(i) wavelength used
(ii) working range
(iii) achievable accuracy

(a) infrared
(b) laser
(c) microwaves

• Infrared: Systems employing these frequencies allow use of optical corner reflectors (special type of reflectors to return the signal, explained later) but need optically clear path between two stations. These systems use transmitter at one end of line and a reflecting prism or target at the other end.
• Laser: These systems also use transmitter at one end of line and may or may not use a reflecting prism or target at the other end. However, the reflectorless laser instruments are used for short distances (100 m to 350 m). These use light reflected off the feature to be measured (say a wall).

• These systems have receiver/transmitter at both ends of measured line. Microwave instruments are often used for hydrographic surveys normally up to 100 km. Hydrographic EDMIs have generally been replaced by Global Positioning System (GPS) (GPS has been explained in a separate module in these lectures).
• These can be used in adverse weather conditions (such as fog and rain) unlike infrared and laser systems. However, uncertainties caused by varying humidity over measurement length may result in lower accuracy and prevent a more reliable estimate of probable accuracy.
• Existence of undesirable reflections and signal leakage from transmitter to the receiver requires the use of another transmitter at the remote station (also called the slave station). The slave station is operated at different carrier frequency in order to separate two signals. This additional transmitter and receiver add to weight of equipment. Multipath effects at microwave frequency also add to slight distance error which can be reduced by taking series of measurements using different frequency.

• long range radio wave equipment for ranges up to 100 km
• medium range microwave equipment with frequency modulation for ranges up to 25 km
• short range electro-optical equipment using amplitude modulated infra-red or visible light for ranges up to 5 km

• Accuracy of EDMI is generally stated in terms of constant instruments error and measuring error proportional to the distance being measured: ± (a mm + b ppm).
  • The first part in this expression indicates a constant instrument error that is independent of the length of the line measured.
  • The second component is the distance related error.
• Here, a is a result of errors in phase measurements (θ) and zero error (z), whereas b results from error in modulation frequency (f) and the group refractive index (n_g). The term group index pertains to the refractive index for a combination of waves- carrier wave and multiple modulated waves in EDMI. θ and z are independent of distance but f and   n_g are functions of distance and are expressed as (Schoffield, 2001):

• In above equations, σ indicates the standard error. Most EDMI have accuracy levels from ± (3 mm + 1 ppm) to ± (10 mm + 10 ppm). For short distances, part a is more significant; for long distances b will have large contribution. Table 1.1 lists details of a few EDMIs