What is levelling in surveying?
Levelling is a branch of surveying aimed at the determination of elevation of a given point with respect to some assumed datum or some standard reference.
In simple words, it is nothing but the process of finding a height of an object or a point relative to another point on, above, or below the earth’s surface.
Levelling is an important process in many engineering works like
- Finding elevation difference between any two given points
- Determining cutting and filling of earthwork in road construction
- Preparing a contour map of a site
- Planning and constructing bridges, pipelines, etc.
Objectives of levelling
- To find the elevation of a given point with respect to some established datum (reference point)
- To establish a point at the required elevation from the datum.
Terminologies used in levelling
Any surface which is parallel to the earth’s surface is called a level surface. In other words, a level surface is a curved surface with the direction of gravity perpendicular to it at each point.
A line lying on a level surface is called a Level line.
The surface of still water of the lake is a perfect example of a level surface.
Plumb lines are the radial lines which indicate the direction of gravity.
Plumb lines are perpendicular to the level surface.
All plumb lines are meeting at the center of the earth.
A surface which is tangent to the level surface is known as a horizontal surface. A line lying on the horizontal surface is called a Horizontal line.
4.Mean Sea Level (MSL)
Mean sea level is the average height of the sea for all the stages of its tides.
MSL of a sea is obtained by averaging all the heights of the sea which is taken over the period of 19 years.
In most of the works, MSL is taken as a reference level in levelling.
5.Reduced Level (R.L)
The reduced level of the point is nothing but the elevation of a point with respect to MSL.
Since our ultimate goal is to find the elevation difference between two points. Instead of measuring the vertical distance of a point from the center of the earth, we reduced it to measuring from the Mean Sea Level since it was easy.
6.Bench Mark (B.M)
A point whose R.L is known and fixed is called Bench Mark.
In simpler words, Bench Mark is a point whose elevation is known and fixed and it doesn’t vary throughout the levelling process.
7.Line of sight
It is a line joining the intersection of crosshairs of the diaphragm to the optical center of object glass and its continuation. It is also known as the Line of collimation.
8.Back sight (B.S)
It is the first staff reading taken after setting up the instrument
9.Fore sight (F.S)
It is the last staff reading taken at an instrument station.
10.Intermediate sight (I.S)
All staff reading that has taken between B.S and F.S are intermediate sights.
Instruments used in levelling
The instrument used in the levelling are,
- A Level
- A Levelling staff
The purpose of a level is to provide a Horizontal line of sight. The various types of levels are,
- Dumpy level
- Wye level
- Tilting level
- Transit level
- Auto level etc.
Read each of the level in detail : 5 types of level used in levelling
A levelling staff is a straight rectangular rod having graduations imprinted on them. The foot of the staff represents zero reading and the reading increases as the height of the staff increases.
The purpose of levelling staff is to determine the amount of height by which the station is above or below the line of sight.
The reading staff are also of various types,
- Self-reading staff
- Telescopic staff
- Folding staff
- Target staff
Types of levelling
- Direct levelling
- Differential levelling
- Fly levelling
- Reciprocal levelling
- Profile levelling/Longitudinal sectioning
- Cross sectioning
- Precise Levelling
- Indirect / Trigonometrical Levelling
- Barometric Levelling
A detailed explanation of each topic mentioned above are soon to be published.
Errors in Levelling
All levelling measurements are subjected to 3 main sources of errors. They are,
- Instrumental errors
- Natural errors
- Personal errors
Some of the errors are mentioned below,
1) Instrumental errors
- Error due to imperfect adjustment in the instrument like tilted Line of sight.
- Error in the bubble tube
- Error due to defective joints in an instrument.
2) Natural errors
- Error due to earth’s curvature
- Error due to atmospheric refraction
- Settlement of tripod
- Variations in temperature etc.
3) Personal errors
- Mistake in instrument handling
- Mistake in levelling staff holding
- Mistakes in recording the data on the field book etc.
Hope this article gives you a brief outline of the levelling performed in the engineering survey and if you have any queries leave it in the comment section.