Any substantial object may be used as a benchmark. Figure 15-48, view A, shows typicalmonuments set to mark important alignment points,but which may also be used as bench marks. Spikesmay be driven into posts or power poles, as shown infigure 15-48, view B, or chiseled into stone or concretestructures, as shown in figure 15-48, view C. Forclarity, the marks are shown on the wing wall; but inpractice, one mark only is usually chiseled orspray-painted on a flat surface.The location, elevation, and description of benchmarks are usually shown on the project drawings or inthe surveyor’s field notes.Determining ElevationsOnce a bench mark is established, certainformulas are used for determining elevations. Youfirst must figure the height of the instrument. This isdone by taking a reading on a level rod that is placedon a known elevation, such as a bench mark. This isknown as a backsight (BS).To determine the height (HI) of the instrument,add the bench mark (BM) elevation to the backsight(BS) reading from the level rod. This formula iswritten as HI = BM + BS.For example, as shown in figure 15-49, the benchmark elevation is 100.00 feet. The backsight readingFigure 15-48.—Temporary bench marks.15-28
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