4. Distance measuring instruments 



Two electronic instruments are used for trilateration and the 

 measurement of base lines and long traverse lines.. Both instruments 

 determine distance between points and eliminate the necessity to 

 traverse between them. The component units of both instruments are 

 portable; the time required for a measurement is short; and the accura- 

 cy of each is high. These instruments are the geodimeter and the 

 tellurometer. 



a. Geodimeter 



The geodimeter is a precise instrument for measuring dis- 

 tances by employing the known velocity of propagation of light waves. 

 A modulated beam of light is transmitted to an unattended passive 

 prism reflector which returns the light to the transmitter. The time 

 interval is measured indirectly, and the distance then is computed. 



Four models of this instrument are available, each having certain 

 advantages. The first, Model NASM-1, is large and bulky and is consid- 

 ered unsuitable for the use except where it can be transported by 

 vehicle; it was designed for the measurement of base lines, and for 

 that purpose has proven excellent. The second, Model NASM-2, is 

 more of a refinement of Model NASM-1 than a separate model. Mod- 

 els NASM-3 and NASM-4 are designed for field service and, as such, 

 are compact and portable; however, with the reduction in bulk, some 

 accuracy has been lost. All the instruments are used for line-of-sight 

 measurements and need a power source at only one end of the line. 



(1) Model NASM-2 



This model has a maximum range of 30 nautical miles and, 

 because of its size and weight, is used primarily for base line work 

 and for checking lines in existing arcs of triangulation. It is made up 

 of the two basic units, the transmitter-receiver and a reflector. The 

 weight of the transmitter is about 220 pounds. The reflector is a 

 group of prisms that weighs approximately 35 pounds. The disadvantages 

 of this model, in addition to its size, are that it must have an available 

 power source and that it must be operated at night. Power requirements 

 are 140 watts from a 110- or 220- volt, 50- to 60-cycle current which 

 may be supplied by a portable generator. This instrument allows a 

 rapid set-up, and a line can be measured in less than three hours with 

 first-order accuracy. Atmospheric conditions can affect the accuracy 

 somewhat, especially on the longer lines. Shorter lines must be used if 

 observing cannot be done under good weather conditions. It is possible 



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