Page 53 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

SURVEYS AND MEASUREMENTS 2.3

If the angle were being read from right to left, the main scale would read 357° and a fraction.
Reading the vernier to the left, the twentieth division is found to correspond with a line on the circle.
20
The angle is therefore 357° plus ⁄ 30 of 30″, or 357°20″.
The telescope may be locked against swinging by means of a thumbscrew for convenience in
reading the scale, or holding it in a certain direction. Another thumbscrew (tangent screw) will
then move it slowly for fine adjustments.
Telescope. The length and power of the telescope and the length of the spirit level determine the
range of the instrument and, to a considerable degree, its accuracy. Telescopes range from 10 to
18 inches in length, and from 10X to 35X power in magnification. Spirit levels may be 3 to 10
inches long.
The telescope is focused by means of a knob on the side or top, and possibly by a turning eye-
piece also.
The field of view of the telescope is divided into quarters by the crosshairs, shown in Fig. 2.3(A),
which are held in a frame or diaphragm inside the telescope. Provision is usually made to make these
visible or invisible by focusing the eyepiece. The horizontal hair is used for taking levels. If it is cor-
rectly placed in the telescope, and the telescope is properly leveled, it indicates the slice of the field
of view which is level with the observer’s eye. The vertical hair is used to sight a given point or line,
and indicates the exact center of the field of view for determining horizontal angles.

Stadia Hairs. Stadia hairs (B) may be fitted into the same frame. These are horizontal and are
located above and below the center hair. The distance between the stadia hairs is fixed at a ratio with
the telescope, usually 1 to 100, so that if a measuring rod is sighted through the scope, the inches or
feet seen between the stadia hairs may be multiplied by 100 to give the distance of the rod from the
instrument.
Amateurs are apt to confuse one or the other stadia hair with the crosshair in taking levels, with
resultant serious error. If this trouble persists, additional hairs may be installed, as in (C) in the
form of a letter X, which should make the center hair easy to distinguish.
Base. The leveling head is mounted on the turntable or base by means of a center pin, on which
it can both tip and rotate, and four leveling screws. These screws are threaded into the leveling
head and rest on the leveling plate or turntable. They are expanded into knurled wheels for con-
venience in turning with the fingers, and have expanded feet which do not turn with the screw and
which protect the plate.
The turntable base has internal threads by means of which it can be screwed on to the tripod
head. It may have a hook on the bottom, at the center, from which a plumb bob (pointed-tip
weight) may be hung by a chain and string. The base may be made to slide a limited distance hor-
izontally, relative to the tripod head, for convenience in centering the unit directly over a mark.
Tripod. A tripod consists of three metal or wood legs, hinged together by a top plate which is
threaded for the instrument, as seen in Fig. 2.4. These threads should be protected by a cap when-
ever the instrument is not mounted. The legs may be one piece, or two pieces sliding on each other
and locked by a screw clamp.
The base may be set directly on a flat rock or stump, if it is not possible to set up the tripod,
but this is not recommended.
Compass. Compasses are standard equipment in transits, and can usually be obtained for other
types of instruments that have a horizontal scale. They are not necessary for the work to be
described in this chapter, although it is often convenient to know the general directions of lines.

FIGURE 2.3 Sighting hairs. (Courtesy of David White,
LLC.)