Figure 11-40.-Types of plumb bobs.Figure 11-41.-Plumb bob, cord, and target.plumb bobs are 6, 8, 10, 12, 14, 16, 18, and 24OZ; the 12- and the 16-oz are the most popular.Typical plumb bobs are shown in figure 11-40.A plumb bob is a precision instrument andmust be cared for as such. If the tip becomes bent,the cord from which the bob is suspended will notoccupy the true plumb line over the pointindicated by the tip. A plumb bob usually has adetachable tip, as shown in figure 11-40, so if thetip becomes damaged, it can be renewed withoutreplacing the entire instrument.Each survey party member should be equippedwith a leather sheath, and the bob should beplaced in the sheath whenever it is not in use.The cord from a plumb bob can be made moreconspicuous for observation purposes by theattachment of an oval form aluminum target (fig.11-41, view A). The oval target has reinforcededges, and the face is enameled in quadrantsalternately with red and white. Also, a flatrectangular plastic target may be used (fig. 11-41,view B). It has rounded corners with alternate redand white quadrants on its face. These plumb bobstring targets are pocket size with approximatedimensions of 2 by 4 in.Optical Plumbing AssemblyThe optical plumbing assembly, or plummet,is a device built into the alidade or the tribrachof some of the instruments to center theinstrument over a point. Its working principle isshown in figure 11-42. The plummet consists ofa small prismatic telescope with a cross wire orFigure 11-42.-Optical plumbing assembly.11-40
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
