3
tangent screw, and then center the bubble by using the two leveling screws. Rotate the
telescope again 180° in azimuth. The bubble should remain centered. If it does not,
repeat the process of bringing it halfway to center with the tangent screw and the rest of
the way with the leveling screws. When the bubble remains centered after the telescope
is rotated, bring the telescope over the other pair of leveling screws and bring the bubble
to center by using the leveling screws only. This makes the vertical axis of the transit
truly vertical.
•
You may still have an error if the sighting is not parallel with the telescope bubble, or the
vertical vernier is off. You can detect this by making two sightings of the same distant
feature with the telescope in normal position and reverted position (rotate the telescope
vertically so the telescope bubble is on top) and comparing the readings. Part of this can
be fixed by adjusting the telescope bubble level, but the remaining
index error
(see
below) is probably best resolved by making two readings, normal and reversed.
6. Get the transit reading magnetic or true azimuths (though absolute accuracy is limited by the
compass to about a degree – with transits and theodolites the absolute accuracy is not as
important as the relative accuracy of angles) by:
•
Using the upper clamp and tangent screw to set the direction to zero. You'll need to
understand how to read the horizontal circle vernier. The lower scale shows the reading
to a half degree; the upper shows the minutes above or below that reading. For minutes
find which mark lines up with a degree or half degree mark below. The figure shows a
reading of 33° 45'. For now though, you just need to put the zero on the zero, but check
the vernier to make sure the 0° mark is lined up with the 0' mark.
•
With the upper clamp closed, and the compass released (unscrew its lock until it moves
freely) open the lower clamp to rotate the telescope until the north arrow is pointing to
north on the scale, or to the magnetic declination (may be around 15.5°) to get true
directions. Then close the lower clamp.
