ELECTRONIC DISTANCE MEASUREMENT

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ELECTRONIC DISTANCE MEASUREMENT
AGA Geodimeter NASM-2A

• Robert Burtch
• Surveying Engineering Department
• Ferris State University

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INTRODUCTION

• Introduced in 1950s evolved into total stations
  today
• Types
• Infrared or laser light utilize transmitter at
  one end and reflecting prism at other
• Some are reflectorless
• Long range 10-20 km
• Medium range 3-10 km
• Short range 0.5-3 km
• Microwave utilize receiver/transmitter at both
  ends of line

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PRINCIPLES

• Wave travels along x-axis at velocity of
  299,792.5 0.4 km/s
• Frequency time taken for one complete
  wavelength
• Relationship
• ? wavelength in meters
• c velocity, in km/s
• f frequency, in hertz (one cycle per second)

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• PRINCIPLES OF EDM MEASUREMENT

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PRINCIPLES

• Modulated wave leaves EDM then reflected back to
  instrument measures double distance (2L)
• Partial wavelength measured from phase delay
  between transmitted and reflected

• n? whole number of
• wavelengths
• f partial wavelength

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PRINCIPLES

• EDM can send 3-4 modulated waves at different
  frequencies
• Find n by substituting these into distance
  equation
• Some EDM use pulsed laser emissions
• Require to determine distance by measuring travel
  time to and from EDM

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PRINCIPLES

• Velocity of light affected by
• Temperature
• Atmospheric pressure
• Water vapor content
• Correction determined using nomograph or
  automatically in automatic processor by inputting
  temperature and pressure

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ATMOSPHERIC CORRECTION

• Atmospheric correction graph

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ATMOSPHERIC CORRECTION

• Insignificant for short-wave light-wave EDM
• Important for long range, especially microwave

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EDM INSTRUMENT CHARACTERISTICS

• Distance range 800 1,000m with average
  atmospheric conditions and single prism
• Short-range can be extended to 1,300m with 3
  prisms
• Long-range can be extended to 15 km with 11
  prisms
• Accuracy range
• Short-range (15 mm 5 ppm)
• Long-range (3 mm 1 ppm)
• Measuring time - 1.5s short-range, 3.5s
  long-range
• Accuracy and time reduced when in tracking mode

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EDM INSTRUMENT CHARACTERISTICS

• Slope reduction manual or automatic depending
  on model
• Average of repeated measurements available on
  some models
• Battery capability 1,400 4,200 measurements
  depending on size of battery and temperature
• Temperature range - -20?C - 50?C
• Nonprism measurements available on some models
• Distances from 100 350 m

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EDM Prisms

• Reflect transmitted signal back to EDM
• Retrodirect capabilities
• Mounted on tripod or attached to prism pole
• Forced centering capabilities

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EDM INSTRUMENT ACCURACIES

• Given in terms of constant error and proportional
  term based on distance
• Most fall in range
• Both EDM instrument prism corrected for
  off-center location
• Usually determined by manufacturer

?(3 mm 1 ppm) to ?(10mm 10 ppm)
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GEOMETRY OF EDM

• Using EDM when optical target and prism at same
  height

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GEOMETRY OF EDM

• EDM instrument mounted on theodolite and target
  located below prism

From which,
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EXAMPLE

• EDM has slope distance AB of 561.276 m. EDM
  instrument is 1.820 m above station A, and the
  prism is 1.986 m above station B. The EDM is
  mounted on a theodolite whose optical center is
  1.720 m above the station. The theodolite
  measured a vertical angle of 6º 21 38 to
  target on prism pole the target is 1.810 m above
  station B. Compute both the horizontal distance
  AB and elevation of station B given an elevation
  at A of 186.275 m.

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Data given in following figures