SUBGRADE—The soil prepared to support astructure or a pavement system. It is thefoundation for the pavement structure. Thesubgrade soil is sometimes called “basement soil”or “foundation soil.”SYSTEMATIC ERRORS—Errors that, as longas conditions are unchanged, will always have thesame magnitude and the same algebraic sign.TANGENT—The ratio between the side oppositeand the side adjacent an angle in a right triangle:tan = opp/ad.TANGENT LINE—A line that touches thecircumference of a circle at one point and isperpendicular to the radius at the point oftangency.TARGET—Any object to which the instrumentis pointed. A target may be a plumb bob or plumbbob cord, a nail in the top of a stake, a tapingarrow, a ranging pole, a pencil, or any otherobject that will provide a sharply defined,stationary point or line. A target is usually placedvertically over an unoccupied transit station.TECHNICAL ENGINEERING CHARTS—Charts based on a series of measurements oflaboratory experiments or work activities.TEMPLATES—Timesaving devices used to drawvarious shapes and symbols. Templates areavailable for all types of drawings.THIRD-ANGLE PROJECTION—Multiviewprojection commonly used in the United States.The top view projects above the front view, andthe sides and bottom automatically project intotheir proper positions.TITLE BLOCK—Block drawn in the lower rightcorner of a drawing; it should contain all theinformation necessary to identify the drawing.TRACING PAPER—High-grade, white, trans-parent paper that takes pencil well; used whenreproductions are to be made of drawings; alsoknown as tracing vellum.TRACING VELLUM—See TRACING PAPER.TRANSIT STATION—A marked point overwhich the instrument is, has been, or will be,accurately positioned for use.TRANSPORTED SOIL—A soil that has beenmoved by natural forces to a location other thanits origin.TRAVERSE THE INSTRUMENT—Rotate theinstrument about its vertical axis; that is, turn theinstrument in azimuth.TRIANGLE—A polygon of three sides.TRIANGULAR PRISM—A solid figure whosebase is a triangle.TRIGONOMETRY—That branch of mathematicsthat deals with certain constant relationships thatexist in triangles and with methods of applyingthese relationships to compute unknown valuesfrom known values.TRAPEZOID—A quadrilateral with only onepair of opposite sides parallel, the other pair beingnot parallel.TRAPEZIUM—A quadrilateral with no sidesparallel.TRIM LINES—Lightly drawn lines used as guidesto trim a drawing to standard size.VERTICAL ANGLE—An angle between twointersecting lines in a vertical plane. It should beunderstood that one line lies on the horizontalplane, and the angle originates from theintersection of the two planes.VERTICAL CONTROL—Established benchmarks.VERTICAL LINE—A line that lies in the verticalplane and is perpendicular to the plane of thehorizon, such as the direction of a plumb line.V E R T I C A L P L A N E — A p l a n e t h a t isperpendicular to the horizontal plane.VIEWING PLANE LINES—Thick, heavy linesused to indicate the plane or planes from whicha surface or several surfaces are viewed.VISIBLE LINES—Solid, thick lines indicating theedges of the object being drawn.WATER-CEMENT RATIO—The ratio of theamount of water, exclusive only of that absorbedby the aggregates, to the amount of cement in aconcrete mixture. This ratio is variously stated asfollows: (1) by bulk volume of cement (assumingcement to weigh 94 lb per cu ft); (2) by absolutevolume of cement; (3) by weight; and (4) in termsof gallons of water per 94-lb sack of cement.AI-14
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