Surveying Instruments - Practical Boundary Surveying - page 54

Fig. 5.3 Wild T1A optical theodolite with peepsight
metal circles with Verniers . A vernier is a specially divided scale which allows
more precise reading of the arc. Transits can be read to about 1 min or 30 seconds
of arc or even a little less. Theodolites have glass circles with markings and verni-
ers etched on the glass. Unlike a transit where the circles are more or less open to
the environment, on a theodolite the circles are internal. Theodolite circles, on the
more precise instruments, can be read to 1 second of arc or less. Modern total sta-
tions have optical encoders on the glass scales enabling angles to be read digitally.
As noted, the most precise instruments are capable of resolving an angle to 1
second of arc or less. To put this into perspective, 1 second of arc at 1,000 feet