Surveying - Carpenter's and Builder's Math, Plans, and Specifications - page 221

4. Measure the height of the instrument. The difference between
this and the last rod reading should equal the difference of
elevation as previously determined. If it does not, correct the
error halfway by means of the attached level-adjusting screw.

5.

Repeat the operation, and verify the adjustment.

Fifth Adjustment
Make the vernier of the vertical circle read zero when the bubble of
the attached level is centered.

1. Level up the instrument.
2. Sight on a well-defined point, and take note of the reading on
the vertical circle.
3. Turn the vernier plate one-half of a revolution, and also turn
the telescope vertically one-half of a revolution.
4. Again, sight on the same point.
5. Read and record the reading on the vertical circle.

One-half the difference of the two readings is the index error. The
error may be corrected by moving either the vernier or the vertical
circle, or the error may be noted and applied as a correction to all
measurements of vertical angles.

Adjustments of the Compass
It will sometimes be necessary to adjust the compass. When an ad-
justment is required, it may be accomplished by using the following
procedure.

First Adjustment
Straighten the needle.

1. Examine to see if the ends of the needle are set on opposite
divisions. If not, fix the pivot so that they are.
2. Revolve the box by one-half of a revolution. If the needle does
not set on opposite divisions, bend both ends by one-half the
difference.

Second Adjustment
Place the pivot in the center of the plate. If the needle is straight,
move the pivot until the needle sets on opposite divisions at points
such as 0 ◦ ,45 ◦ , and 90 ◦ .