(measured  upward  from  the  horizon)  is  termed an  angle  of  elevation,  and  a  negative  altitude (measured   downward   from   the   horizontal)   is termed  an  angle  of  depression.  2.  Altitude  is sometimes  used  to  apply  to  elevation  above  a datum; for example, the altitude of an airplane. ANGLE—A figure formed by two lines or planes extending  from  or  diverging  at  the  same  point. ANGLE OF DEPRESSION—A negative altitude. ANGLE  OF  ELEVATION—A  positive  altitude. ANGLE   OF   INCLINATION—A   vertical   angle of  elevation  or  depression. ANNUAL  VARIATION—The  annual  change  in the  magnetic  declination. ANTILOG—The   result   when   a   logarithm   is converted  to  a  number. ARC—A portion of the circumference of a circle. ARCHITECT’S   SCALE—Scale   used   when dimensions or measurements are to be expressed in feet and inches. ARITHMETIC—The  art  of  computation  by  the use  of  positive  real  numbers. ASPHALT—A dark brown to black cementitious material  in  which  the  predominating  constituents are  bitumens  that  occur  in  nature  or  are  obtained in petroleum processing. Asphalt is a constituent in varying proportions of most crude petroleums. AUXILIARY PLANE—A plane (NOT one of the normal planes) from which the auxiliary view is projected. AUXILIARY  VIEW—A  view  that  is  not  on  one of the normal planes of projection. It is used to show  features  of  objects  that  do  not  appear^- in their true size and shape in the normal views. AXONOMETRIC—A   single   view   of   an   object depicting  all  three  dimensions.  The  projection lines  are  parallel  to  each  other  and  perpendicular to the plane of projection. The object is inclined to  the  plane  of  projection,  thereby  allowing  the viewer to see three dimensions. AZIMUTH   (Surveying)—The   horizontal   direc- tion  of  a  line  measured  clockwise  from  a  reference plane,   usually   the   meridian;   often   called FORWARD   AZIMUTH   to   differentiate   from BACK  AZIMUTH.  In  the  basic  control  surveys of  the  United  States,  azimuths  are  measured clockwise  from  south  following  the  continental European   geodetic   practice.   However,   this practice  is  not  followed  in  all  countries. AZIMUTH  MARK—A  marked  point  visible from  a  survey  station,  the  azimuth  to  which  is determined  for  use  in  dependent  surveys. BACK  AZIMUTH—As  the  azimuth  of  the  line from  A  to  B  is  known  as  the  forward  azimuth, the azimuth of the same line from B to A is known as the back azimuth. BACKSIGHT—1. In traversing, a backsight (BS) is a sight on a previously established traverse or triangulation station, that is not the closing sight of  the  traverse.  2.  In  leveling,  a  backsight  is  a reading on a rod held on a point whose elevation has   been   previously   determined   and   not   the closing  sight  of  a  level  line. BALANCING  A  SURVEY—Distributing  correc- tions through a closed traverse to eliminate the error  of  closure  and  to  obtain  an  adjusted  position for  each  traverse  station. BASE   COURSE—The   layer   of   material   im- mediately  beneath  the  surface  or  intermediate course.  It  may  be  composed  of  crushed  stone, crushed  slag,  crushed  or  uncrushed  gravel  and sand, or combinations of these materials. It also may  be  bound  with  asphalt. BASE  LINE—A  surveyed  line  established  with more  than  usual  care  as  the  known  length  of  a triangle side for computing other triangle sides. BASE   CONTROL—Horizontal   or   vertical control,  the  positions  of  whose  stations  have  been accurately  coordinated  and  correlated,  forming a  framework  to  which  other  surveys  are  adjusted. BEARING—The   direction   of   a   line   within   a quadrant, with respect to the meridian. Bearings are measured clockwise or counterclockwise from north  or  south,  depending  on  the  quadrant. BENCH  MARK-A  relatively  permanent  object, natural or artificial, bearing a marked point whose AI-2