Introduction to tacheometric surveying

 

Tacheometric surveying is a method of angular surveying in which the horizontal distance from the instrument to the staff stations are determined from instrumental observations only.

Thus the chaining operations are eliminated.

Field Work can be completed very rapidly Tacheometry is mainly used for preparing the contour plans of areas.

As chaining is avoided, as far as possible, this method of surveying is best suited in broken and hilly areas, areas covered with stretches of water, Swamps, etc. where chaining will become very slow, tedious and inaccurate.

Even though this method is not very accurate owing to the above-mentioned advantages, it is employed in location surveys of lines of communications such as roads, railways, etc.

What is Tacheometer?

A tacheometer is similar to an ordinary transit theodolite fitted with stadia wires in addition to the central cross-hairs.

As accuracy and speed are necessary, the telescope fitted with a tacheometer must fulfill additional requirements. Also, the vertical circle should be more refined.

The telescope of the tacheometer is usually longer than that of the Ordinary theodolite and has a higher power of magnification.

The object glass is of greater diameter, and the lens system is of better quality. The magnification power should not be less than 20-25.

The effective aperture should not be less than 3.5-4.5 cm in diameter facilitating the obtaining of a bright image.

The multiplying constant of the instrument (f/I) is generally kept as 100. Sometimes an additional pair of cross-hairs is provided such that the multiplying constant (f/I) is 50.

This is used for close sights. As the top and bottom hairs are read, arrangements may be provided by rack-and-pinion for the movement of the eyepiece in the vertical plane.

It should be possible to take direct readings up to an angle of 5 degrees on the vertical circle.

A Sensitive spirit level should be fixed on the “underarm.” An instrument fulfilling the above requirements is used in fixed hair method of stadia surveying.

For use in the movable hair method of stadia surveying, the above tacheometer will have a different type of diaphragm.

Generally, the fixed central cross-hair and the vertical hair are stretched in the diaphragm frame.

The movable top and bottom cross-hairs are mounted on different slides which can be moved by a micrometre screw.

The diaphragm and the slides carrying the stadia wires are located in different vertical planes to prevent wear and tear.

Micrometre screws that operate the stadia hairs are provided with milled heads and drum scales. The drums are divided into 100 parts.

Readings may be taken up to 0.001 of the pitch of the screws. This instrument is also called as a subtense theodolite.