Many different types and variations of levels are used for surveying. The following procedure for setting up an engineer's level will also work with most other types. When not in use, the instrument is housed in a case for protection. To be used, it must be removed from this case and carefully threaded on the tripod head. The tripod legs are set about 4 ft apart and pushed firmly into the ground, thereby
Whole foot mark
Whole foot mark
FIGURE 15.7. Reading the rod using a target.
providing a stable base for the instrument and helping to ensure an accurate survey. If the tripod is set up with the head nearly horizontal, leveling of the instrument will be much easier and faster.
To level a four-legged (four leveling screws) instrument, the telescope is lined up over one pair of leveling screws, and the bubble is centered. The screws are turned in opposite directions. The leveling process is expedited when the left thumb rule is used, the bubble in the level moves in the direction of the left thumb. Then the telescope is lined up over the other pair of leveling screws and centered again. The procedure is repeated until the bubble remains centered for any position of the telescope. The leveling screws should not be over tightened, or they will bind and put excessive strain on the instrument frame, or left loose, the instrument will rock and not remain level.
To level a three-legged instrument, the telescope is aligned over any one of the three leveling screws. The instrument is leveled with the adjusting screws and the spirit level. Once leveled, the telescope is should be slowly rotated to check the accuracy of the leveling.
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