Basic Surveying BSU 22205 Notes - UNIT 2

 

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Unit-2 Chain and cross staff survey 

Principle of chain surveying:
-The principle of chain surveying is triangulation
- this means that the area to be surveyed is divided into a number of small triangles which should be well conditioned
- no angle smaller than 30° are greater than 120° are known as 'ill conditioned' triangles
- in chain surveying the sides of the triangles are measured directly on the field by chain or tape and no angular measurements are taken

Chain surveying is recommended when:
1) the ground surface is level (more or less)
2) the small area is to be surveyed
3) small-scale map is to be prepared
4) formation of well conditioned triangle is easy

Chain surveying is unsuitable when:
1) The area crowded with many details
2) Area consists of two many undulations
3) Area is very large 
4) formation of well conditioned triangle is difficult

Method of linear measurement:
1) direct measurement
2) measurement by optical mean
3) method based on electronics

Method of Direct measurement
A) Chaining
B) Instrument for Chaining
1) Chain or tape
2) Pegs
3) Arrows
4) Ranging rods
5) Plumb bob
6) Digital tape

Chaining: in the method of chaining to denote horizontal distance either a chain or tape is used for making direct measurements
1) Chain:
- Chain is made of straight links of galvanized mild steel
- Diameter - 4mm
- For flexibility links are bent into rings
- Brass handles are provided at both ends
- Length of chain include both handles
- At every one metre brass ring is provided
- At every 5 metre brass tallies are provided
(each tally has different shape)
- chain are available in 20m and 30m lengths
- 20m chain ->100 links of 20cm each
- 30m chain ->150 links of 20cm each
Testing of chain:
- before start of changing work it is necessary to ensure that the chain is of correct length
- If chain is too long
a) opened up joints are closed by Hammer
b) some of links may removed
- If chain is too short
a) smaller size links replaced by larger size links
b) bent links are made straight

2) Pegs:
- Pegs are made of either steel or timber
- used to mark positions of stations
- Pegs are 15cm long 

3) Arrows
- for longer distance chaining to count number of chains arrows are insected into ground after every chain length
- at the end of work number of arrows utilised multiplied by length of chain
- made of good quality steel
- one end is made sharp and other end is bent

4) Ranging rods
- are circular in section and 2 to 3m length
- painted with alternate bonds of black and white or red and white

5) Plumb bob
- used to transfer points on ground
- with the help of plumb bob centering in prismatic compass, theodolite, plane table is done
- to check verticality

6) Digital Tape
- Have top mounted LCD screen
- Easy measurement

Basic Surveying All Units Notes :

• Unit-1 Notes
• Unit-2 Notes
• Unit-3 Notes

Methods of linear measurement
1) Pacing 
- method of walking over a line and counting number of paces (steps) is known as pacing
- read distance may be obtained by multiplying number of laces by average length of pace
- average pace length - 80cm
- length of pace varies with
a) Individual
b) Age, height, physical condition
c) Nature of ground
2) Speedometer
- Used in automobile
3) Chaining
- Accurate method
- distance are directly measured in field by chain or tape
4) Digital tape
- easily convert metric to imperial unit
- hold and save the measurement
- measuring length up to 4.8m
Survey stations
- these are the points at the beginning and end of chain line
a) Main stations
b) Subsidiary stations
c) Tie stations

1) Main stations
- station taken along the boundary of an area as controlling points are known as 'main stations'
- main survey line should cover whole area to be surveyed
- denoted by '∆' with letters A,B,C,D etc
- Chains are denoted by "__...__...__..."

2) Subsidiary stations
- station which are on the main survey line or any other survey line
- for dividing area into triangles
- checking accuracy of triangles
- denoted by 'O' and letters S1, S2, S3, etc.

3) Tie stations
- Line joining tie stations
- to locate interior details

A) base line
- line on which framework of survey is built
- most important line
- longest survey line passing through the centre of the area

B) Tie line
- line joining some fixed points as tie stations on main survey line
- sometimes tie line is provided so so as to locate interior details
- helps to check accuracy of the framework

C) Check Line 
- accuracy can be check by 'check line'
- line joining between apex of triangles and some fixed point on survey Line or on baseline
Survey line
- Line joining survey stations
Offset
- lateral measurement taken from an object to the chain line is known as offset
- offsets are taken to locate objects with reference to the chain line
1) Perpendicular offset
2) oblique offset

1) Perpendicular offset
- When the lateral measurements are taken perpendicular to the chain line they are known as perpendicular offset
- offsets are normally measured by metallic tape

2) oblique offset
- any offset not perpendicular to the chain line
- other than 90°

3) Short offset
- when offset set out by eye or swinging the tape
- it should not be more than 15cm

4) Long offset
- when offset set out by cross staff for optical square
- as
- offsets which are more than 15m
Instruments used for setting offset
1) Ranging rod
2) Chain
3) Metallic tape
4) Open cross-staff
Instruments for laying offset
1) Optical square
2) Indian optical square
3) Open cross-staff
4) French cross-staff
5) Adjustable cross staff

1) Open cross-staff
- simple instrument for setting out right angle
- open cross staff consist of four metal arms with vertical slits
- vertical slits are meant for slighting object and ranging road
- cross staff is mounted on wooden pole with iron shoe at bottom
- for setting out a perpendicular on a chain line, the cross staff is held vertically at approximate position
- suppose slits A and b are directed to the ranging roads (R,R1)
- slits C & D are directed to the object 'O'
- looking through slits A and b the ranging rods are bisected at the same time looking through slits C and D the object 'O' is bisected
- To bisects and ranging rods simultaneously the cross staff may be moved forward or backward along the chain line

2) Optical square
- used to set perpendicular offsets on chain line
- works on principle of optics
Principle- according to the principle of reflecting surfaces the angle between the first incident ray and last reflected ray is twice the angle between the mirrors
- in this case the angle between mirror is fixed at 45° angle between the horizon site and index sight will be 90°

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