4
Reading horizontal azimuths
From now on when you rotate the telescope with the upper clamp open, or fine adjust the upper
tangent screw, you'll get readings in azimuths. You can get angles by subtracting one angle from
another.
Reading vertical angles
Also uses a vernier to get degrees and minutes. The reading below is 0° 04'. Note that the
minutes are now on the bottom, in contrast to the horizontal circle – a potential source of
confusion and blunder.
A single reading may be off by 10' or more. To get a more accurate reading – to a minute –
average two readings: (1) the normal reading, and (2) a reversed reading, with the telescope
reversed 180° in azimuth and the telescope
plunged
(rotated vertically 180° so that the telescope
bubble is on top). Half of the difference is the
index error
, which should remain reasonably
consistent during a given use, so you could also add this index error to the normal reading
instead of making two readings; but it's probably a good idea to check this on multiple readings.
Reading distances, using the stadia rod
If reading is at level, the stadia readings can be converted to distances by subtracting the lower
from the upper stadia reading and multiplying the resulting difference (s) by the
stadia interval
factor
(K) to get distance: H = Ks. K should be approximately 100, but you can derive a more
accurate value by previously calibrating using a known horizontal distance – your derived K
should be recorded on paper and kept with the instrument.
If you're not reading a level reading, you can use the following formula to get horizontal distance
using the vertical angle
a
:
H = Ks cos(a) cos(a)
This is the general formula for any angle including zero, which should make sense as cos(0) = 1.
