Correct Answer - Option 3 : Cyclic Error
Explanation:
EDM (Electronic Distance Measurement) Survey:
- Electronic distance measurement (EDM) is a method of determining the length between two points using electromagnetic waves. EDM is commonly carried out with digital instruments called theodolites.
- EDM instruments are highly reliable and convenient pieces of surveying equipment and can be used to measure distances of up to 100 kilometers.
The three distinct systematic errors that may occur in EDM instruments are:
- index (or zero) error
- scale error
- cyclic or short periodic error
Additive Constant (correction for zero or index error):
(i) All distances measured by a particular EDM/reflector combination are subject to a constant error. It is caused by three factors:
- electrical delays, geometric detours, and eccentricities in the EDM;
- differences between the electronic center and the mechanical center of the EDM;
- and differences between the optical and mechanical centers of the reflector
Scale Error:
(i) Scale error is proportional to the length of the line measured and is caused by:
- the drift in frequency of the quartz crystal oscillator in the instrument; errors in the measured temperature, pressure, and humidity which affect the velocity of the propagation; and non-homogeneous emission/reception patterns from the emitting and receiving diodes (phase inhomogeneities). The scale frequency can be checked by:
- direct comparison against frequency testing apparatus; and measurement over a base of known distance.
Cyclic Error (or Short Periodic Error):
- Cyclic error is caused by the non-linearity in amplitude modulation of the carrier wave and phase measurement.
- This cyclic error varies across the modulated wavelength. For an instrument in good adjustment, this error is normally small.
- However, its presence must be determined as an indication of the instrument's adjustment.
- Cyclic error is usually sinusoidal in nature with a wavelength equal to the unit length of the EDM.
