globe  at  an  altitude  of  approximately  12,400  miles above the earth. These satellites transmit unique signals that are encoded with information that enables ground receivers to measure the travel time of the signals from satellite to receiver. That travel time is then converted to distance using the velocity of electromagnetic energy through  the  atmosphere. Determining point locations using GPS procedures consist  essentially  of  measuring  distances  from  points at  unknown  locations  to  satellites  whose  positions  are known at the instant of observation. In concept this is identical  to  performing  resection  (chapter  9  of  this TRAMAN),  using  distances  that  are  measured  from  a point of unknown location to three or more stations whose positions are known. For  a  more  thorough  discussion  of  both  global positioning  systems  and  Doppler  positioning  systems, you can refer to commercial books, such as  Elementary Surveying, by Wolf and Brinker. TRIANGULATION In your previous studies you learned that a principal method  of  locating  points  in  horizontal  control  is traversing.  As  you  know,  traversing  requires  that distances and angles be measured at all stations. In this chapter  you  will  learn  another  method.  This method—  triangulation —requires  that  distances  be measured only at the beginning, at specified intervals, and at the end of the survey. Both the triangulation method and the traverse method of control are based on the character of the terrain, and not on the degree of precision to be attained; that  is,  each  system  is  equally  precise  under  the conditions   in   which   each   is   used.   Discussion   of triangulation in this chapter normally is limited to triangles having sides less than 3,000 yards in length and to triangulation nets that do not extend more than 25,000 yards. The triangulation method is used principally in situations where the chaining of distances is impossible or infeasible except with the use of electronic measuring devices. Suppose you want to locate a point, say, point C, which is offshore; and the measured baseline, AB, is located  on  the  shore.  In  this  situation  the  triangulation method is used because the chaining of distances is impossible.  The  chaining  of  long  distances,  especially in rough country, also is not always possible; therefore, triangulation is used to establish horizontal control in large-area   surveys. In   some   large-area   surveys   conducted   by triangulation,  you  must  consider  factors  involving  the curvature of the earth; hence, in such cases,  geodetic triangulation  is  involved.  Whether  or  not  the  curvature of the earth must be considered depends upon the area covered  and  the  precision  requirements  of  the  survey. The  error  resulting  in  horizontal  measurements  when you ignore the curvature of the earth amounts to about 1 foot in 34 1/2 miles. This means that in most ordinary surveying,  an  area  of  100  square  miles  may  be plane-triangulated without significant error. In this discussion  we  are  concerned  with  plane  triangulation only. For a discussion of geodetic triangulation, you should refer to commercial publications. This section contains information on the three types of  triangulation  networks  and  the  usual  procedure  for conducting a triangulation survey. Also covered are primary and secondary triangulation stations, types of signals used in marking triangulation stations, and checking for precision and locations of points. SUPERVISION AND TRIANGULATION SURVEYS In triangulation surveys, the duties of the EA1 are those of party chief; that is, he directs the triangulation survey. He keeps the triangulation notes and should be at  the  spot  where  any  important  measurement  is  made so that he can verify the readings personally. He is responsible  for  selecting  triangulation  stations  and erecting triangulation signals and towers. He determines the degree of precision to be attained. He also performs the computations necessary to determine horizontal locations  of  the  points  in  the  triangulation  system  by bearing  and  distance. Triangulation is used extensively as a means of control  for  topographic  and  similar  surveys.  A triangulation system consists of a series of triangles. At least one side of each triangle is also aside of an adjacent triangle;  two  sides  of  a  triangle  may  form  sides  of adjacent triangles. By using the triangulation method of control, you do not need to measure the length of every line.   However,   two   lines   are   measured   in   each system—one line at the beginning and one at the closing of the triangulation system. These lines are called  base lines and are used as a check against the computed lengths  of  the  other  lines  in  the  system.  The recommended length of a base line is usually one sixth to  one  fourth  of  that  of  the  sides  of  the  principal triangles. The transcontinental system established by the U.S. Coast and Geodetic Survey (now the National Geodetic  Survey)  is  an  example  of  an  extensive 15-24