should be exercised to ensure that any apparent lack of adjustment is actually caused by the instrument’s condition and not by test deficiencies. To properly test and adjust leveling instruments in the field, the following rules should be followed:
- Choose terrain that permits solid setups in a nearly level area enabling sights of at least 200 ft to be made in opposite directions.
- Perform adjustments when good atmospheric conditions prevail, preferably on cloudy days free of heat waves. No sight line should pass through alternate sun and shadow, or be directed into the sun.
- Place the instrument in shade, or shield it from direct rays of the sun.
- Make sure the tripod shoes are tight and the instrument is screwed onto the tripod firmly. Spread the tripod legs well apart and position them so that the tripod plate is nearly level. Press the shoes into the ground firmly.
Standard methods and a prescribed order must be followed in adjusting surveying instruments. Loosening or tightening the proper adjusting nuts and screws with special tools and pins attains correct positioning of parts. Time is wasted if each adjustment is perfected on the first trial, since some adjustments affect others. The complete series of tests may have to be repeated several times if an instrument is badly off. A final check of all adjustments should be made to ensure that all have been completed satisfactorily.
Tools and adjusting pins that fit the capstans and screws should be used, and the capstans and screws handled with care to avoid damaging the soft metal. Adjustment screws are properly set when an instrument is shipped from the factory. Tightening them too much (or not enough) nullifies otherwise correct adjustment procedures and may leave the instrument in worse condition than it was before adjusting.
Adjusting for Parallax
The parallax adjustment is extremely important, and must be kept in mind at all times when using a leveling instrument, but especially during the testing and adjust ment process. The adjustment is done by carefully focusing the objective lens and eyepiece so that the crosshairs appear clear and distinct, and so that the crosshairsdo not appear to move against a background object when the eye is shifted slightly in position while viewing through the eyepiece.
Testing and Adjusting Level Vials
For leveling instruments that employ a level vial, the axis of the level vial should be perpendicular to the vertical axis of the instrument (axis about which the instrument rotates in azimuth). Then once the bubble is centered, the instrument can be turned about its vertical axis in any azimuth and the bubble will remain centered. Centering the bubble and revolving the telescope 180° about the vertical axis can quickly check this condition. The distance the bubble moves off the central position is twice the error. To correct any maladjustment, turn the capstan nuts at one end of the level vial to move the bubble halfway back to the centered position. Level the instrument using the leveling screws. Repeat the test until the bubble remains centered during a complete revolution of the telescope.
Preliminary Adjustment of the Horizontal CrossHair
Although it is good practice to always sight an object at the center of the crosshairs, if this is not done and the horizontal crosshair is not truly horizontal when the instrument is leveled, an error will result. To test for this condition, sight a sharply defined point with one end of the horizontal crosshair. Turn the telescope slowly on its vertical axis so that the crosshair moves across the point. If the crosshair does not remain on the point for its full length, it is out of adjustment.
To correct any maladjustment, loosen the four capstan screws holding the reticle. Rotate the reticle in the telescope tube until the horizontal hair remains on the point as the telescope is turned. The screws should then be carefully tightened in their final position.
Testing and Adjusting the Line of Sight
For tilting levels, described in Section 4.9, when the bubble of the level vial is centered, the line of sight should be horizontal. In other words, for this type of instrument to be in perfect adjustment, the axis of the level vial and the line of sight must be parallel. If they are not, a collimation error exists. For the automatic levels, described in Section 4.10, after rough leveling by centering the circular bubble, the automatic compensator must define a horizontal line of sight if it is in proper adjustment. If it does not, the compensator is out of adjustment, and again a collimation error exists. The collimation error will not cause errors in differential leveling as long as backsight and foresight distances are balanced. However, it will cause errors when backsights and foresights are not balanced, which sometimes occurs in differential leveling, and cannot be avoided in profile leveling, and construction staking.