Module 1 : Modern Surveying Equipment
 Lecture 1 : Introduction
Electronic Distance Measuring Instruments (EDMIs)
  • EDMIs were first introduced in 1950's by Geodimeter Inc. Early instruments were large, heavy, complicated and expensive. Improvements in electronics have given lighter, simpler, and less expensive instruments. EDMIs can be manufactured for use with theodolites (both digital and optical) or as an independent unit. These can be mounted on standard units or theodolites or can also be tribrach mounted.
  • The electronic methods depend on the value of velocity of Electromagnetic radiation (EMR), which itself is dependent upon measurement of distance and time. Hence, there is no inherent improvement in absolute accuracy by these methods. The advantage is mainly functional - precise linear measurement can now be used for longer base lines, field operations can be simplified and trilateration can replace or augment triangulation.
Principle of EDMI
  • The general principle involves sending a modulated Electro-magnetic (EM) beam from one transmitter at the master station to a reflector at the remote station and receiving it back at the master station. The instrument measures slope distance between transmitter and receiver by modulating the continuous carrier wave at different frequencies, and then measuring the phase difference at the master station between the outgoing and the incoming signals. This establishes the following relationship for a double distance (2D):
  • Where m is unknown integer number of complete wavelengths contained within double distance, Φ; is the measured phase difference and λ is modulation wavelength, and k is constant. Multiple modulation frequencies are used to evaluate m , the ambiguity .
Figure 1.3 Principle of EDMI (Wolf and Ghilani, 2002)
Various EDMIs in use are based on two methods:
  • using timed pulse techniques such as those used in variety of radar instruments.
  • using measurements of a phase difference which may be equated to one part of a cycle expressed in units of time or length.

Pulse methods have advantages over the phase difference methods but their weight and power requirement is such that they cannot be classed lightweight portable instruments.

(i) Pulse techniques
  • All such measurements incorporate a very precise measurement of time usually expressed in units of nanoseconds (1x10-9 s), which a EM wave takes to travel from one station to another. In this method, a short, intensive pulse radiation is transmitted to a reflector target, which is immediately transmitted back to the receiver. As shown in Figure 1.4, the distance (D) is computed as the velocity of light (V) multiplied by half the time (Δt/2) the pulse took to travel back to the receiver (D = V x Δt/2).
Figure 1.4 Principle of EDMI based on pulse measurement (Schoffield, 2002)
(ii) Phase difference techniques
  • The relationship between wavelength and associated phase difference can be illustrated by the Figure 1.5 which shows that for a given complete cycle of EM wave, the phase difference can be expressed both in terms of angular (degrees) and linear (fraction of wavelengths) units. In phase difference method used by majority of EDMI, the instrument measures the amount δλ by which the reflected signal is out of phase with the emitted signal (Figure 1.6).
Figure 1.5 Relationship between Wavelength and phase difference (Wolf and Ghilani, 2002)
Figure 1.6 Principle of phase measurement (Schofield, 2001)