# SETTING OUT RIGHT ANGLE IN SURVEYING

Upendra Kumar

Can't imagine the work of surveying without a right angle. The right angle is used in surveying in the fieldwork, area volume calculation, and preparation of plan and section.

Method of setting out  right angles  on the ground

1. SETTING RIGHT ANGLE WITH CHAIN OR TAPE
2. SETTING RIGHT ANGLES WITH INSTRUMENT

1. SETTING RIGHT ANGLE WITH CHAIN OR TAPE:- This method is based on a triangle whose sides are 3m,4m, and 5m. this type of triangle is a right-angled triangle.
 RIGHT ANGLE TRIANGLE

In the above picture, AB is the chain line or baseline which is progressing A to B. Dis the point where one offset chainage is required. spread the chain or tape and measure the length CD 3m on the line AB.
Keep the zero end of the chain or tape one chainman and another chainman keep 3m mark of the chain or tape on the point D. Another chainman spreading the chain or tape to the direction of point E. Stop the spreading after got a 7m point mark in the chain or tape. Keep the 7m mark with the hand by the chainman.spreading the chain or tape until 12m mark to the direction of C by the other chainman. keep the 12m mark of the chain or tape by the first chainman. Drop the ring on point C, D, and E. Extend the line DE with the help of the ranging rods.
2. SETTING RIGHT ANGLES WITH INSTRUMENT:-
A. CROSS STAF:-The cross-staff is a simple instrument for setting the right angle.
There are three types of the cross-staff
i. Open
ii. French
i. Open cross-staff:-
The open cross-staff is made of four metal arms with vertical slits. The two pairs of arms AB and CD are at the right angle to each other.  The vertical slits are used in ranging and sighting the object. The cross-staff is mounted on a wooden pole. In the ranging work, two arms are used in ranging along the chain line, and the other two arms are used in setting a right angle.

ii. French cross-staff:-

A French cross-staff is an octagonal form of the cross-staff. It consists of an octagonal brass tube with slits on all eight sides. It has an octangle brass tube with slits on all eight sides. It has an alternate vertical sight slit and an opposite vertical window with a vertical fine wire on each of the four sides. these are used for setting out right angles.

It consists of a brass cylindrical tube about 8 cm in diameter and 10 cm deep and is and is divided in the center. The upper cylinder can be rotated relative to the lower one by a circular rack and pinion arrangement. It has a magnetic compass on the top for taking the bearing of a line.

B. OPTICAL SQUARE:-

An optical square is an instrument and it is also used for setting a right angle. It consists of a metal box that has a diameter of 2.5cm and depth is 1.5cm. There are two glass is fitted in the optical square which makes a 45-degree angle to each other. one glass is called horizon glass which is half silver and half unsilvered and it is fixed bottom of a metal box. The other glass is called index glass and it is completely silvered. It also fixed in a metal box.

Principle:- Principle of the optical square is based on the principle of the reflecting light properties,  the first incident ray and the last reflected ray is twice the angle between mirrors. So the angle between the horizon sight and index sight will be 90 degrees.

PROCEDURE:-The observer kept the optical square in hand and stand on the chain line where the perpendicular to be setup.The observer sight the ranging rod at position B from the unsilvered portion of the horizon glass and then the observer looks through the upper portion of the horizon glass to see the image of the object O.
C. THEODOLITE:-Theodolite is also used for setting the right angle. In this method, the theodolite is set up on that point where the right angle is required. Make a temporary adjustment and set both verniers zero. Sight the ranging rod on along the chain line. Make a fine target ranging with the help of a lower tangent screw.   Unclamp the upper vernier screw and rotate the theodolite in clockwise and clamp the upper vernier after reach 90 degrees. Make the final set with the help of the upper vernier screw tangent. The advantage of this method sets the right angle on both sides of the chain line of a single setting of the instrument.

D TOTAL STATION:-
TOTAL STATION
The most accurate way to set out a right-angle is to use a total station. Position the instrument on the point along the survey line from which the right-angle is to be set out, target the endpoint of the survey line, set the horizontal circle to zero (see user manual), and turn the total station until the horizontal circle reading is90 °.
USE OF RIGHT ANGLE IN THE SURVEYING

I. Taking offset
II. Computation area and volume.
III.Griding of the map and ground
IV.Contouring, etc