direct sun rays as these may cause the bubble to run off.
2.2 Levelling up with the automatic index
Proveded the instrument set-up is very stable, it is possible to level up with the
assistance of the automatic index. With this method, levelling up to about ±1″ to
±2″is possible, it is especially useful for horizontal angle measurement with steep
Level up with plate level then proceed as follows:
a. With alidade in any position, tighten vertical clamp and then read vertical circle.
Vertical clamp and drive must not be touched.
b. Turn alidade through 180°. Read vertical circle. Compute the mean of the vertical
circle readings taken in steps a and b. Now, set micrometer to give the minutes and
seconds of this mean reading.
c. Turn alidade until telescope is parallel to the line joining any two footscrews A and
B. Turn A and B very carefully, by equal and opposite rotations, until the circle
graduation lines in top window of reading microscope coincide. That is until mean
vertical circle reading calculated in step b is set.
d. Turn alidade through 90°. Turn footscrew C until circle graduation lines coincide.
e. Repear until graduation lines are in coincidence for all positions of the alidade, i.e.
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