levelling and contouring

LEVELLING AND

CONTOURING

BY:- ANAND R. JIBHAKATECIVIL ENGINEER

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LEVELLINGAccording to science

• Leveling is a branch of surveying which deals with the measurement of relative heights of different points on, above or below the surface of the earth. Thus in leveling, the measurements (elevations) are taken in the vertical plane.

Simple Definition• Leveling is the process used to determine a difference in elevation

between two points.

LEVELLING

LEVELLING 2

Definitions• Station:- A point where the levelling staff is kept.• Height of instrument:- It is the elevation of the plane of sight with

respect to assumed datum. It is also known as plane of collimation.• Datum line ( M.S.L. ) :- Is the level (line) which are attributed to it

points levels on the surface of the Earth. Which is the average sea level.

• Reduced level ( R.L) :- Is the high point from datum line.• Benchmark (B .M ) :- Are fixed points information site and attributed

placed in different places until you start racing them when conducting settlement .

• Back sight ( B.S.) :- Is the first reading taken after placing the device in any position so that we see the greatest possible number of points required to find the elevation .

DEFINITIONS

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• BENCHMARKS

LEVELLING 4

DEFINITIONS

• Fore sight (F.S) :- Is the last reading taken before the transfer device.

• Change point(CP) or turning point(TP): The point at which both BS and FS are taken.

• Intermediate sight ( I.S.) :- Is reading taken between the back sight and fore sight reading .

• Elevation of line of sight ( H.I) :- Is the imaginary vertical level determined by the line of sight to the amount of increase or decrease for sea level .

DEFINITIONS

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• Tripod stand :- is a portable three-legged frame, used as a platform for supporting the weight and maintaining the stability of some other object.

PLUMB BOBTRIPOD STAND TRIPOD STAND

DEFINITIONS

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• Leveling Staff :-Is a wooden or metal ruler one side runway to meters and centimeters. And is a ruler of solid wood 2 , 3 , 4 , 5 meters in length and usually 4 meters .

DEFINITIONS

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• Leveling of the instrument is done to make the vertical axis of the instrument truly vertical. It is achieved by carrying out the following steps:

• Step 1: The level tube is brought parallel to any two of the foot screws, by rotating the upper part of the instrument.

• Step 2: The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward. (The bubble moves in the same direction as the left thumb.)

• Step 3: The level tube is then brought over the third foot screw again by rotating the upper part of the instrument.

• Step 4: The bubble is then again brought to the centre of the level tube by rotating the third foot screw either inward or outward.

LEVELING OF THE INSTRUMENT

LEVELLING 9

• Step 5: By rotating the upper part of the instrument through 180 ° , the level tube is brought parallel to first two foot screws in reverse order. The bubble will remain in the centre if the instrument is in permanent adjustment.

LEVELING OF THE INSTRUMENT

LEVELLING 10

TYPES OF LEVELLING

1] Simple levelling2] Differential levelling3] Fly levelling4] Profile levelling5] Cross sectional levelling6] Reciprocal levelling

There are two methods for obtaining the elevations at different points:

1] Height of instrument (or plane of collimation) method2] Rise and fall method

TYPES OF LEVELLING

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Simple levelling:-• When the difference in the elevation of two nearby points is

required then simple levelling is performed.

TYPES OF LEVELLING

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Differential levelling:-• Performed when the final point is very far from the final point.

TYPES OF LEVELLING

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Fly levelling• Performed when the work site is very far away from the bench

mark.• The surveyor starts by taking BS at BM and proceed towards

worksite till he finds a suitable place for temporary BM. All works are done with respect to temporary BM.

• At the end of the day the surveyor comes back to original BM.• This is called fly levelling.

TYPES OF LEVELLING

LEVELLING 14

Profile levelling:-•Profile levelling, which yields elevations at definite points along a reference line, provides the needed data for designing facilities such as highways, railroads, transmission lines.•Reduced levels at various points at regular interval along the line is calculated.•After getting the RL of various points the profile is drawn. Normally vertical scale is much larger than horizontal scale for the clear view of the profile.

TYPES OF LEVELLING

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Reciprocal levelling:-When levelling across river is required then this method is applied to get rid of various errors.

TYPES OF LEVELLING

LEVELLING 16

METHODS1] Height of Instrument method

• The basic equations are• Height of instrument for the first setting= RL of BM + BS(at BM)• Subtract the IS and FS from HI to get RL of intermediate stations and

change points. • Checking: ΣBS -ΣFS = Last RL –First RL. This is –ve for FALL and +ve

for RISE.

METHODS OF LEVELLING

LEVELLING 17

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METHODS2] Rise and Fall method

• In this method the difference of the present staff reading is subtracted from the previous staff reading.

• Previous reading –present staff reading = +ve, denotes RISE• Previous reading –present staff reading = -ve, denotes FALL• Checking: ΣBS -ΣFS = Last RL –First RL= ΣRise -ΣFall

METHODS OF LEVELLING

LEVELLING 19

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BENCH MARK ELEVATEDe.g. 100m

?TOP OF SLOPE

FIND THE DIFFERENT BETWEEN BENCH MARK AND TOP OF SLOPE

Divide hill into 3 spots to make easierOtherwise too far to see or level staff not long enough

Back sight9 m

Rotate dumpy round

Move level staff to new spot

Fore sight2 m

Move dumpy level to new spot

Back sight6 m

Fore sight1 m

Rotate dumpy round

Move level staff to new spot

CONCEPT

LEVELLING 21

Ground level

Back sight [from dumpy level]

Height of instrument [ground level+ back sight from dumpy level]

Fore sight [from dumpy level]

Height of new point [height of instrument fore sight]

Bench mark

100 9 109 2 107

107 6 113 1 112

Back sight9 m

Fore sight2 m

Back sight6 m

Fore sight1 m

Level of dumpy=109m

Level of dumpy=113m

9m

100m+9m

6m

6m+107m1m

2m

BENCH MARK ELEVATEDe.g. 100m

SO THE DIFFERENCE IN HEIGHT IS112-100=12M

CONCEPT

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CONTOURING

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• Contours are those lines you can see on OS maps.

• A contour line is an imaginary line that joins points of equal height above sea level.

• They can be used to learn about the shape of the land (the relief).

• A map with only a few contour lines will be flat (and often low lying)

• If a map has lots of contours it is a mountainous or hilly area.

• The actual pattern of the lines will tell you more detail about the area too.

Steep slopes

Area that is flatter with only a gentle slope

CONTOURS

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CONTOURS• Contour An imaginary line on the ground surface joining the points

of equal elevation is known as contour. • In other words, contour is a line in which the ground surface is

intersected by a level surface obtained by joining points of equal elevation. This line on the map represents a contour and is called contour line.

• Contour Map :-A map showing contour lines is known as Contour map.

• A contour map gives an idea of the altitudes of the surface features as well as their relative positions in plan serves the purpose of both, a plan and a section.

CONTOURS

CONTOURS 25

Contouring • The process of tracing contour lines on the surface of the earth is called

Contouring. • PURPOSE OF CONTOURING • Contour survey is carried out at the starting of any engineering

project such as a road, a railway, a canal, a dam, a building etc. i) For preparing contour maps in order to select the most

economical or suitable site. ii) To locate the alignment of a canal so that it should follow a ridge

line. iii) To mark the alignment of roads and railways so that the

quantity of earthwork both in cutting and filling should be minimum.

CONTOURS

CONTOURING

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iv) For getting information about the ground whether it is flat, undulating or mountainous.

v) To find the capacity of a reservoir and volume of earthwork especially in a mountainous region.

vi) To trace out the given grade of a particular route. vii)To locate the physical features of the ground such as a pond

depression, hill, steep or small slopes.

CONTOURS

CONTOURING

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METHODS OF CONTOURING

• There are mainly two methods of locating contours:-

(1)Direct Method and (2) Indirect Method.

CONTOURS

METHODS OF CONTOURING

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1] Direct Method: • In this method, the contours to be

located are directly traced out in the field by locating and marking a number of points on each contour.

• These points are then surveyed and plotted on plan and the contours drawn through them.

• This method is most accurate but very slow and tedious as a lot of time is wasted in searching points of the same elevation for a contour.

• This is suitable for small area and where great accuracy is required

CONTOURS

METHODS OF CONTOURING DIRECT METHOD

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DIRECT METHOD OF CONTOURING

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48

B.M.

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Procedure:• To start with, a temporary B.M is established near the area to be

surveyed with reference to a permanent B.M by fly levelling. • The level is then set up in such a position so that the maximum

number of points can be commanded from the instrument station. • The height of instrument is determined by taking a back sight on the

B.M. and adding it to the R.L. of bench mark. • The staff reading required to fix points on the various contours is

determined by subtracting the R.L. of each of the contours from the height of instrument.

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• Example:

• If the height of instrument is 82.48m., then the staff readings required to locate 82, 81 and 80m contours are 0.48, 1.48 and 2.48m respectively. The staff is held on an approximate position of point and then moved up and down the slope until the desired reading is obtained. The point is marked with a peg.

• Similarly various other points are marked on each contour. The line joining all these points give the required contour. It may be noted that one contour is located at a time. Having fixed the contours within the range of the instrument, the level is shifted and set up in a new position.

CONTOURS

DIRECT METHODMETHODS OF CONTOURING

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Procedure (Contd…..)• The new height of instrument and the required staff readings are then

calculated in a similar manner and the process repeated till all the contours are located.

• The positions of the contour points are located suitably either simultaneous with levelling or afterwards.

• A theodolite , a compass or a plane table traversing is usually adopted for locating these points.

• The points are then plotted on the plan and the contours drawn by joining the corresponding points by dotted curved lines.

CONTOURS


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Direct Method By Radial Lines

• This method is suitable for small areas, where a single point in the centre can command the whole area. Radial lines are laid out from the common centre by theodolite or compass and their positions are fixed up by horizontal angles and bearings.

RADIAL LINES METHOD OF CONTOURING

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65

60

55

CONTOURS


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Direct Method By Radial Lines (contd.):• Temporary bench marks are first established at the centre and near

the ends of the radial lines .• The contour points are then located and marked on these lines and

their positions are determined by measuring their distances along the radial lines.

• They are then plotted on the plan and the contours drawn by joining all the corresponding points with the help of a plane table instrument.

CONTOURS


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2. INDIRECT METHOD• In this method the points located and surveyed are not necessarily

on the contour lines but the spot levels are taken along the series of lines laid out over the area .

• The spot levels of the several representative points representing hills, depressions, ridge and valley lines and the changes in the slope all over the area to be contoured are also observed.

• Their positions are then plotted on the plan and the contours drawn by interpolation.

• This method of contouring is also known as contouring by spot levels.

CONTOURS

METHODS OF CONTOURING INDIRECT METHOD

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• This method is commonly employed in all kinds of surveys as this is cheaper, quicker and less tedious as compared to direct method. There are mainly three method of contouring in indirect method:

• (I) BY SQUARES [SQUARE METHOD]:- • In this method, the whole area is divided into number of squares,

the side of which may vary from 5m to 30m depending upon the nature of the ground and the contour interval. The square need not be of the same size throughout.

• The corners of the squares are pegged out and the reduced levels of these points are determined with a level.

CONTOURS


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SQUARE METHOD

CONTOURS


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(II) By Cross- Sections: • This method is most suitable for the survey of long narrow strips

such as a road, railway or canal etc.

CONTOURS


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• (III) By Tacheometric method:• A techeometer is a transit theodolite having a diaphragm fitted with

two stadia wires, one above and other below the central wire. • The horizontal distance between the instrument and staff station

may be determined by multiplying the difference of the staff readings of the upper and lower stadia wires with the stadia constant of the instrument, which is usually 100.

• Thus the techeometer is used for both the vertical as well as horizontal measurements.

Stadia Wires at Diaphragm

CONTOURS


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iii) By Tacheometric method (contd…):

• This method is most suitable in hilly areas as the number of stations which can be commanded by a techeometer is far more than those by a level and thus the number of instrument settings are considerably reduced. A number of radial lines are laid out at a known angular interval and representative points are marked by pegs along these radial lines. Their elevations and distances are then calculated and plotted on the plan and the contour lines are then interpolated.

CONTOURS


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THANK YOU

BY:- ANAND R. JIBHKATECIVIL ENGINEER 41

Education

levelling and contouring