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rotated separately. The bubble will move towards any screw turned clockwise. It
always must be kept in mind that turning any screw on a three-screw level slightly
changes the HI. Never turn a leveling screw of a three-screw leveling head once a
BS reading has been taken and an HI established.

Fig.5.10 Leveling a three-screw instrument

C. Leveling Mistakes and Errors
As with any surveying operation, blunders must be eliminated and errors
minimized while running levels. Misreading the rod is a common blunder; it can be
avoided by always having the rod person check the reading with pencil point or
target. Note-keeping mistakes can be particularly troublesome. The computations
of HI and turning point (TP) elevation should be done in the field, as the work
progresses. A simple arithmetic check at the end of the leveling run can be made to
avoid addition or subtraction errors.

Random Errors
Unavoidable accidental errors may occur when running levels, for several
reasons. For example:
The level rod may not be precise when the reading is taken.
Heat waves from the ground make it difficult to read. On windy day, slight
vibration of the cross hair can cause small errors in the reading.
The instrument may be slightly out of level if the spirit level is not perfectly

centered.
Accidental errors can be minimized with a properly maintained and adjusted
instrument if the following steps are taken:
1. Make sure the tripod legs are secure and firmly anchored before leveling the
instrument.
2. Check to see that the bubble is centered before each reading; re-center it if
necessary.
3. Do not lean on the tripod legs when reading the rod.
4. Have the rod person use a rod level, to make sure it is held vertically.
5. Try to keep the line of sight about 0.5m above the ground when positioning
the instrument.
6. Do not use very long BS and FS reading.

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