The velocity V of electromagnetic waves in air is a function of the speed of light in vacuum (Vo = 299,792.5 Km / s) and the refractive index (n) of air and is given by
Thus velocity of the modulated waves get altered as atmospheric conditions vary resulting in corresponding change in the modulated wavelength and hence the basic measuring unit of the EDM instruments. The refractive indices of electromagnetic waves in air are functions of air temperature, atmospheric pressure and the partial pressure of water vapor. But, light waves and microwaves react somewhat differently to varying atmospheric conditions.
For light wave, the index of refraction ng of standard air (i.e., for an atmosphere at 0o C, 760 mm Hg pressure and 0.03 percent carbon-di-oxide) as given by Barell and Sears Equation
in which l is the wavelength of the carrier beam of light in micrometers.
Owing to changes in temperature pressure and humidity the refractive index of air becomes na, is given by
Where p = atmospheric pressure in mm Hg
t = temperature in oC
e = vapor pressure, mmHg.
For microwave , the refractive index ( n r ) of the atmosphere is
Where p = atmospheric pressure, mmHg
E = Vapor pressure, mmHg
t = temperature, oC
The method of correcting the obscured distance to account for varying atmospheric conditions is achieved through recording of temperature and atmospheric pressure at each end of the line. Once the meteorological data are recorded, corrections to observed distances are carried out either by calculating through charts and nomographs provided with the instrument or by dialing in the parameters for automatic compensation during measurement. In case of, microwave EDM, the partial pressure of water vapor is also determined along with temperature and atmospheric pressure.