globe at an altitude of approximately 12,400 milesabove the earth. These satellites transmit unique signalsthat are encoded with information that enables groundreceivers to measure the travel time of the signals fromsatellite to receiver. That travel time is then convertedto distance using the velocity of electromagnetic energythrough the atmosphere.Determining point locations using GPS proceduresconsist essentially of measuring distances from pointsat unknown locations to satellites whose positions areknown at the instant of observation. In concept this isidentical to performing resection (chapter 9 of thisTRAMAN), using distances that are measured from apoint of unknown location to three or more stationswhose positions are known.For a more thorough discussion of both globalpositioning systems and Doppler positioning systems,you can refer to commercial books, such as ElementarySurveying, by Wolf and Brinker.TRIANGULATIONIn your previous studies you learned that a principalmethod of locating points in horizontal control istraversing. As you know, traversing requires thatdistances and angles be measured at all stations. In thischapter you will learn another method. Thismethod— triangulation —requires that distances bemeasured only at the beginning, at specified intervals,and at the end of the survey.Both the triangulation method and the traversemethod of control are based on the character of theterrain, and not on the degree of precision to be attained;that is, each system is equally precise under theconditions in which each is used. Discussion oftriangulation in this chapter normally is limited totriangles having sides less than 3,000 yards in length andto triangulation nets that do not extend more than 25,000yards.The triangulation method is used principally insituations where the chaining of distances is impossibleor infeasible except with the use of electronic measuringdevices. Suppose you want to locate a point, say, pointC, which is offshore; and the measured baseline, AB, islocated on the shore. In this situation the triangulationmethod is used because the chaining of distances isimpossible. The chaining of long distances, especiallyin rough country, also is not always possible; therefore,triangulation is used to establish horizontal control inlarge-area surveys.In some large-area surveys conducted bytriangulation, you must consider factors involving thecurvature of the earth; hence, in such cases, geodetictriangulation is involved. Whether or not the curvatureof the earth must be considered depends upon the areacovered and the precision requirements of the survey.The error resulting in horizontal measurements whenyou ignore the curvature of the earth amounts to about1 foot in 34 1/2 miles. This means that in most ordinarysurveying, an area of 100 square miles may beplane-triangulated without significant error. In thisdiscussion we are concerned with plane triangulationonly. For a discussion of geodetic triangulation, youshould refer to commercial publications.This section contains information on the three typesof triangulation networks and the usual procedure forconducting a triangulation survey. Also covered areprimary and secondary triangulation stations, types ofsignals used in marking triangulation stations, andchecking for precision and locations of points.SUPERVISION AND TRIANGULATIONSURVEYSIn triangulation surveys, the duties of the EA1 arethose of party chief; that is, he directs the triangulationsurvey. He keeps the triangulation notes and should beat the spot where any important measurement is madeso that he can verify the readings personally. He isresponsible for selecting triangulation stations anderecting triangulation signals and towers. He determinesthe degree of precision to be attained. He also performsthe computations necessary to determine horizontallocations of the points in the triangulation system bybearing and distance.Triangulation is used extensively as a means ofcontrol for topographic and similar surveys. Atriangulation system consists of a series of triangles. Atleast one side of each triangle is also aside of an adjacenttriangle; two sides of a triangle may form sides ofadjacent triangles. By using the triangulation method ofcontrol, you do not need to measure the length of everyline. However, two lines are measured in eachsystem—one line at the beginning and one at the closingof the triangulation system. These lines are called baselines and are used as a check against the computedlengths of the other lines in the system. Therecommended length of a base line is usually one sixthto one fourth of that of the sides of the principaltriangles. The transcontinental system established by theU.S. Coast and Geodetic Survey (now the NationalGeodetic Survey) is an example of an extensive15-24