FUNDAMENTALS OF SURVEYING : Instruments for setting out angles

Instruments for setting out angles

a.      Cross staff

Open cross staff: can set two lines at right angle to each other. French cross staff: can set lines at 45^o and 90^o.

Adjustable cross staff: can set any angle.

b.      Optical Square

This is more convenient and precise than cross staff for setting right angles. Consist of two mirrors which make an angle of 45^o with each other.

One mirror is totally silvered and another is top silvered & bottom unsilvered.

c.       Prism Square

Working principle is similar to that of optical square, but, it is more modern and precise than

optical square.

c. Site Square: it is designed for setting out straight lines and offset lines at 90^o.

Obstacles in Chaining

a.              Obstacle to ranging but not to chaining Example: forest, high intervening ground, etc.

b.             Obstacle to chaining but not ranging Example: small pond, river, etc.

c.               Obstacle to both chaining and ranging

Example: big building

Degree of precision in measuring offsets

Normally limit ofprecision in plotting is 0.25 mm.

Example: if the scale of plotting is 1 cm : 5 m, so 0.25 mm on paper will correspond to

5×0.25	= 0.125m on the ground. Hence, the offsets should be measured to the nearest of 12.5
10

cm, for this particular scale.