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 dista...