PERMANENT BENCH MARK—A bench markof as nearly permanent character as it ispracticable to establish; usually designated benchmark.PERSPECTIVE—A single-view drawing of anobject showing three dimensions. Lines andsurfaces are foreshortened to make it appearoptically correct. Perspective drawing is usedwhen the end product is to be of an illustrativenature.PHANTOM LINE—Thin broken line consistingof one long and two short evenly spaced dashes;used to indicate the alternate position of a movingpart.PICTORIAL PROJECTION—Any projectionthat shows three dimensions in a single view asit would be seen by the eye.PLAN/PROFILE PAPER—Upper half (plan) isplain white paper used to draw the plan view;lower half is gridded and used to draw profiles.PLANE COORDINATES—See GRID COOR-DINATES.PLANE OF PROJECTION—The theoretical,transparent plane placed between the point ofsight and the object in any type of projection. Theobserver sees the features of an object as they lieon the plane of projection; also known as imageplane.PLANE SURVEY—A survey in which the effectof the curvature of the earth is almost entirelyneglected, and computations of the relativepositions of the stations are made usingthe principles of plane geometry and planetrigonometry.PLASTIC LIMIT (PL)—The lower limit of theplastic state, expressed as the minimum moisturecontent, at which a soil can be rolled into a threadone-eighth in, in diameter without crumbling.PLASTICITY—The property of a fine-grainedsoil that allows it to be deformed beyond the pointof recovery without cracking or changing volumeappreciably.PLASTICITY INDEX (PI)—The numericaldifference between the moisture content at theliquid limit and the moisture content at the plasticlimit.PLUMB LINE—The line indicated by a plumbbob cord. The direction in which gravity acts.PLUNGE THE INSTRUMENT—Turn theinstrument telescope end-for-end about itshorizontal axis.POINT OF SIGHT—The position of the observerin relation to the object and the plane ofprojection in any type of projection; also knownas station point.POLYGON—A plane figure that is bounded bystraight-line sides.POROSITY—The ratio, expressed as a per-centage, of the intergranular space in a given soilmass to the total volume of the soil mass.POSITION— 1. Data that define the location ofa point with respect to a reference system. Thecoordinates that define the location of a point ona grid. 2. A circle position.POWER—The number of times, as indicated byan exponent, a number occurs as a factor; forexample, 24 = 2<2.2.2=2 to the 4th power=16.POZZOLAN—A siliceous or aluminous materialthat in itself possesses little or no cementitiousvalue but will, in finely divided form and in thepresence of moisture, chemically react withcalcium hydroxide at ordinary temperaturesto form compounds possessing cementitiousproperties.PRECISION—The degree of refinement in theperformance of an operation or the degree ofperfection in the instruments and methods usedwhen making measurements. Precision relates tothe quality of the operation by which a result isobtained and is distinguished from accuracy thatrelates to the quality of the result.PROJECTION CHART—Chart showing thecomparison of scheduled progress and actualprogress.PROJECTION LINES—The imaginary linesfrom the eye of the viewer to the points on theobject in any type of projection; also known aslines of sight.PROPORTION—An equation in which tworatios are set equal to each other; for example,3/4 = 9/12,AI-11
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