Surveying is the art of making suitable measurements in horizontal or vertical planes. This is one of the important subjects of civil engineering. Without taking a survey of the plot where the construction is to be carried out

Arc and Angle Method

It is the method of land surveying in which only linear measurements are made.

Chain surveying requires chain, tape and ranging rods.

Chain surveying is not suitable for large areas having many details.

(The term “details” means a natural or manmade features at or near the ground surface).
Chain Surveying

It consists of the following:

Hard details

Hard details include buildings, roads, walls etc.

Soft details include river, vegetation, trees etc.

Overhead details include power and telephone lines.

Underground details include survey of water mains, sewer etc.

A triangle is a simple figure which can be plotted from the lengths of three sides even if the angles are not known.

In chain survey, the area to be surveyed is divided into a framework consisting of triangles.

Selection of survey station and Survey lines

• 
  The main station should be mutually intervisible so that ranging can be done easily.
  

• 
  Survey lines should be as few as possible so that framework of triangles can be plotted easily.
  

• 
  Survey lines should pass through a level open ground as far as possible so that chaining can be done easily.
  

• 
  The frame should have atleast one long baseline that runs through the middle of the area.
  

• 
  The main survey lines should form well conditioned triangles with no angles less than 30 degrees or more than 120degrees.
  

• 
  The survey lines should be selected such that there are no obstacles in chaining and ranging.
  

• 
  The survey lines will be close to the details to avoid long offsets.
  

• 
  If required, number of subsidiary stations should be selected and subsidiary lines run to avoid long offsets.
  

• 
  Each triangle should have a check line to detect mistakes in measurements and plotting.
  

• 
  While selecting main stations and survey lines, the basic principles of surveying of working from whole to the part should be followed.
  

Earlier, we discussed “Principles of Chain Surveying” and the considerations that are taken for carrying out Chain Surveying… In this article, we will discuss various instruments that are essential for carrying out Chain Surveying.

1. 
   Reconnaissance Survey
   
2. 
   Marking stations
   
3. 
   Running survey lines
   
4. 
   Taking offsets
   

1. 
   Chain, 20m or 30m long
   
2. 
   Tape
   
3. 
   Arrows
   
4. 
   Ranging rods
   
5. 
   Wooden or Iron Pegs
   
6. 
   Plumb bob
   
7. 
   Cross staff
   

20m chain – 100 links

30m chain – 150 links

1 links – 20cm

Links of chain are made by mild steel wire – 4mm diameter.

Chain used for Chain Surveying

Each link at the end is bent into a loop and is connected to adjoiningly by means of three oval rings which offers flexibility to the chain. Ends of chains carry brass handle.

The length of chain is the distance between the outside of handles.

Brass Tags in Chains

Brass tags are fixed at every 5m length.

Tape used for Chain Surveying

Cloth tapes reinforced with brass, copper and bronze wires to prevent it from stretching.

They are available in lengths 10, 15, 20 and 30m.

Tape is provided in leather case fitted with winding device.

It is used for small linear measurements and for taking offsets.

Steel Tapes

Outer end of the tape carries better links for its easy handling and the length of tape is inclusive of this ring.

It is provided in a leather case or a rust-proof metal case with a suitable winding device.

Steel tapes are light, delicate and are used for measurements of distance with high degree of accuracy.

Arrows

400mm in length are pointed at one end for intersecting into the ground and bend into a ring at the other end for facility of carrying.

A piece of white or red tape tied to the ring so that they can be made easily visible at a distance.

Ranging Rods

An iron shoe provided at its bottom enable it to the fixed at a required point in the ground.

Ranging rods

A coloured flag is provided at its tops so that it can be easily seen from a long distance.

They are used for ranging intermediate points along a given survey line and are also used to mark the positions of survey stations clearly.

Wooden or iron pegs

Iron peg – 15cm long (1cm diameter)

Wooden or Iron Pegs

There are two methods for determining horizontal distance on sloping ground.

1. 
   Direct Method
   
2. 
   Indirect Method
   

This method is also known as “Stepping Method”.

The horizontal distances are directly measured by the process of stepping.

Procedure

A path of chain or tape is stretched out from ‘P’.

The path length of chain or tape depends on the steepness of the ground.

The follower holds the zero end of the chain at ‘P’ and directs the leader at P1 to be in the line of PQ and stretch the chain or tape above the ground in horizontal line.

Direct Method | Chain Surveying

The leader then transfers the point ‘P1’ to P2 on the ground by means of plumb bob or dropping a pebble or an arrow,

Now the followers take the new position ‘P2’ and directs the leader to move forward and stretch the tape or chain in a line of PQ.

Now the followers take the new position ‘P2’ and directs the leader to move forward and stretch the tape or chain in a line of PQ and the new position is P3.

Again the leader transfers the point P3 to P4 on the ground as done earlier.

This process is repeated till the point Q is reached.

Horizontal distance PQ = S1 + S2 + S3 + S4 + S5

Indirect Method of Chain Surveying

In this method, the sloping side is measured on the ground and later it is converted into the horizontal equivalent using geometrical condition.

• 
  By measuring angle of inclination
  

• 
  By measuring difference of levels
  

• 
  By hypotenusal allowance
  

In measuring a survey line, the chain has to be laid out on the ground between the stations.

If the line is short, the chain could be put in alignment easily but if it is long or the end station is not clearly visible, then intermediate points has to be established in line with end points to know the directions of the line by ranging.

Types of Ranging

There are two types of ranging:

1. 
   Direct ranging
   
2. 
   Indirect ranging
   

Direct ranging is possible when the stations are intervisible.

Ranging is done by eye-judgement. Ranging rods are erected vertically beyond each end of survey line.

The surveyor stands 2m beyond the ranging rod while the assistant folds the ranging rod vertically in the intermediate stations.

The ranging rod is held roughly in line by the thumb and fore-finger.

The surveyor directs the assistant to move the rod to the left or right until the three ranging rods appear to be in a straight line.

To avoid errors due to the ranging rods not being vertical, the lower end of the rod are cited for alignment.

Indirect Ranging

1. Indirect Ranging is possible when the ends of a line are not inter-visible as in the case when a hill ground or when the distance between the stations are so large that they are not clearly inter-visible.

Indirect Ranging

Intermediate points are fixed by the process of reciprocal ranging as explained below.

Let A and B be the ends of a survey line to be measured as a rising ground between them.

Two chain men with ranging rods take the positions M1 and N1 such that they are as nearly in line with A and B as they could judge the chain men at M1 could N1 and B.

And the chain men at N1 could see M1 and A.

First chain men at N1 directs M1 to M2 so that he comes in the line with A and N.

Then the chain man at M2 directs N1 to N2 such that he comes in line with B and M2.

The process is repeated so that they align each other successively directing each other until they are both finally in the line AB.

Types of Errors occurring in Chain Surveying

There are two types of Errors that are commonly seen to occur in Chain Surveying. For students studying the concept of Chain Surveying, study of the occurrence of different types of Errors in Chain Surveying is important. In this article, we will briefly discuss different types of Errors in Chain Surveying and the situations in which they occur.

1. 
   Cumulative error
   
2. 
   Compensative error
   

These errors always accumulate in one direction and are serious in nature. They affect the survey work considerably.

They make measurements too long or too short.

These errors are of two types and are known as systematic errors.

1. 
   Positive error
   
2. 
   Negative error
   

These errors make the measured length more than the actual length which results into wrong calculations by the Surveyor.

• 
  The length of chain is shorter than the standard length due to bending of links, removal of connecting rings and knots in links.
  
• 
  The temperature is lower than at which the tape was calibrated.
  
• 
  Not applying sag correction.
  
• 
  Sag takes place due to self weight of the chain.
  
• 
  Incorrect alignment
  

These errors make the measured length less than the actual length.

Length of chain or tape greater than its standard length due to flattening of rings, opening of ring joints and temperature being higher than at which it was standardised.

These errors occur in either direction and are likely to compensate.

• 
  Incorrect holding of chain
  
• 
  Displacement of arrows
  
• 
  Adding or omitting a full length of chain
  
• 
  Reading wrongly
  
• 
  Booking wrongly
  

ape Correction, Sag Correction, Pull Correction and Temperature Correction

Tape Correction

Correction for absolute length

Absolute length of tape is its actual length under specified condition

Ca = Lc/L

Where, Ca = correction to be applied to the tape in ‘m’

Lc = measured length in ‘m’

L = nominal length of tape in ‘m’

Correction for temperature

Length of tape is increased with the increase of temperature and decrease with decrease of temperature.

Ct = A (Tm – Ts)

Where,

A = coefficient of thermal expansion per one degree Kelvin

Tm = temperature during the measurement in Kelvin

Ts = temperature at which the tape standardised in Kelvin

Correction for Sag

When the tape is suspended from two ends in air

Sag correction, Cs = L/24(W/P)2

Where, L = horizontal distance between supports in meters

W = weight of tape in Newtons

P = Pull applied in Newtons

Correction for Pull

If the pull during measurement in not the same at which the tape is standardised then the correction for pull is necessary.

Cp = [(P – P0)/AE) * L]

Where,

P = pull in Newtons

L = length of measured line in ‘m’

P0 = standard pull in Newtons

E = Young’s Modulus in N/mm2

Correction due to incorrect length of chain

This is like a formula list which is to be kept in mind while making Calculations:

True distance = L’/L*measured distance

True area = (L’/L)2*measured area

True Volume = (L’/L)3 * measured volume

Where, L’ = incorrect length of chain

L = correct length of chain

The length of a line measured with 20m chain was found to be 500m. It was subsequently found that the chain was 0.04m too long. What is the length of line?

Correct length of chain, L’ = 20 + 0.04 = 20.04m

Length, L = 20m

Measured length, = 500m

True length = (L’/L) * measured length

= (20.04/20) * 500

= 501m

Related posts:

Scales | Engineering Surveying

Scale is a fixed ratio that every distance on the plan bears with corresponding distance on the ground.

Rulers bearing a certain specific scale for measurement

Scale can be represented by the following method

One centimetre on the plan represents some whole number of the metres on the ground.

1cm = 10m

This type of scale is called engineering scale.

1 unit of length on the plan represents some number of same units of length on the ground.

1/1000 etc…

This ratio of map distance to the corresponding ground distance independent of units of measurement is called Representative Factor (RF).

For example,

1cm = 50m

RF = 1/(50×100) = 1/5000

Above two types of scales are also known as numerical scales.

There are four main types of scales which are used for different purposes for measurements:

1. 
   Plain Scale
   
2. 
   Diagonal Scale
   
3. 
   Vernier Scale
   
4. 
   Scale of chords
   

Scales | Engineering Surveying

Scale is a fixed ratio that every distance on the plan bears with corresponding distance on the ground.

Rulers bearing a certain specific scale for measurement

Scale can be represented by the following method

One centimetre on the plan represents some whole number of the metres on the ground.

1cm = 10m

This type of scale is called engineering scale.

1 unit of length on the plan represents some number of same units of length on the ground.

1/1000 etc…

This ratio of map distance to the corresponding ground distance independent of units of measurement is called Representative Factor (RF).

For example,

1cm = 50m

RF = 1/(50×100) = 1/5000

Above two types of scales are also known as numerical scales.

There are four main types of scales which are used for different purposes for measurements:

1. 
   Plain Scale
   
2. 
   Diagonal Scale
   
3. 
   Vernier Scale
   
4. 
   Scale of chords
   

Importance of Contouring in the field of Surveying

Contouring is an imaginary line on the ground obtained by joining points having same elevation.

Characteristics of Contours

Contour lines are closed, however they may be close on the map itself or outside the map depending upon the topography.

The spacing between contour lines depends upon the slope of the ground.

In steep slopes, the spacing is small, for gentle slopes the spacing is large.

If the contour lines are equally spaced, they indicate uniform slope.

Contour Analysis

If the contour lines are parallel, straight or equally placed, they represent plane surface.

In a series of contour lines on the plan or map indicates either a hill or depression.

In case of the hill, the values of the elevation go on increasing towards the centre whereas in case of depression, the values go on decreasing towards the centre.