INTERSECTION—A   method   of   locating   the horizontal  position  of  a  point  by  observations from two or more points of known position, thus measuring directions that intersect at the station being  located.  A  station  whose  horizontal  position is  located  by  an  intersection  is  known  as  an intersection  station. IRRATIONAL   NUMBER—Real   number   that cannot be expressed in the ratio of two integers; for  example,  3,  n. IRREGULAR   POLYGON—A   nonequilateral polygon. ISOMETRIC  AXIS—Axis  used  in  isometric projections  and  drawings.  Each  line  in  the  axis forms  an  angle  of  120⁰ with  the  adjacent  line, easily   constructed   with   a   straightedge   and   a 30°/60°   triangle. ISOMETRIC  DRAWING—Same  as  an  isometric projection  except  that  the  dimensions  of  the  object drawn  are  scaled  and  not  projected. ISOMETRIC  PROJECTION—A  single  view projection of an object showing three dimensions. The object is inclined so all faces make the same angle with the plane of projection, making all lines and  surfaces  foreshortened  in  the  same  ratio.  This allows  one  scale  to  be  used  throughout. ISOSCELES TRIANGLE—A triangle having two equal sides. LATERAL  FACES—Faces  or  surfaces  forming the sides of a solid figure; also known as lateral surfaces. LATERAL SURFACES—See LATERAL FACES. LATITUDE—In   plane   surveying,   the   amount that one end of a line is north or south of the other end.  As  the  plane  coordinates  of  a  point  are known as the casting and northing of the point, the latitude is the difference between the northings of the two ends of the line, which may be either plus  or  minus.  (See  DEPARTURE.) LAW OF COSINES—A law of mathematics that states that, in any triangle, the square of one side is  equal  to  the  sum  of  the  squares  of  the  other two  sides  minus  twice  the  product  of  these  two sides multiplied by the cosine of the angle between them. This statement may be expressed in formula form  as  follows: a2=b2+c2–2bccos    A bz=a2+c2–2accos    B cz=a2+b2–2abcos    C LAW  OF  SINES—A  law  of  mathematics  that states that the lengths of the sides of any triangle are  proportional  to  the  sines  of  their  opposite angles. It is expressed in formula form as follows: LAW  OF  TANGENTS—A  law  of  mathematics that  states  that,  in  any  triangle,  the  difference between two sides is to their sum as the tangent of half the difference of the opposite angles is to the  tangent  of  half  their  sum.  For  any  pair  of sides—as,  side  a  and  side  b—the  law  may  be expressed  as  follows: LEADER  LINES—Thin  unbroken  lines  used  to connect  numbers,  references,  or  notes  to appropriate  surfaces  or  lines. LEGEND—A  description,  explanation,  table  of symbols,  and  so  on,  printed  on  a  map  or  chart for a better understanding and interpretation of it. LEVEL—1.  Pertaining  to  a  level  surface;  2.  To make   horizontal   at   the   point   of   observation; 3.  An  instrument  for  leveling. LEVEL   DATUM—A   level   surface   to   which elevations  are  referred.  The  generally  adopted level datum for leveling in the United States is mean  sea  level.  For  local  surveys,  an  arbitrary level  datum  is  often  adopted  and  defined  in  terms of an assumed elevation for some physical mark (bench  mark). LEVEL  LINE—1.  A  line  in  a  horizontal  plane; 2.   A   line   over   which   leveling   operations   are accomplished. LEVEL NET—Lines of spirit leveling connected together  to  form  a  system  of  loops  or  circuits extending  over  an  area.  This  is  also  called  a vertical  control  net. AI-8