SUBGRADE—The   soil   prepared   to   support   a structure  or  a  pavement  system.  It  is  the foundation  for  the  pavement  structure.  The subgrade soil is sometimes called “basement soil” or   “foundation   soil.” SYSTEMATIC  ERRORS—Errors  that,  as  long as  conditions  are  unchanged,  will  always  have  the same magnitude and the same algebraic sign. TANGENT—The ratio between the side opposite and the side adjacent an angle in a right triangle: tan  =  opp/ad. TANGENT  LINE—A  line  that  touches  the circumference  of  a  circle  at  one  point  and  is perpendicular  to  the  radius  at  the  point  of tangency. TARGET—Any  object  to  which  the  instrument is pointed. A target may be a plumb bob or plumb bob  cord,  a  nail  in  the  top  of  a  stake,  a  taping arrow,  a  ranging  pole,  a  pencil,  or  any  other object   that   will   provide   a   sharply   defined, stationary  point  or  line.  A  target  is  usually  placed vertically  over  an  unoccupied  transit  station. TECHNICAL   ENGINEERING   CHARTS— Charts  based  on  a  series  of  measurements  of laboratory  experiments  or  work  activities. TEMPLATES—Timesaving devices used to draw various  shapes  and  symbols.  Templates  are available  for  all  types  of  drawings. THIRD-ANGLE  PROJECTION—Multiview projection commonly used in the United States. The top view projects above the front view, and the  sides  and  bottom  automatically  project  into their  proper  positions. TITLE BLOCK—Block drawn in the lower right corner  of  a  drawing;  it  should  contain  all  the information  necessary  to  identify  the  drawing. TRACING   PAPER—High-grade,   white,   trans- parent  paper  that  takes  pencil  well;  used  when reproductions  are  to  be  made  of  drawings;  also known  as  tracing  vellum. TRACING   VELLUM—See   TRACING   PAPER. TRANSIT   STATION—A   marked   point   over which  the  instrument  is,  has  been,  or  will  be, accurately  positioned  for  use. TRANSPORTED  SOIL—A  soil  that  has  been moved by natural forces to a location other than its origin. TRAVERSE   THE   INSTRUMENT—Rotate   the instrument about its vertical axis; that is, turn the instrument in azimuth. TRIANGLE—A   polygon   of   three   sides. TRIANGULAR   PRISM—A   solid   figure   whose base is a triangle. TRIGONOMETRY—That branch of mathematics that  deals  with  certain  constant  relationships  that exist in triangles and with methods of applying these  relationships  to  compute  unknown  values from  known  values. TRAPEZOID—A  quadrilateral  with  only  one pair of opposite sides parallel, the other pair being not parallel. TRAPEZIUM—A  quadrilateral  with  no  sides parallel. TRIM LINES—Lightly drawn lines used as guides to  trim  a  drawing  to  standard  size. VERTICAL  ANGLE—An  angle  between  two intersecting lines in a vertical plane. It should be understood  that  one  line  lies  on  the  horizontal plane,   and   the   angle   originates   from   the intersection  of  the  two  planes. VERTICAL   CONTROL—Established   bench marks. VERTICAL LINE—A line that lies in the vertical plane  and  is  perpendicular  to  the  plane  of  the horizon,  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    is perpendicular  to  the  horizontal  plane. VIEWING  PLANE  LINES—Thick,  heavy  lines used to indicate the plane or planes from which a surface or several surfaces are viewed. VISIBLE  LINES—Solid,  thick  lines  indicating  the edges  of  the  object  being  drawn. WATER-CEMENT  RATIO—The  ratio  of  the amount of water, exclusive only of that absorbed by the aggregates, to the amount of cement in a concrete  mixture.  This  ratio  is  variously  stated  as follows: (1) by bulk volume of cement (assuming cement to weigh 94 lb per cu ft); (2) by absolute volume of cement; (3) by weight; and (4) in terms of  gallons  of  water  per  94-lb  sack  of  cement. AI-14