of 10 mi is only 0.04 greater that its subtended chord; that a plane surface tangent to the spherical arc  has  departed  only  about  8  in.  at  1  mi  from the  point  of  tangency;  and  that  the  sum  of  the angles of a spherical triangle is only 1 sec greater than the sum of the angles of a plane triangle for a triangle having an area of approximately 75 sq mi on the earth’s surface, it is just reasonable that the  errors  caused  by  the  earth’s  curvature  be considered only in precise surveys of large areas. In  this  training  manual,  we  will  discuss primarily  the  methods  used  in  plane  surveying rather than those used in geodetic surveying. GEODETIC  SURVEYING GEODETIC  SURVEYING  is  a  process  of surveying in which the shape and size of the earth are considered. This type of survey is suited for large areas and long lines and is used to find the precise   location   of   basic   points   needed   for establishing control for other surveys. In geodetic surveys, the stations are normally long distances apart, and more precise instruments and surveying methods  are  required  for  this  type  of  surveying than  for  plane  surveying. The  shape  of  the  earth  is  thought  of  as  a spheroid, although in a technical sense, it is not really a spheroid. In 1924, the convention of the International  Geodetic  and  Geophysical  Union adopted 41,852,960 ft as the diameter of the earth at the equator and 41,711,940 ft as the diameter at  its  polar  axis.  The  equatorial  diameter  was computed on the assumption that the flattening of the earth caused by gravitational attraction is exactly 1/297. Therefore, distances measured on or  near  the  surface  of  the  earth  are  not  along straight lines or planes, but on a curved surface. Hence,  in  the  computation  of  distances  in  geodetic surveys,  allowances  are  made  for  the  earth’s minor and major diameters from which a spheroid of  reference  is  developed.  The  position  of  each geodetic  station  is  related  to  this  spheroid.  The positions  are  expressed  as  latitudes  (angles  north or south of the Equator) and longitudes (angles east or west of a prime meridian) or as northings and castings on a rectangular grid. The methods used in geodetic surveying are beyond  the  scope  of  this  training  manual. TYPES OF SURVEYS Generally, surveys can be classified by names descriptive   of   their   functions.   Functionally, surveys are classed as construction, topographic, route,  and  special.  Special  surveys,  such  as photogrammetry, hydrography,  and  property surveys, are conducted either with special equip- ment or for a special purpose. Some of the types of  surveys  that  you  may  perform  as  an  EA  are discussed  in  the  following  paragraphs. CONSTRUCTION  SURVEYS CONSTRUCTION   SURVEYS   (sometimes called   engineering   surveys)   are   conducted   to obtain  data  essential  for  planning,  estimating, locating,  and  layout  for  the  various  phases  of construction  activities  or  projects.  This  type of  survey  includes  reconnaissance,  preliminary, location,  and  layout  surveys. The objectives of engineering or construction surveying  include  the  following: 1.  The  obtaining  of  reconnaissance  informa- tion and preliminary data required by engineers for  selecting  suitable  routes  and  sites  and  for preparing structural designs 2.   The   defining   of   selected   locations   by establishing  a  system  of  reference  points 3.   The   guidance   of   construction   forces   by setting  stakes  or  otherwise  marking  lines,  grades, and   principal   points   and   by   giving   technical assistance 4.  The  measuring  of  construction  items  in place  for  the  purpose  of  preparing  progress reports 5. The dimensioning of structures for prepa- ration  of  as-built  plans All  of  the  above  objectives  are  called  engineer- ing  surveys  by  the  American  Society  of  Civil Engineers  (ASCE),  and  the  term  construction surveys  is  applied  to  the  last  three  objectives  only, The Army Corps of Engineers, on the other hand, generally applies the term construction surveying to  all  of  the  objectives  listed  above. Engineering  and/or  construction  surveys,  then, form part of a series of activities leading to the construction of a man-made structure. The term structure is usually confined to something that is built  of  structural  members,  such  as  a  building or  a  bridge.  It  is  used  here  in  a  broader  sense, however, to include all man-made features, such as graded areas; sewer, power, and water lines; roads and highways; and waterfront structures. Construction  surveys  normally  cover  areas considered small enough to use the plane survey- ing  methods  and  techniques. 11-2