Sexagesimal or North American System In the sexagesimal or North American system, the circle is divided into 360 equal parts known as  DEGREES  of  arc,  each  degree  into  60  equal parts  known  as  MINUTES  of  arc,  and  each minute into 60 equal parts known as SECONDS of  arc.  As  an  example,  angles  in  this  system are  written  as  263047’16’’.48  which  is  read  as “two   hundred   sixty-three   degrees,   forty-seven minutes, and sixteen point four eight seconds of arc. ”   In  the  United  States,  this  is  the  most commonly used system of angular measurement. MORE  UNITS  OF  MEASUREMENT Aside  from  the  units  of  measurement discussed  above,  the  EA  must  also  deal  with other  units  of  measurement,  such  as  TIME, TEMPERATURE,  PRESSURE,  and  so  forth. He  must  use  exact  time  in  computing  problems in  astronomy  and  some  laboratory  works.  He must be able to apply temperature corrections to his  tape  readings.  He  must  also  evaluate  the effect   of   atmospheric   pressure   at   different elevations   and   get   involved   in   some   other types  of  measure  that  will  be  discussed  in  the following   paragraphs. Centisimal or Metric System Time Measurement In  the  centisimal  or  metric  system,  the  full circle  is  divided  into  four  quadrants,  and  each quadrant is divided into 100 equal parts known as  GRADS  or  GRADES.  Each  grad  is  further divided  into  decimal  parts.  As  an  example,  angles in  this  system  are  written  as  376^g.7289,  or 376^g72´89"   which   is   read   as   “three   hundred seventy-six point seven two eight nine grads,” or as “three hundred seventy-six grads, seventy-two centisimal  minutes,  and  eighty-nine  centisimal seconds. ” Mil System In  the  mil  system,  the  circle  is  divided  into 6,400  equal  parts  known  as  MILS.  The  mil  is divided  into  decimal  parts.  As  an  example,  angles in this system are written as 1728.49 roils, which is read as “one thousand seven hundred twenty- eight point four nine mils.” This system is used principally by the artillery people. The significance of this unit of angular measure is the fact that 1 mil is the angle that will subtend 1 yd at a range of  1,000  yd. The  relationship  among  values  in  the  three systems  of  angular  measure  are  as  follows: For  practical  purposes  in  everyday  affairs  and in surveying, the measurement of time intervals is  of  great  concern.  The  time  used  in  everyday  life is known as STANDARD TIME and is based on the mean apparent revolution of the sun around the  earth  because  of  the  earth’s  rotation  on  its axis. Standard time is used in surveying to regulate the  normal  day’s  operations.  But,  when  it  is necessary  to  observe  the  sun  or  the  stars  to determine  the  azimuth  of  a  line  or  the  position of a point on the earth’s surface, the surveyor uses three other kinds of time. They are APPARENT (true) SOLAR TIME, CIVIL (mean solar) TIME, and SIDEREAL (star) TIME. You will learn more about these different times when you study the chapter  on  “Geodesy  and  Field  Astronomy”  in Engineering  Aid  1  &  C. In  all  four  kinds  of  time,  the  basic  units  of measure are the YEAR, DAY, HOUR, MINUTE, and  SECOND  of  time.  The  duration  of  any  one of these units is not the same for all kinds of time. For example, the sidereal day is approximately 4 min  shorter  than  a  standard-  or  civil-time  day. In  the  practice  of  surveying,  it  is  customary to  say,  or  write,  the  time  of  day  as  the  number of  hours,  minutes,  and  seconds  since  midnight. Then   the   recorded   time   would   appear,   for example,   as   16h37*52S.71   which  is  read  as “sixteen  hours,  thirty-seven  minutes,  and  fifty- two  point  seven  one  seconds  of  time.” Units of time measure are sometimes used to designate the sizes of angles. The longitude of a point on the earth’s surface is often expressed in this manner. The relationship between the units 1-30