vertical circle images into a single field of view.
In the DEGREE-GRADUATED SCALES (fig.
11-16), the images of both circles are shown as
they would appear through the microscope of the
1-min theodolite. Both circles are graduated from
0° to 360° with an index graduation for each
degree on the main scales. This scale’s graduation
appears to be superimposed over an auxiliary that
is graduated in minutes to cover a span of 60 min
(1°). The position of the degree mark on the
auxiliary scale is used as an index to get a direct
reading in degrees and minutes. If necessary, these
scales can be interpolated to the nearest 0.2 min
of arc.
The vertical circle reads 0° when the
theodolite’s telescope is pointed at the zenith, and
180° when it is pointed straight down. A level line
reads 90° in the direct position and 2700 in the
reverse. The values read from the vertical circle
are referred to as ZENITH DISTANCES and not
vertical angles. Figure 11-17 shows how these
zenith distances can be converted into vertical
angles.
Figure 11-17.-Converting zenith distances into vertical
angles (degrees).
In the MIL-GRADUATED SCALES (fig.
11-18), the images of both circles are shown as
they would appear through the reading micro-
scope of the 0.2-mil theodolite. Both circles are
graduated from 0 to 6,400 mils. The main scales
are marked and numbered every 10 mils, with the
Figure 11-16.-Degree-graduated scales.
Figure 11-18.-Mil-graduated scales.
11-23
