Surveying Instruments - Practical Boundary Surveying - page 52

5.1 Transits and Theodolites
Angles were traditionally measured using optical instruments such as Transits and
Theodolites . These instruments typically have a telescope of 20-30 power mag-
nification. The telescope is fitted with a Reticle . A reticle is a set of cross hairs
similar to that found on a rifle scope. The reticle consists of horizontal and vertical
lines, crossing in the center of the telescope. The reticle allows a target to be pre-
cisely sighted both horizontally and vertically.
Surveying instruments also have a short set of horizontal Stadia hairs, above
and below the horizontal cross hair, which allow Rangefinding . Rangefinding is
the ability to optically measure the distance from the instrument to a target. Cross
hairs and stadia hairs are shown in Fig. 5.1 . On instruments found in the United
States, the stadia hairs are spaced so the vertical distance between them is exactly
1 foot at 100 feet. A graduated vertical rod, as shown in the image, can be used to
read off the approximate distance from the instrument to the rod. Most rods are
graduated in hundredths of a foot (0.01′) so distances measured using stadia are
accurate to about 1 foot. In the image in Fig. 5.1 the distance would be about 21
feet. Stadia measurement was handy for rough measurements before electronic
measuring devices became common but it is rarely used today.
Transits and theodolites have clamps and fine adjustment screws, called
Tangent Screws , which allow very precise positioning of the telescope in both
the horizontal and vertical axes. A tangent screw and clamp are shown in Fig. 5.2 .
Each axis has its own clamp and adjustment screw. When sighting a target, the
instrument is manually turned and aimed at the target. This is best accomplished
by visually sighting over the telescope with both eyes open, not by looking
through the telescope.
Some telescopes have crude metal sights or collimators on the outside of the
telescope for this purpose. The Wild T1A shown in Fig. 5.3 has a mechanical peep
sight just above the eyepiece. The front sight, which is a bit difficult to see in the
image, is a short black post just above the objective lens. (The feature mounted
Fig. 5.1 A reticle with stadia hairs