for AC and CD, and triangle BCD for BD and CD. Thesolutions for each of the overlapping triangles aresummarized as follows:secondary triangulation station is one that is sightedfrom primary stations but is not itself used as aninstrument station. Only the primary stations are used toextend the system of figures.Each triangulation station must be marked in a waythat will make it visible from other stations from whichit is sighted. A mark of this kind is called a triangulationsignal. For a secondary station, the signal may berelatively simple, such as a pole set in the ground or ina pile of rocks, or a pole set on the ground and held erectby guys. An object already in place, such as a flag pole,a church spire, or a telegraph pole, will serve thepurpose. When the instrument itself must be elevated forvisibility, a tower is used.TargetsAs you can see, for each of the unknown sides ofthe quadrilateral (AC, CD, and BD), values have beenobtained by two different routes. You can also see thatthere are discrepancies in the values, almost the samefor AC and BD and smaller for CD. All the discrepanciesshown are much larger than would be tolerable in actualpractice; they reflect the high imprecision of the originalprotractor measurement of the angles. The example hasbeen given here only to illustrate the basic principles andprocedures of chain-of-quadrilateral triangulation.Later in this chapter you will see how observed angles(measured in the field with the required precision) areadjusted to ensure that values computed by differentroutes will be practically close enough to each other tosatisfy precision requirements.TRIANGULATION STATIONS, SIGNALS,AND INSTRUMENT SUPPORTSAll triangulation stations of third order or highermust be identified on the ground with a station marker,at least two reference markers, and, if necessary, anazimuth marker. These markers are usually embeddedin or etched on a standard station monument. Stationmarkers, monuments, and station referencing arediscussed in the EA3 TRAMAN. For low-order surveys,unless otherwise required, the stations may be markedwith 2-inch by 2-inch wooden hubs.A primary triangulation station is both a sightedstation and an instrument station; that is, it is a pointsighted from other stations and also a point where aninstrument is set up for sighting other stations. AA target is generally considered to be anonilluminating signal. Target requirements can be metby three general types—tripods, bipeds, and poles—allof which may incorporate variations. The targets areconstructed of wood or metal framework with clothcovers.SIZE OF TARGET.— For a target to be easilyvisible against both light and dark backgrounds, itshould be constructed in alternating belts of red andwhite or red and yellow. For ready bisection, it shouldbe as narrow as possible without sacrificing distinctness.A target that subtends an angle of 4 to 6 seconds of arcwill fulfill this purpose. Since 1 second of arc equals 0.5centimeters at a 1-kilometer distance, an angle of 6seconds requires a target 3 centimeters wide at 1kilometer or 30 centimeters at 10 kilometers. Underadverse lighting conditions, the target width will haveto be increased. Flags of an appropriate size may beadded to aid in finding the target. All cloth used ontargets should be slashed after construction to minimizewind resistance.TRIPOD TARGET.— The tripod target is themost satisfactory from the standpoint of stability,simplicity of construction, durability, and accuracy. Itranges from a simple hood of cloth, cut and sewn into apyramid shape and slipped over the instrument tripod,to the permanent tripod with the legs embedded inconcrete, sides braced, a vertical pole emplaced, and theupper part boarded up and painted. Temporary tripodtargets may be constructed of 2-inch by 2-inch lumber,pipes, poles, or bamboo joined at one end by wire orbolts threaded through drilled holes. The tripod must be15-27
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