Question

i. List the different types of horizontal distance measurement methods. 

ii. Among the list which method is fastest one and why, explain the principles followed in the same method.

Answer 1

Horizontal distance measurement methods :

1) DIRECT METHOD

In direct methods, distance is actually measured in field using chain or tape. It is the most commonly used method.

2) OPTICAL METHODS

Distance is not actually measured in field, it is computed indirectly using instruments like tacheometer and subtense bar.

3) ELECTRONIC DISTANCE MEASUREMENT METHODS

Distances are measured with instruments relying on propagation, reflection and subsequent reception of either radio waves, light waves or infrared waves.

4) APPROXIMATE METHODS

a) Pacing

b) Passometer

c) Pedometer

d) Odometer/Perambulator

e) Speedometer

Among the above methods Electronic distance measurement(EDM) is the fastest method.

EDM are fastest because they work on electromagnetic radiations and are equipped with microprocessor which perform various mathematical operations - for example averaging multiple measurements, calculation of slope corrections, atmospheric and instrumental corrections and in some cases store observations directly using internal memory.

EDM are instruments that rely on propagation, reflection, and subsequent reception of different electromagnetic waves.

They are based on principle of calculating phase difference between emitted and reflected rays to calculate distance between points.

EDM equipments are broadly classified into:

1) Electro-optic Instruments - uses light waves

2) Microwave Instruments - uses radio waves

3) Infrared emitting instruments - uses infrared waves

Microwaves emitting instruments adopts frequency modulation.

Where as, Electro- optic and Infrared adopts amplitude modulation technique.

Generally, EDM are combined and interfaced with electronic theodolite and electronic data collectors, they become Total Stations or Electronic tacheometers.

A total Station measures slope distance and the microprocessor uses the vertical angle recorded by theodolite along line of sight to calculate horizontal distance.