LEVEL  SURFACE—A  surface  that  is  parallel with the spheroidal surface of the earth, such as a  body  of  still  water. LEVELING—The   operations   of   measuring vertical   distances,   directly   or   indirectly,   to determine elevations. LINE  OF  SIGHT—1.  The  straight  line  between two points. This line is in the direction of a great circle, but it does not follow the curvature of the earth. 2. The line extending from an instrument along  which  distant  objects  are  seen  when  viewed with  a  telescope  or  other  sighting  device. LINE  OF  SYMMETRY—A  line  that  divides  an object  into  two  equal  identical  parts;  used  only when  the  two  halves  of  an  object  are  identical. LIQUID  LIMIT—The  upper  limit  of  the  plastic state, expressed as the moisture content at which the flow curve intersects the “25 blows” ordinate. LOGARITHM—The  exponent  or  the  power  to which  a  fixed  number,  called  the  base,  must  be raised  to  produce  a  given  number;  for  example, common  log LOOP TRAVERSE—A closed traverse that starts and ends at the same station. MAGNETIC  AZIMUTH—An  azimuth  measured with  reference  to  the  direction  indicated  by  a magnetic  compass  needle.  Magnetic  azimuth  is measured from magnetic north, which is east or west  of  true  north,  as  shown  by  the  magnetic declination. MAGNETIC   DECLINATION—The   angular amount  that  a  magnetic  compass  needle  points eastward  or  westward  from  true  north. MAP  MEASURE—Instrument  used  when  lengths of  irregular  outlines  are  measured. MATCH  LINES—Lines  used  when  an  object  is too large to fit on a single drawing sheet and must be continued on another sheet. The points where the   object   stops   on   one   sheet   and   continues on   the   next   sheet   must   be   identified   with corresponding   match   lines.   They   are   medium weight lines indicated with the words match line and  referenced  to  the  sheet  that  has  the  cor- responding  match  line. MATHEMATICS—The  science  that  deals  with the relationships that exist between quantities and operations,  and  with  methods  by  which  these relationships   can   be   applied   to   determine unknown  quantities  from  given  or  measured  data. MEAN SEA LEVEL—The average height of the sea  for  all  stages  of  the  tide.  Mean  sea  level  at numerous tide-gaging stations usually forms the basis  of  a  level  datum  for  large  areas. MEASURED  ANGLES—Angles  that  are  either vertical  or  horizontal. MECHANICAL   DRAFTING—Any   drawing   in which the pen or pencil is guided by a mechanical device. MERIDIAN—A   north-south   line   from   which longitudes  (or  departures)  and  azimuths  are reckoned. MIDPOINT—That point on the arc of a circular curve  that  is  the  same  distance  from  both  ends of  the  arc. MILITARY    STANDARDS    (MIL-STDs)— Instructions  set  forth  by  the  Department  of Defense that members of all armed services are required to follow. Only a few of these standards refer  directly  to  drafting. MINUTE—A 60th part of a degree used to define the  size  of  an  angle. MOISTURE   CONTENT   (w.)—The   ratio, expressed as a percentage, of the weight of water in a given soil mass to the weight of solid particles. MONUMENT—Any   object   or   collection   of objects that indicates the position on the ground of a survey station. In military surveys, the term monument  usually  refers  to  a  stone  or  concrete station marker containing a special bronze plate on  which  the  exact  station  point  is  marked. NONCIRCULAR  CURVE—A  curve  composed of  a  series  of  extremely  small  circular  arcs  of varying  radii. NON-NORMAL LINE—A line that is oblique to one  or  more  of  the  planes  of  projection. AI-9