Levelling In Survey Engineering

What is Levelling

Levelling 

Definition

Levelling is the art of determining the relative height of different points on or below the surface of the earth is known as levelling. Thus, Levelling deals with measurements in the vertical plane. The aim of Levelling is to determine the relative heights of different objects on or below the surface of the earth.

Read more: Chain Surveying- Definition & Principle

Level Surface

Any surface parallel to the mean spheroidal surface of the earth is said to be a level surface. Such a surface is obviously curved. The water surface of a still lake is also considered a level surface.

Level Line

Any line lying on a level surface is called a level line. This line is normal to the plumb line at all points.

Horizontal Plane

Any plane tangential to the level surface at any point is known as the horizontal plane. It is perpendicular to the plumb line which indicates the direction of gravity.

Horizontal Line

Any line laying on the horizontal plane is said to be a horizontal line. It is a straight line tangential to the level line.

Vertical line

The direction indicated by a plumb line is known as the vertical line. This line is perpendicular to the horizontal line.

Vertical Plane

Any plane passing through the vertical line is known as the vertical plane.

Datum Surface

This is an imaginary level surface or level line from which the vertical distance of different points is measured. In India, the datum adopted for the Great Trigonometrical Survey(GTS) is the Mean Sea Level(MSL) at Karachi.

Reduced Level(RL)

The vertical distance of a point above or below the datum line is known as the reduced level(RL) of that point. According to the point, the RL of a point may be positive or negative as the point is above or below the datum.

Line Of Collimation

It is an imaginary line passing through the intersection of the cross-hairs at the diaphragm and optical center of the object-glass and its continuation. It is also known as the line of sight.

 

Axis Of The Telescope

This axis is an imaginary line passing through the optical center of the object-glass and the optical center of the eyepiece.

Axis Of Bubble Tube

It is an imaginary line tangential to the longitudinal curve of the bubble tube at its middle point.

Benchmarks(BM)

These are fixed points or marks of known RL determined about the datum line. These are very important marks. They serve as reference points for finding the RL of new points or for conducting Levelling operations in projects involving roads, railways, etc.

Bench-marks maybe four types-

A) GTS.

B) PERMANENT.

C) TEMPORARY

D) ARBITRARY

A) GTS Bench Marks

These benchmarks are established by the survey of the India Department at large intervals all over the country. The values of reduced levels, the relevant positions, and the number of bench-marks are given in a catalog published by this department.

B) Permanent Benchmarks

These are fixed points or marks established by different Government departments like PWD, Railways, Irrigation, etc. The RL of this pointwise determined about the GTS bench-mark and kept on permanent points like the plinth of a building, parapet of a bridge or culvert, etc. Sometimes they are kept on underground pillars.

C) Arbitrary Benchmarks

When the RL of some fixed points is assumed, they are termed arbitrary bench-marks. These are adopted in small survey operations when only the undulation of the ground surface is required to be determined.

D) Temporary Benchmarks

When the benchmarks are established temporarily at the end of a day's work, they are said to be temporary benchmarks. They are generally made on the root of a tree, the parapet of the nearby culvert, a furlong post, or on a similar place.

 

Backsight Reading(BS)

This is the first staff reading taken in any set-up of the instrument after the Levelling has been perfectly done. This reading is taken on a point of known RL, I.E on a bench-mark or changing point.

Foresight Reading(FS)

It is the last staff reading in any set-up of the instrument and indicates the shifting of the latter.

Intermediate Sight Reading(IS)

It is any other staff reading between the BS and FS in the same set-up of the instrument.

Changing Points(CP)

These points indicate the shifting of the instrument. At this point, an FS is taken from one setting and BS from the next setting.

Height Of Instrument(HI)

When the Levelling instrument is properly leveled, the EL of the line of collimation is known as the height of the instrument. This is obtained by adding the BS reading to the RL of the BM or CP on which the staff reading was taken.

Focusing

The operation of setting the eyepiece and the object-glass a proper distance apart for a clear vision of the object is known as focusing. This is done by turning the focussing screw clockwise or anticlockwise.

Parallax

The apparent movement of the image relative to the cross-hairs is known as parallax. This occurs due to imperfect focusing when the image does not fall in the plane of the diaphragm.

 

Levelling

Different Types Of Levelling Operations

I. Simple Levelling.

II. Different Levelling.

III. Fly Levelling.

IV. Longitudinal or profile Levelling

V. Cross-sectional Levelling.

VI. Check Levelling.

VII. Reciprocal Levelling.

 

I. Simple Levelling

When the difference of level between two points is determined by setting the levelling instrument midway between the points, the process is called simple levelling.

II. Differential Levelling

Differential levelling is adopted when 

(i) the points are a great distance apart. 

(ii) the difference in elevation between the points is large. 

(iii) there are obstacles between the points.

III. Fly Levelling

When differential levelling is done to connect a bench-mark to the starting points of the alignment of any project, it is called fly levelling. Fly levelling is also done to connect the BM to any intermediate point of the alignment to check the work's accuracy. In such levelling, only the backsight and foresight readings are taken at every set up of the level and no distances are measured along the direction of levelling. The level should be set up just midway between the BS and the FS.

IV. Longitudinal or Profile Levelling

The operation of taking levels along the centerline of any alignment is known as longitudinal levelling. In this operation, the backsight, intermediate sight, and foresight readings are taken at regular intervals, at every set-up of the instrument. The chainages of the points are noted in the level book. This operation is undertaken to determine the undulations of the ground surface along the profile line.

V. Cross-Sectional Levelling

The operation of taking levels transverse to the direction of longitudinal levelling is known as cross-sectional levelling. The cross-sections are taken at regular intervals along the alignment. Cross-sectional levelling is done to know the nature of the ground across the centerline of any alignment.

VI. Check Levelling

The fly levelling done at the end of a day work to connect the finishing points with the starting point on that particular day is known as check levelling. It is undertaken to check the accuracy of the day work.

 

Different types of levels

The following are the different types of levels-

1. Dumpy level.

2. Wye level.

3. Cooke reversible level.

4. Cushing level.

5. Modern tilting level.

6. Automatic level.

1. Dumpy Level

The telescope of the dumpy level is rigidly fixed to its supports. It cannot be removed from its supports, nor can it be a permanent adjustment for a long time. This instrument is commonly used.

2. Way Level

The telescope is held in two ‘y’ supports. It can be removed from the supports and reversed from one end of the telescope to the other end. The ‘y’ supports consist of two curved clips which may be raised. Thus, the telescope can be rotated about its longitudinal axis.

3. Cooke Reversible Level

This is a combination of the dumpy level and the Y-level. It is supported by two rigid sockets. The telescope can be rotated about its longitudinal axis, withdrawn from the socket, and replaced from one of the telescopes to the other end.

4. Cushing Level

The telescope cannot be removed from the sockets and rotated about its longitudinal axis. The eye-piece and object-glass are removable and can be interchanged from one end of the telescope to the other end.

5. Modern Tilting Level

The telescope can be tilted slightly about its horizontal axis with the help of a tilting screw. In this instrument, the line of collimation is made horizontal for each observation using the tilting screw.

6. Automatic Level

This is also known as the self-aligning level. This instrument is leveled automatically within a certain tilt range using a compensating device.

 

Sources Of Error In Levelling

The following are the different sources of error in levelling-

1. Instrumental Errors

(a) The permanent adjustment of the instrument may not be perfect. That is, the line of collimation may not be parallel to the axis of the bubble tube.

(b) The internal arrangement of the focussing tube is not perfect.

(c) The graduation of the levelling staff may be perfect.

 

2. Personal Errors

(a) The instrument may not be leveled perfectly.

(b) The focussing of the eyepiece and object glass may not be perfect and the parallax may not be perfect and the parallax may not be eliminated entirely.

(c) The position of the staff may be displaced at the change point at the time of taking FS and BS readings.

(d) The staff may appear inverted when viewed through the telescope. By mistake, the staff readings may be taken upwards instead of downwards.

(e) The reading of the stadia hair rather than the central collimation hair may be taken by mistake.

(f) A wrong entry may be made in the level book.

(g) The staff may not be properly and fully extended.

 

3. Errors Due To Natural Causes

(a) When the distance of sight is long, the curvature of the earth may affect the staff reading.

(b) The effect of refraction may cause a wrong staff reading to be taken.

(c) The effect of high winds and shining sun may result in a wrong staff reading.