Interior angles may be improved by averaging equal numbers of direct
and reversed readings. As a check, exterior angles may also be observed to
close the horizon (see Section 8.10). In the traverse of Figure 9.1(a), a reference
line A-Az MK of known direction exists. Thus, the clockwise angle at A from
Az Mk to E must also be observed to enable determining the directions of all
other lines.This would not be necessary if the traverse contained a line of known
direction, like AB of Figure 7.2, for example.