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. 6-3