Figure 11-19.-Vertical angles from zenith distances (mils).
last zero dropped. The auxiliary scales are graduated
from 0 to 10 roils in 0.2-mil increments. Readings on
the auxiliary scale can be interpolated to 0.1 mil.
The vertical circle reads 0 mil when the telescope
is pointed at the zenith, and 3,200 mils when it is pointed
straight down. A level line reads 1,600 roils in the direct
position and 4,800 roils in the reverse. The values read
are zenith distances. These zenith distances can be
converted into vertical angles as shown in figure 11-19.
The 1-sec theodolite is a precision direction
type of instrument for observing horizontal and
vertical directions. This instrument is similar to,
Figure 11-20.-A 1-second theodolite.