How to Use Optical Square and Open Cross Staff

Offsets can be accurately set up, particularly long offsets, by use of optical instruments like cross staff or optical square etc.

  a) Cross Staff:

            Two pairs of vertical slits, mounted orthogonally on a pole staff constitutes a simple instrument for setting out normal offsets as depicted in Figure. Each pair of slits can form a line of sight. It is fixed on a stand. Two persons are required to operate cross staff.

Use of cross staff:

1. To find a foot of perpendicular on chain line

2. To set out the perpendicular from a given point.

 b) Optical Square:

It may be noted that at least two surveyor’s are required to set normal offsets while using cross staff. This disadvantage can be overcome if optical square is used for this purpose as only single surveyor can perform this task.

Principle of optical square: 

The angle between the first incident ray and the final reflected ray is twice that of the angle between a pair of plane mirrors.

Construction: 

The optical square consists of a circular box 50mm in diameter and 12.5 mm in depth. The box has three slits as E, F, and G. Two mirrors M₁ and M₂ are mounted on a frame such that the included angle between them is 45^o. The incident ray from object D is reflected from mirror M₁ in line M₁ M₂ to mirror M₂ and finally along AM₂ by mirror M₂. Since included angle between mirrors M₁ and M₂ is 45^o, the angle between rays DM ₁ and AM₂ will be 2 × 45 = 90^o.

Optical Square

Use of Optical Square:

1.   To find a foot of perpendicular on chain line

2.   To set out the perpendicular from a given point.

Procedure:

a) For cross staff:

1.   First of all, the point on the survey line is fixed.

2.   The cross-staff is held vertically at this point.

3.   Then cross staff is turned till the ranging rod at A and B is sighted by one pair of opposite slits.

4.   Let C be the fixed point on the survey line as shown in fig.

5.   Through the other pair of slight point ‘D’ is seen

6.    If it is not seen the ranging pole at D is moved forwarded and backward parallel to survey line till it is bisected.

7.   After bisection, CD is exactly perpendicular to AB.

                                                  

 b) For optical square:

1.   In order to set the offset at point C on survey line AB, the optical square is held horizontally over the station C with the help of a plumb bob.

2.    Line of sight is aligned along line AB by observing ranging rod at B through opening at G.

3.   Next the ranging rod at D is observed through mirrors. The image of rod D is made to coincide with the image of rod B by moving ranging rods sideways.

4.   When the two images coincide, then the line CD becomes perpendicular to line AB.

Offesting by Optical Square

Conclusion:

1.   Offsets can be accurately set up, particularly long offsets, by use of optical instruments like cross staff or optical square etc.

2.   Two persons are required to operate cross staff.

3.   This disadvantage can be overcome if optical square is used for this purpose as only single surveyor can perform this task.