Instruments for Setting out Right Angles
The cross staff is used for
a) Finding out foot of the perpendicular from a given point to a line
b) Setting right angle at a given point on a line
There are three forms of cross staff
Open cross staff
The French and
Prism square, the first one is commonly use.
1. Open Cross Staff:
It is a simple. It consists of two parts: – 1) the head; 2) the leg. The common type of cross staff consists of 4 metal arms with vertical slits for sighting through.
The head is fixed to the top of an iron stand about 1 .2 to 1.5 m long this is driven in to the ground.
For setting perpendiculars lines, one pair of opposite arms is aligned with the chain line. It is specially used for setting off and marking contour lines.
To find the foot of perpendicular from a given point to a given lines:
To take offset, the cross staff is held vertically on chain where the offset is likely to occur, and turn until one pair of opposite slits directed to arranging rod at the forward end of the chain line, the offset is taken is bisected. If not the cross staff is moved forward or back word the chain line until the line of sight through the pair of slits at right angle the chain lines, does bisect the point.
In setting out a right angle at point on the chain line:
The cross staff is held vertically over the given point on the chain and turn until the ranging road at either end of the chain line is bisected the line of slight through 1 pair of the opposite slits. Then the line through the other pair of slits which is at right angles to the chain line through the other pair of slits which is at right angles to the chain line. Line may be marked by a ranging rod at inconvenient point on the sighted.
2. French Cross Staff:
The octagonal form cross staff is the French cross staff. It consists of an octagonal brass tube with slits on all eight sides. If has an alternate vertical slit and an opposite vertical window with a vertical horse hair or affine wire on each of the four sides. These are used for setting our right angles. On the other side are vertical slits, which are at 45 degree to those previously mentioned, for setting out angles of 45 degree.
The base carries a socket so that it may be fitted on the pointed staff when the instrument is to be used. The sight being too close (only 8 cm apart) it is inferior to the open type.
3. Adjustable Cross Staff:
It consists of a brass cylindrical tube about 8 cm in diameter and 1 cm in deep, and is divided in the centre. The upper cylinder can be rotated relatively to the lower one by a circular rack and pinion arrangement actuated by screw. Both are provided with sighting slits. The lower part is graduated to degrees and sub divisions, while the upper one carries a vernier. Thus it may be use for setting out angles of any magnitude. It has a magnetic compass at the top, which may serve to take the bearing of the line.
4. Optional Square:
An optional square is an compact hand instrument used in setting out right angles with greater accuracy than a cross staff. It consists of a circular box about 5 cm in diameter and 1.25 cm deep in which two mirrors are fitted at right angles to the plane of instruments . The mirror ‘h’ called the horizontal glass which is half silvered and half uncovered. The mirror ‘I’ is known as index glass is known as wholly silvered. There are three openings on the side of box at e, b and ,c. the opening e is pin whole for eye , b is small rectangular slot placed opposite to pin hole and c is large rectangular slot placed at right angle to line joining e & b
The surveyor simply turns the optical square upside down which throws the aperture for the object on that side.
5. Prism Square:
It is a modern instrument and is a very use full for setting out right angles. It is based on the same principles as the optical square and is used in same manner. It requires no adjustment, since the angle between reflecting surface of the prism (45 degree) is fixed. It is unaffected by dust & can be used in poor light. For taking offset to an object the observer holds the instrument in his hand & slights directly over the prism at ringing of station.