What is field work / field book and optical square /prism square /cross staff

 Content

• Field work
• Field book 
• Single line field book 
• Double line field book 
• Instruments used for setting out right angle 
• Cross staff
• Open cross staff 
• French cross staff
• Adjustable cross staff
• Optical square 
• Prism square 
• Cross staff survey

Field work

The following are the first of equipment required for chain survey 

1. 20 m /30 m chain
2. 10 Arrows
3. 3 Ranging rods
4. Offset rod 
5. Peg (wooden hammer / chalk)
6. Tape 10 m / 20 m
7. Pencil, field book
8. Plumb bob 
9. An instrument for setting out right angle (optical square) 

Field book 

The notebook within which field measurements are noted is termed as Field book.The size of field book is 20 cm * 12 cm.  

There are two types of field book :-

1. Single line field book 
2. Double line field book 

1. Single line field book 

In this field book a single line is drawn through the center of the each page. The line represents a chain line, and chainage are written on it. The chainage is recorded with sketch to the bottom page, and it's left or right to the chain line. 

Single line field book is used for large project and more detail dimension work. 

2. Double line field book

In this field book lines are draw  about the 1.5 cm - 2 cm to the middle of each page. The column represents the chain line, and chainage are written in it. 

Field book is usually used for ordinary work.

Note:-

The main stations are marked by a triangular shape and tie stations are marked by oval shape. 

Instruments used for setting out right angle 

There are following instruments which are used for setting out right angle :-

1. Cross staff
2. Optical square 
3. Prism square 

1. Cross staff

It is an instrument which is made of frame or box having two or more pairs of vertical slits.

There are following types of cross staff:-

A. Open cross staff

                                    

It consists of two pairs of vertical slits in which two line of sight cross each other at right angle, for setting out right angle is placed on the survey line in such a way that one pair of slit a ranging rod is bisected and at the same time from another pair of slit rod is bisected. Hence, right angle is setting out.

B. French cross staff 

                                       

It consists of an octagonal hollow box in which slits are made in the middle of each  faces. It may be used for drawing oblique offset 45° and perpendicular offset at 60°.

C. Adjustable cross staff 

                                               

It is made up of a circular box and provided with some special graduations are made there it is possible to set out any angle with these instrument.

2. Optical square 

It is made of a circular box in which three slits are made and it has two plane mirrors which is fixed at 45° angle to each other. First mirror is not fully silvered that is half silvered and another mirror is fully silvered. It's supported the principle of double reflection. 

3. Prism square 

It is an instrument which is used to set out the perpendicular offset. Prism square is working same as optical square ,but it is differs as in prism square pentagonal prism is used in place of two mirrors. Prism square is also more accurate instrument.

Cross staff survey 

It is used for finding the area of the field. For this purpose a chain line is drawn in the middle or center of the field which is divided into a number of triangle and trapezoidals. The instruments used for the cross staff survey are chain, tape, arrow, pegs, ranging rod, offset rod, and cross staff etc.