Plane trigonometry with practical applications, by Leonard E. Dickson.

Ch. VII]

LAND SURVEYING

99

mine the line of collimation. This line indicates the point toward
which the telescope is pointed.
The telescope may possess a pair of horizontal stadia wires fastened to movable slides so adjusted that the wires subtend say one foot on a graduated
stadia rod standing 100 feet away, and hence subtend three feet on a rod 300
feet away, etc. With the stadia wires we can therefore rapidly measure distances approximately, which is especially useful over rough ground.'
73. Measuring angles with a transit. To measure a horizontal angle ABC,
place the transit over the vertex B of the angle, level the transit, and set the
zero of the vernier exactly opposite to the zero of the limb. Bring the line of
collimation to bear approximately on A, clamp the lower plate to the leveling
head and make the line of collimation bear exactly on A by means of the lower
tangent screw. Unclamp the upper plate and turn it until the line of collimation bears approximately on C, clamp, and make the line of collimation bear
exactly on C by means of the upper tangent screw. Then angle ABC is the
arc over which the zero of the vernier has passed and can be read by observing the point on the limb at which the zero of the vernier stops. If it stops
exactly opposite to a graduation mark of the limb, the angle is read without
using the vernier. But usually it stops between graduation marks and then
the vernier reading tells how far it has passed beyond the last mark (cf. Art. 67).
To measure the angle of elevation or depression of an object relative to the
horizontal plane (Art. 5), make the line of collimation horizontal (as shown
by the level bulb attached to the telescope), whence the vertical circle or arc
reads zero if in proper adjustment. Then sight approximately at the object,
tighten the set screw, and, by turning the tangent screw, bring the line of
collimation exactly on the object. Then read the vertical circle.
74. Traverse. A series of connected courses AB, B C, CD,...
is called a traverse. It is called closed if it re- B
turns to the starting point. Thus a survey of
a field is a closed traverse.,' /
In a traverse with a transit four main types A 10~o' 5
of angles are used: direct angle, deflection angle, /
azimuth, and bearing. The first three types | /
will be defined in turn. The fourth type has C
been defined above. FIG. 55

1For details, see Tracy's Plane Surveying, 1914, pp. 300-317.