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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) EQUIPMENTWhen 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 manufacturers 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 INSTRUMENTSElectromagnetic EDMs, first developed in the 1950s, use high-frequency radio waves. The first generation of this equipment was very precise for measuring long distances; however, it was too bulky and heavy for the practicing surveyors 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 |
 
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