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Plumbing over a Point If intervening low growth or some other circumstance makes it impossible for the instrumentman to sight the point on the ground, the chainman must plumb over the point, using the plumb bob and cord. If the distance is too far for observation of the plumb bob cord, the cord should be equipped with a plumb bob target, or a range pole may be used. In the absence of a target when using the plumb bob, you may tie a piece of colored flagging to the cord, or you may use a handkerchief, as shown in figure 12-5. Some chainmen prefer to hold the plumb bob and cord with the cord running over the forefinger. Others prefer to have the cord running over the thumb. If you are plumbing high (that is, required to hold the cord at chest level or
Figure 12-5.-Using a handkerchief as a substitute for a target on a plumb bob cord. 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. |
 
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