• Home
  • Download PDF
  • Order CD-ROM
  • Order in Print
Chain of Quadrilaterals - 14070_360
Figure  15-18.Tripod  targets. - 14070_362

Engineering Aid 1 - Advanced Structural engineering guide book
Page Navigation
  334    335    336    337    338  339  340    341    342    343    344  
for AC and CD, and triangle  BCD for BD and CD. The solutions  for  each  of  the  overlapping  triangles  are summarized  as  follows: secondary triangulation station is one that is sighted from  primary  stations  but  is  not  itself  used  as  an instrument station. Only the primary stations are used to extend the system of figures. Each triangulation station must be marked in a way that will make it visible from other stations from which it is sighted. A mark of this kind is called a triangulation signal.  For a secondary station, the signal may be relatively simple, such as a pole set in the ground or in a pile of rocks, or a pole set on the ground and held erect by guys. An object already in place, such as a flag pole, a  church  spire,  or  a  telegraph  pole,  will  serve  the purpose. When the instrument itself must be elevated for visibility, a tower is used. Targets As you can see, for each of the unknown sides of the  quadrilateral  (AC, CD, and BD), values  have  been obtained by two different routes. You can also see that there are discrepancies in the values, almost the same for AC and BD and smaller for CD. All the discrepancies shown are much larger than would be tolerable in actual practice; they reflect the high imprecision of the original protractor measurement of the angles. The example has been given here only to illustrate the basic principles and procedures  of  chain-of-quadrilateral  triangulation. Later in this chapter you will see how observed angles (measured in the field with the required precision) are adjusted  to  ensure  that  values  computed  by  different routes will be practically close enough to each other to satisfy  precision  requirements. TRIANGULATION  STATIONS,  SIGNALS, AND INSTRUMENT SUPPORTS All  triangulation  stations  of  third  order  or  higher must be identified on the ground with a station marker, at least two reference markers, and, if necessary, an azimuth marker. These markers are usually embedded in or etched on a standard station monument. Station markers,  monuments,  and  station  referencing  are discussed in the EA3 TRAMAN. For low-order surveys, unless otherwise required, the stations may be marked with 2-inch by 2-inch wooden hubs. A primary triangulation station is both a sighted station and an instrument station; that is, it is a point sighted from other stations and also a point where an instrument  is  set  up  for  sighting  other  stations.  A A  target  is  generally  considered  to  be  a nonilluminating signal. Target requirements can be met by three general types—tripods, bipeds, and poles—all of which may incorporate variations. The targets are constructed of wood or metal framework with cloth covers. SIZE OF TARGET.—  For a target to be easily visible against both light and dark backgrounds, it should be constructed in alternating belts of red and white or red and yellow. For ready bisection, it should be as narrow as possible without sacrificing distinctness. A target that subtends an angle of 4 to 6 seconds of arc will fulfill this purpose. Since 1 second of arc equals 0.5 centimeters at a 1-kilometer distance, an angle of 6 seconds  requires  a  target  3  centimeters  wide  at  1 kilometer or 30 centimeters at 10 kilometers. Under adverse lighting conditions, the target width will have to be increased. Flags of an appropriate size may be added to aid in finding the target. All cloth used on targets should be slashed after construction to minimize wind   resistance. TRIPOD TARGET.— The tripod target is the most  satisfactory  from  the  standpoint  of  stability, simplicity  of  construction,  durability,  and  accuracy.  It ranges from a simple hood of cloth, cut and sewn into a pyramid shape and slipped over the instrument tripod, to the permanent tripod with the legs embedded in concrete, sides braced, a vertical pole emplaced, and the upper  part  boarded  up  and  painted.  Temporary  tripod targets may be constructed of 2-inch by 2-inch lumber, pipes, poles, or bamboo joined at one end by wire or bolts threaded through drilled holes. The tripod must be 15-27







Western Governors University

Privacy Statement
Press Release
Contact

© Copyright Integrated Publishing, Inc.. All Rights Reserved. Design by Strategico.