Principles and planning of theodolite traverse

Principles of theodolite traversing

• Working from the whole to the part: a framework of control stations of high accuracy is fixed first, and detail or minor traverses are fixed from it.
• Closure and checks: the traverse should start and end on known points or return to the start so that angular and linear errors can be checked.
• Balanced accuracy: angular and linear measurements are made to equal accuracy, and errors are distributed over the whole figure (Bowditch or transit rule).
• Independent coordinates: stations are fixed from coordinates referred to a common origin, so errors do not accumulate in plotting.
• Adequate redundancy: angles are measured on both faces and repeated, and lengths measured twice, to detect blunders.

Planning before traversing

1. Study of the purpose and accuracy required (type of map, scale, specifications) and the available maps and existing control.
2. Reconnaissance: walk over the area and decide the route and number of stations.
3. Selection of stations: firm ground, clear intervisibility of the adjacent stations, sides as long and as equal as possible, few stations, easy access to details, free from disturbance and traffic, not on slopes where the instrument cannot be levelled.
4. Marking and referencing with pegs, nails, concrete pillars and witness marks, with a sketch of each location.
5. Selection of instruments and methods: theodolite or total station, tape or EDM, and the method of angle measurement (included angle, deflection angle, direct bearing).
6. Organisation: party size, schedule, safety, field books, and the method of computation and checking.