PRECISE LEVELLING

PRECISE LEVELLING

* Precise levelling is required for establishing the bench marks. Precise levelling is classified as:

1. First order: Permissible error = mm

2. Second order: Permissible error = mm

3. Third order: Permissible error = mm

where k is the total distance of the level line in kilometre.

* For this precise levelling precise levels are used. The levelling procedure followed is:

1. Three-wire levelling is carried out.

2. Two-levelling staffs are used.

3. Use of double set of touring points is preferred.

4. Levelling staffs with two scales are preferred.

Precise theodolites Precision of theodolites may be obtained by:

1. Using optical plummet for precise centring.

2. Optical micrometers are used for accurate reading with excellent light on scales.

3. Many are provided with digital display using LEDs or LCDs.

4. If electronic distance measurement (EDM) system is added to the precise theodolite it is known as

total station, which have become very popular nowadays.

* In EDMs distance measurement is based on propagation, reflection and reception of either radio

or light of infrared waves. The accuracy achieved is l in 1 × 10

for sights upto 100 km.

* Electromagnetic waves travel approximately at 3 × 10

8 m/sec. They are of sinosoidal nature.

* By phase comparison even fraction of wavelength can be found.

* Depending upon the carrier waves, EDM instruments are classified as:

1. Microwave instruments

2. Infrared instruments and

3. Visible light instrument