object in the immediate vicinity, such as the rim of amanhole cover, a rod, or the finish floor of an existingstructure. This object may be given its relative sealevel elevation (if it is known); or it may be given aconvenient, arbitrarily assumed elevation, usually awhole number, such as 100.0 feet. An object of thistype, with a given, known, or assumed elevation,which is to be used in determining the elevations ofother points, is called a bench mark.PRINCIPLES OF DIFFERENTIALLEVELINGFigure 5-15 illustrates the principle of differentialleveling.The instrument shown in the centerrepresents an engineer’s level.This opticalinstrument provides a perfectly level line of sightthrough a telescope, which can be trained in anydirection. Point A in the figure is a bench mark (itcould be a concrete monument, a wooden stake, asidewalk curb, or any other object) having a knownelevation of 365.01 feet. Point B is a ground surfacepoint whose elevation is desired.The first step in finding the elevation point ofpoint B is to determine the elevation of the line ofsight of the instrument. This is known as the height ofinstrument and is often written and referred to simplyas “H.I.” To determine the H.I., you take a backsighton a level rod held vertically on the bench mark(B.M.) by a rodman.A backsight (B.S.) is alwaystaken after a new instrument position is set up bysighting back to a known elevation to get the new H.I.A leveling rod is graduated upward in feet, from 0 atits base, with appropriate subdivisions in feet.In figure 5-15, the backsight reading is 11.56 feet.Thus, the elevation of the line of sight (that is, theH.I.) must be 11.56 feet greater than the bench markelevation, point A. Therefore, the H.I. is 365.01 feetplus 11.56 feet, or 376.57 feet as indicated.Next, you train the instrument ahead on anotherrod (or more likely, on the same rod carried ahead)held vertically on B.This is known as taking aforesight. After reading a foresight (F.S.) of 1.42 feeton the rod, it follows that the elevation at point B mustbe 1.42 feet lower than the H.I. Therefore, theelevation of point B is 376.57 feet minus 1,42 feet, or375.15 feet.GRADINGThe term “grade” is used in several differentsenses in construction. In one sense, it refers to thesteepness of a slope; for example, a slope that rises 3vertical feet for every 100 horizontal feet has a gradeof 3 percent. Although the term “grade” is commonlyused in this sense, the more accurate term forindicating steepness of slope is “gradient.”In another sense, the term “grade” simply meanssurface. On a wall section, for example, the line thatFigure 5-15.—Procedure for differential leveling.5-10