be reset. The rodman reads the target to feet, tenths,and hundredths of the nearest foot gradation below thehorizontal quadrant separation line of the target.Equipment Operator’s seldom use the vernier scale inearthwork operation.LEVELINGThe vertical distance, measured during leveling,is the difference of elevation between two points. Theterm elevation refers to the height of a point or aparticular spot above or below a reference line, calleda datum or datum plane.Datum are of two general types: actual andassumed. An actual datum is mean sea level (fig.15-46). An assumed datum plane is an imaginary levelsurface assumed to have an elevation of zero. It is usedas a convenience in leveling procedures.A reference point whose elevation is known andmarked is called a bench mark (B.M.). It is usedeither as the starting point in leveling or as a point ofclosure in checking the accuracy of your work.Bench MarksBench marks are classified as temporary orpermanent. Temporaary bench marks (T. B. M.) areestablished for the use of a particular job and areretained for the duration of that job. Throughout theUnited States, a series of permanent bench marks hasbeen established by various governmental agencies.These identification markers are set in stone, iron pipe,or concrete and are sometimes marked with theelevation above sea level. Typical markers are shownin figure 15-47.Figure 15-47.—Federal bench marks.15-27
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