in figure 11-51. Note that it is graduated in a manner
that facilitates counting the number of graduations
intercepted between the hairs. Each tenth of a foot is
marked by the point of one of the black, saw-toothed
graduations. The interval between the point of a black
tooth and the next adjacent white gullet between two
black teeth represents 0.05 ft.
Other types of graduations on stadia rods or boards
are shown in figure 11-52.
Turning-Point Pins and Plates
The point on which a leveling rod is held between
a foresight and the next backsight while the instrument
is being moved to the next setup is called a TURNING
POINT. It must be sufficiently stable to maintain the
accuracy of the level line. Where either proper natural
features of man-made construction is not available, a
turning-point pin, a turning-point plate, or a wooden
stake is used. These not only furnish the solid footings
but also identify the same position for both sightings.
Normally, the pins or plates are used for short periods
and are taken up for future use as soon as the instrument
readings are completed. Wooden stakes are used for
longer periods except when wood is scarce or local
regulations require their removal.
A turning-point pin is shown in figure 11-53, view
A. It is made of a tapered steel spike with a round top
with a chain or a ring through the shaft for ease in
pulling. The pin is driven into the ground with a
sledgehammer.
After a turning pin has served its
purpose atone point, it is pulled and carried to the next
turning point.
Turning-point plates (fig. 11-53, view B) are
triangular metal plates with turned-down corners or
added spikes that form prongs and have a projection or
bump in the center to accept the rod. The plates are
devised for use in loose, sandy, or unstable soils. The
Figure 11-52.-Types of graduations on stadia boards.
Figure 11-53.-Turning-point pin and plate.
plate is set by placing it on the ground, points down, and
stepping on it to press it to a firm bearing. After use, it
is lifted, shaken free of dirt and mud, and carried
forward to the next turning point.
Magnifying Glass
A magnifying glass is used mainly to aid the
instrumentman in reading graduations that are provided
with verniers, such as the horizontal and vertical circle
of a transit. Although these graduations can be read
with the naked eye, the use of a magnifying glass makes
the reading easier and decreases the chance of reading
the wrong coincidence.
Two types of magnifying glasses that you will
generally find in the transit box are shown in
figure 11-54. They are usually called pocket
Figure 11-54.-Types of pocket magnifying glasses.
11-45
