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CHAPTER  12 ELECTRONIC SURVEYING EQUIPMENT - 14071_261

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CHAPTER  12 ELECTRONIC SURVEYING EQUIPMENT Chapter 12 of the EA3 TRAMAN introduced you to   electronic   surveying   equipment,   specifically electronic distance-measuring equipment. This chapter is intended to supplement what you learned in the EA3 TRAMAN  discussion,  and,  in  addition,  introduces  you to  the  basic  principles  and  uses  of  other  types  of electronic  surveying  equipment. As a rule, the EA seldom has the need or opportunity to use any of the equipment discussed in this chapter; however, when the need and occasion arise, the EA should have at least a basic familiarity with the different electronic  equipment  used  in  surveying.  This  chapter provides  that  familiarization. ELECTRONIC DISTANCE-MEASURING  (EDM) EQUIPMENT When  electronically  determining  the  straight-line distance (horizontal or slope) between two points or stations, you use equipment that (1) sends an electronic impulse  of  known  velocity  or  rate  of  speed  and  (2) measures the time it takes for the impulse to travel the length of the interval between the points. Then, by using the well-known equation of distance = rate x time, the length of the interval is determined. Two types of electronic distance meters (simply referred to as EDMs) are commonly used. They are the electromagnetic  (microwave) instruments and the electro-optical  (light wave) instruments. In this section, we  will  briefly  discuss  both  types  of  instruments; however, since there are many different makes and models of EDMs on the market and since you should always  study  the  manufacturer’s  operating  instructions before you try to use the equipment, only the basic principles of the operation and use of EDM equipment is covered. For in-depth discussions of EDM principles, you should read publications, such as  Surveying  Theory and Practice, by Davis, Foote, Anderson, and Mikhail. ELECTROMAGNETIC (MICROWAVE) EDM INSTRUMENTS Electromagnetic  EDMs,  first 1950s,  use  high-frequency  radio developed  in  the waves.  The  first generation  of  this  equipment  was  very  precise  for measuring long distances; however, it was too bulky and heavy for the practicing surveyor’s needs. Over the years, the equipment has undergone rapid improvement to  the  extent  that  modern  electromagnetic  EDMs  are smaller, more portable, and are being equipped with direct  readout  capability. When  used,  two  identical  and  interchangeable instruments, such as shown in figure 12-1, are setup at both ends of the line that you are measuring. This line must be unobstructed, but intervisibility is not required; so, you can make observations in fog or during other unfavorable  weather  conditions.  As  illustrated  in  figure 12-2, the sending (master) instrument transmits a series of modulated radio waves to the receiving (remote) instrument. The remote instrument interprets these signals and sends them back to the master unit that measures the time required for the radio waves to make the round trip. The distance is computed based on the velocity of the radio waves. Because this velocity is affected  by  atmospheric  conditions,  corrections  for temperature  and  barometric  pressure  are  applied according  to  the  operating  instructions  provided  with the  equipment. ELECTRO-OPTICAL  (LIGHT  WAVE) EDM INSTRUMENTS Electro-optical  EDMs  use  the  velocity  of  light waves  to  determine  the  distance  between  two  points. The  earliest  of  these  instruments,  typified  by  the Geodimeter, was developed during the same decade as the  electromagnetic  EDMs.  Figure  12-3  shows  an example of a Geodimeter. Like the electromagnetic instruments,  the  first  generation  of  electro-optical instruments were heavy, bulky, and not well suited to the needs of the practicing surveyor; however, through later development,   modern   electro-optical   EDMs   are smaller, lighter, easier to use, and require less power. Modern short-range instruments have ranges from 0.3 miles  to  3  miles.  Longer  range  instruments,  using coherent laser light, have ranges from 50 feet to 36 miles. To  use  an  electro-optical  EDM,  you  set  up  the instrument at one end of the line being measured and a 12-1



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