above), you need to learn to brace your holding
arm with your other arm, and against your body
or head or both, to avoid unsteadiness and
fatigue. When there is a wind, you may find it
difficult to hold the plumb bob suspended over
a point. The plumb bob will tend to swing back
and forth. You can overcome this problem by
bouncing the point of the plumb bob slightly up
and down on the point.
For a long sight, it is much better to plumb
over a point with a range pole. For a short sight,
however, the shaft of a range pole is too thick to
permit exact alignment of the vertical cross hair.
For long sights, or for sights on a point that
is to be sighted repeatedly, it is often desirable
to construct a semipermanent target. There
are no definite rules that can be stated for
constructing targets because they usually must be
built from materials at hand. Use your ingenuity;
but make the target high enough to be seen, strong
enough to withstand prevailing winds, and plumb
over a point. Several types of semipermanent
targets are shown in figure 12-6.
MARKING CONTROL POINTS,
REFERENCE POINTS, AND
MONUMENTS
In general, control surveys deal with estab-
lished points. To define these points, surveyors
Figure 12-6.-Field-constructed semipermanent targets.
have to mark them. Certain points are made
permanent; on the other hand, others are
temporary. A line that will be used for a long
period of time, for example, may be marked at
each end with a bronze disk set in concrete, or
with a center-punched metal rod driven flush with
the ground. For less permanent control points,
wooden stakes or hubs with nails, shiners, and
flaggings can be used.
Placing Driven Markers
A DRIVEN MARKER must be set exactly
vertically on the point it is supposed to mark. If
it is driven on a slant, the top of the marker will
not define the correct location of the point. To
drive the marker vertically, first align it vertically;
then, using a sledgehammer or other type of
driving implement, strike each blow squarely on
the flat end of the hub or stake.
A wooden hub is normally driven to mark
the exact horizontal location of a point, usually
for the purpose of plumbing an instrument
over the point. Consequently, it is not normally
necessary for the top of a hub (or other
markers used for the same purpose) to extend
much above the ground line. The precise location
of the point is marked by a hub tack, punch
mark, or other precise marker driven or set
in the top of the hub. For work on asphalt
roads or runways, you’ll find it easier to use
flagging or a soda pop top and a nail as a marker;
in concrete and other hard surfaces, you can use
orange paint or a star-drilled hole plugged with
lead. The choice of markers to be used depends
on the surveyor’s judgment as well as the purpose
of the survey.
In frozen or otherwise extra hard ground, use
a bull-point to start a hole for a stake or hub.
Remember that the stake or hub will follow the
line of the opening made by the bull-point.
Therefore, if the bull-point is not driven vertically,
the stake or hub will not be vertical either.
Placing Monuments
In surveying, a MONUMENT is a permanent
object or structure used where a point or station
must be retained indefinitely for future reference.
It may simply consist of a conspicuous point
carved on an outcrop of a ledge rock or otherwise
12-7
