Surveying

Height of Collimation and Rise and Fall Method | Methods of Levelling

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Methods of Levelling | Guide to Surveying and Levelling

In this article, we will discuss two important methods of Levelling. We will also study these Methods with the help of Numerical Examples in our successive articles.

There are two Methods of Levelling:

  1. Height of Collimation Method
  2. Rise and Fall Method

Height of Collimation Method

This method is simple and easy.

Reduction of levels is easy.

Visualization is not necessary regarding the nature of the ground.

There is no check for intermediate sight readings;

This method is generally used where more number of readings can be taken with less number of change points for constructional work and profile levellings.

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Principles of Chain Surveying or Linear Surveying | Civil Engineering

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Chain Surveying or Linear Surveying | Surveying and Levelling

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
Chain Surveying

It consists of the following:

Hard details

Hard details include buildings, roads, walls etc.

Soft Details

Soft details include river, vegetation, trees etc.

Overhead details

Overhead details include power and telephone lines.

Underground details

Underground details include survey of water mains, sewer etc.

Principles of Chain Survey

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.

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Methods for Correcting the bearings affected by Local Attraction

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Local Attraction | Methods for Correcting the bearings affected by Local Attraction

The deflection of a magnetic needle from its true position due to the presence of magnetic influencing material such as iron ore, magnetic rock, underground pipeline, electric cables, iron pipes, electric poles in its vicinity is called “Local Attraction”.

Methods of Correcting the bearings

There are two methods of correcting the bearings affected by local attraction:

  1. Included angle Method
  2. Error Computation

Included angle Method

In this method, the included angles of the traverse are calculated first, then starting from the line which is unaffected by local attraction and using the included angles, the corrected bearings of the traverse are computed.

Error Computation Method

In this method, the direction and the amount of local attraction at each survey station is determined.

Then starting from the line which is unaffected by local attraction, the corrected bearing of the traverse are computed.

This method is more accurate than the included angle method.

Hence it is adopted by most of the surveyors.

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Types of Ranging | Chain Surveying

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Methods of Ranging in Chain Surveying | Guide to Surveying and Levelling

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

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.

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Numerical Examples for Errors in Chain Surveying

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A practical Example for Calculation of the True Area of the field | Errors in Chain Surveying

In my previous article, we studied how to measure true distance considering the Errors occurring in Surveying.

Correction formulas to be kept in mind (for incorrect length of Chain)

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 chain was tested before starting the surveying and was found to be 20m. At the end of surveying, it was tested again and was found to be 20.12m. The area of the plan of the field drawn to a scale 1cm = 6m was 50.4sqm. Find the true area of the field in sqcm.

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Chaining on Sloping Ground | Guide to Surveying and Levelling

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Chaining on Sloping Ground | Surveying and Levelling

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

  1. Direct Method
  2. Indirect Method

Direct Method of Chain Surveying

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
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

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Types of Levelling Instruments | Definitions in Levelling

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Various types of Levelling Instruments and Important Definitions | Surveying

  1. Dumpy level
  2. Y-level
  3. Modern Tilting level
  4. Automatic level

The levelling instruments essentially consist of the following:

A levelling head with three foot screws which enables to bring the bubble at its centre.

Telescope that provides line of sight to bisect distinct objects.

A bubble tube to make the line of sight horizontal either mounted on top or side of the telescope.

A tripod for supporting the levelling instrument.

Important Definitions in Levelling

Line of Collimation or line of sight

The line joining the point of intersection of the cross wires of the diaphragm to the optical centre of the objective and its imaginary continuation.

Reduced Level

The vertical distance measured above or below the mean sea level or benchmark is called as reduced level.

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Sources of Errors in Surveying | Civil Engineering

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Sources of Errors in Surveying

In this article, we will discuss three major types of errors that are found to be very common in Surveying.

Types of Errors
Types of Errors

Types of Errors

  1. Instrumental errors
  2. Personal errors
  3. Natural errors

Instrumental errors

Error may arise due to imperfection or faulty adjustment of the instrument with which measurement is being taken.

For example:

A tape may be too long or an angle measuring instrument maybe out of adjustment. Such errors are known as Instrumental erros.

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Classification of surveys according to their Purpose | Civil Engineering

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Classification of  Surveys according to their Purpose

In our previous article, we discussed division and classification of surveys based on various factors such as Purpose, instruments, field of work, method of work etc.

In this article, we will discuss different types of surveys carried out for various purposes in the field of Surveying…

Mine Survey

Mine survey involves exploration of mineral wealth in the country. This would include minerals such as Gold, copper, coal, silver etc. between the earth’s crust.

Mine Survey
Mine Survey

Geological Survey

Geological Survey is carried out by geologists. It is usually for the purpose of the study of earth’s rock structure. It helps to determine different strata in the earth’s crust.

Archaeological Survey

As the name suggests, Archaeological Survey is carried out by the Archaeologists for the purpose of their study concerning the past. This survey helps to trace the relics of the past.

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Types of Scales in Engineering Surveying

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Types of Scales | Engineering Surveying

In the previous article, we discussed briefly on the topic of “Scales in Surveying” where we came across an important term “Representative factor” which forms an important part in understanding the scales in Surveying.

In this article, we will discuss different types of scales used in Surveying…

The scales are classified into four categories:

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

Lets go on with the discussion of types of scales briefly for our better understanding…

Plain Scale

Plain Scale is one on which it is possible to measure two dimensions only. For example, measurements such as units and lengths, metres and decimetres etc.

Plain Scale
Plain Scale

Six different plain scales in metric used by engineers, Architects and Surveyors.

Diagonal Scale

On diagonal scale, it is possible to measure three dimensions such as metres, decimetres and centimetres, units , tens and hundreds; yards, feet and inches etc.

A short length is divided into number of parts using the principle of similar triangle in which sides are proportional.

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