NORMAL  LINE—A  line  that  is  parallel  to  two planes  of  projection  and  perpendicular  to  the third. A line or plane that forms a 90⁰ angle with another  line  or  plane  is  normal  to  that  line  or plane. NORTHING—One  of  the  two  values  indicating the  position  of  a  point  on  a  grid  system.  The northing coordinate is abbreviated N. (See GRID COORDINATES.) OBLIQUE  PROJECTION—A  single  view  of  an object showing three dimensions (length, width, and height). OBLONG—A   nonequilateral   rectangle. OBTUSE  ANGLE—An  angle  greater  than  90°. OCCUPIED  STATION—A  traverse  or  triangu- lation   station   over   which   a   theodolite   or   an engineer transit is set up for the measurement of angles  at  this  station.  Also,  a  station  at  which angles have been so measured. OCTAGON—A   polygon   of   eight   sides. OFFSET  LINE—A  supplementary  line  close  to, and usually parallel to, a main survey line to which it is referenced by measured offsets. When the line for which data are desired is in such a position that it is difficult to measure over it, the required data are obtained by running an offset line in a convenient  location  and  measuring  offsets  from it  to  salient  points  on  the  other  line. OGEE  CURVE—Any  curve  composed  of  two consecutive  tangent  circular  arcs  that  curve  in opposite directions; also known as reverse curve. OPEN  TRAVERSE—A  traverse  that  starts  at  a point of known or assumed position and ends at a point whose relative position is unknown with respect to the starting point. ORDER   OF   ACCURACY—A   mathematical ratio  defining  the  general  accuracy  of  the measurements  made  in  a  survey.  The  orders  of accuracy for surveys are divided into four classes named first-order, second-order, third-order, and fourth-order. ORGANIC   SOIL—Soil   that   contains   mineral grains and a more or less conspicuous admixture of  vegetable  matter. ORIENT—To  establish  the  correct  relationship in  direction  with  reference  to  the  points  of  the compass;  to  bring  into  correct  relationship  in direction   with   reference   to   the   points   of   the compass. ORTHOGRAPHIC   PROJECTION—The   pro- jection  of  height,  width,  and  depth  of  an  object into various single planes so as to depict the true size  and  shape  of  the  object  as  seen  from  each individual  plane,  each  plane  showing  only  two dimensions, thereby necessitating a minimum of two  planes  to  show  all  three  dimensions. PARALLAX—The  apparent  displacement  or  the difference  in  apparent  direction  of  an  object  as seen from two different points not on a straight line  with  the  object.  In  testing  the  focus  of  a telescope,  the  head  of  the  observer  must  move from side to side or up and down while sighting through the eyepiece. Any apparent movement of the  cross  hairs  in  relation  to  the  object  image means that parallax is present. Parallax can be practically  eliminated  by  careful  focusing. PARALLEL   OF   LATITUDE—A   line   on   the surface of the earth having the same latitude at every  point. PARALLELEPIPED—A  solid  figure  whose  base is  a  parallelogram. PARALLELOGRAM—A quadrilateral with each pair  of  opposite  sides  parallel. PARTIAL   AUXILIARY   VIEW—An   auxiliary view in which only the features of an object that are  specifically  desired  are  shown. PARTIAL   SECTION—A   sectional   view   con- sisting of less than a half section; used to show the  internal  structure  of  a  small  portion  of  an object;  also  known  as  a  broken  section. PAVEMENT   STRUCTURE—All   courses   of selected  material  placed  on  the  foundation  or subgrade  soil,  other  than  any  layers  or  courses constructed  in  grading  operations. PENTAGON—A   polygon   of   five   sides. PERCENT—Portion  in  one  hundred  parts. PERIMETER—The   sum   of   the   sides   of   a polygon. AI-10