Contouring - Surveying...for civil Engineering and Architecture..

CHAPTER 7

Er. Pramesh Hada, Assistant Professor, nec

Er. Pramesh Hada•BE Civil Engineering (2002-2006)

•MSC Urban planning (2009-2011)

•Assistant ProfessorNepal Engineering College,

Changunarayan,Bhaktapur

By:-

CHAPTER 7

1

CHAPTER 7. CONTOURING (4 HOUR)

� 7.1 Basic Definitions in Contouring –pu 2010

� 7.2 Contour Interval (pu 09,10,14, 15), Horizontal

Equivalent .

� 7.3 Characteristics of Contours – pu 08,09,10,11,13,14)

7.4 Methods of Contouring – pu 09,10,11,13,14


� 7.4 Methods of Contouring – pu 09,10,11,13,14

� 7.5 Interpolation of Contours (PU 2015)

� 7.6 Uses of Contour Maps – pu 09,11,13

2


3

•Contour An imaginary line on the ground surface joining

the points of equal elevation is known as contour.

•It facilitates depiction of the relief of terrain in a twodimensional plan or map.

•In other words, contour is a line in which the ground

7.1 Basic Definitions in Contouring –pu 2010Er. Pramesh Hada, Assistant Professor, nec

•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.

•Contouring is the science of representing the vertical dimension of the terrain on a two dimensional map.

2CONTOURINGCONTOURING 24

Contours


Contour interval = 30 m5


Different contour mapsDifferent contour maps

6


7

RELIEF OR SHAPE OF THE LAND IS

SHOWN BY CONTOURS� Contours are those light brown or orange lines that can see on maps.

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

The lines are numbered � The lines are numbered giving the height above sea level in metres.

� On 1:50000 map the contours will be drawn for every 10 metre rise.

� Although on this map they are every 3m so always remember to check. Er. Pramesh Hada, Assistant

Professor, nec8

� It is important to remember that the closer

together the contour lines the steeper the

slope.

Steep slopes


Area that is flatter with only a gentle slope

XXXX

9


� 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.and a section.

� Contouring : The process of tracing contour lines

on the surface of the earth is called Contouring.

10

� Contour LineContour LineContour LineContour Line : A Contour line is an imaginary outline of the terrain obtained by joining its points of equal elevation.

PuPuPuPu 09,10,1409,10,1409,10,1409,10,14

� Contour Interval (CI) Contour Interval (CI) Contour Interval (CI) Contour Interval (CI) – It is the vertical distance between any two consecutive contours.

Suppose a map includes contour lines of 100m, 98m ,96 m and so on .The contour interval here is 2 m.

This interval depends upon

(i) the nature of the ground (i.e. whether flat or sleep).


(i) the nature of the ground (i.e. whether flat or sleep).

(ii) the scale of the map

(iii) the purpose of the survey.

Contour intervals for flat country are generally small, e g. 0.25 m, 0.5 m, 0.75m. etc. Contour interval for a steep slope in a hilly area is generally greater. e.g. 5m. 10 m, 15 m etc.

It should be remembered that the contour interval for a particular map is Constant.

11


12

� Horizontal Equivalent (HE)

Horizontal equivalent is the horizontal distance

between two consecutive contour lines measured

to the scale of the map.

� It is not constant. It varies according to the steepness

of the ground.

� For steep slopes, the contour lines run close together,


� For steep slopes, the contour lines run close together,

and for flatter slopes they are widely spaced.

13

CHARACTERISTICS OF CONTOURS

(pu 08,09,10,11,13,14)

i) All points in a contour line have the same elevation.

ii) Flat ground is indicated where the contours are widely

separated and steep-slope where they run close together.


separated and steep-slope where they run close together.

iii) A uniform slope is indicated when the contour lines are

uniformly spaced.

iv) A plane surface when they are straight, parallel and

equally spaced.

CONTOURINGCONTOURING 1314

EXAMPLES


15


16


v) A series of closed

contour lines on the

map represent a hill

, if the higher values

are inside 65

70

75

80

HILL


are inside

A HILL

60

65

60

65

707580

HILL



vi) A series of

closed contour

lines on the map

indicate a

depression if the

DEPRESSION

60

65

70

75

80


depression if the

higher values are

outside

A DEPRESSION

70

60

6570

75

80

60



vii) Contour line cross ridge or valley line at

right angles. •If the higher values are

inside the bend or loop

in the contour, it

indicates a Ridge.

100

90

80


RIDGE LINE

70

60

50



vii) Contour line cross ridge or valley line at

right angles.If the higher values are

outside the bend, it

represents a Valley100

90


VALLEY LINE

90

80

70

60

50



VALLEY 21

viii). Contours cannot end anywhere but close

on themselves either within or outside the limits

of the map.




ix).Contour lines cannot

merge or cross one

another on map except

in the case of an

overhanging cliff.


10

20

30

40


40 30 20 10

OVERHANGING CLIFF

10


OVERHANGING CLIFF


24

x) Contour lines never run

into one another except in the

case of a vertical cliff. In this

case ,several contours

coincide and the horizontal

equivalent becomes zero.


10

20

30

40

VERTICA

CLIFF

50


40302050

OVERHANGING CLIFF

10

10 50


CHARACTERISTICS OF CONTOURSXI) Depressions between

summits is called a saddle. It

is represented by four sets of

contours as shown. It

represents a dip in a ridge or

the junction of two ridges.

And in the case of a mountain

range ,it takes the form of a

SADDLE70

80

90

7080

90


range ,it takes the form of a

pass .

90

110100

90

21CONTOURINGCONTOURING

Line passing through the

saddles and summits gives

water shed line.

26

A SADDLE


27

Think where is the steepest slope on this island?

150

200


XXXX

50

100

28

Steepest slope –contour lines are closest together

Sparrow point 227

. 76

150

200


Sometimes spot heights can be shown as a dot with a note of the height of that particular place.

A trigonometrical point (or trig point) shows the highest point in an area (in meters) and is shown as a blue triangle

. 76

50

100

29

THE SHAPE AND PATTERN OF THE CONTOURS


30


31


32


33


34


35

PURPOSE (uses) OF CONTOURING/CONTOUR MAP

(pu2009,2011,2013)

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) contour maps is prepared in order to select the most

economical or suitable site.


economical or suitable site.

ii) It helps to locate the alignment of a canal so that it

should follow a ridge line.

iii) It helps to mark the alignment of roads and railways so

that the quantity of earthwork both in cutting and filling

should be minimum.


iv) It helps for getting information about the ground

whether it is flat, undulating or mountainous.

v) It helps to find the capacity of a reservoir and volume

of earthwork especially in a mountainous region.

vi) It helps to trace out the given grade of a particular

route.

PURPOSE(uses) OF CONTOURING/CONTOUR

MAP


route.

vii) It helps to locate the physical features of the ground

such as a pond depression, hill, steep or small slopes.

(Undulating – rising & falling)


USES OF CONTOUR MAP (Pu,09,11,13)

- A contour map furnishes information regarding the features

of the ground , whether it is flat, undulating or mountainous.

- From a contour map , sections may be easily drawn in any

direction

-Intervisibility between two ground points plotted on map can be

ascertained

- It enables an engineer to approximately select the most



- It enables an engineer to approximately select the most

economical or suitable site for an engineering project such as

a road, a railway, a canal or a pipe line etc.

- A route of a given grade can be traced on the map.

- Catchment area and capacity of a reservoir may be determined

from the contour map.

- Contour map may be used to determine the quantities of earth

work.

38

FACTORS ON WHICH CONTOUR -INTERVAL

DEPENDS

The contour interval depends upon the following

factors:-

i) The Nature of the Ground - In flat and

uniformly sloping country, the contour


uniformly sloping country, the contour

interval is small , but in broken and

mountainous region the contour interval

should be large otherwise the contours will

come too close to each other.


FACTORS ON WHICH CONTOUR -

INTERVAL DEPENDS

ii) The Purpose and extent of the survey.

• Contour interval is small if the area to be surveyed

is small and the maps are required to be used for

the design work or for determining the quantities of


the design work or for determining the quantities of

earth work etc.

• while wider interval shall have to be kept for large

areas and comparatively less important works.


FACTORS ON WHICH CONTOUR -

INTERVAL DEPENDS

iii) The Scale of the Map-

The contour interval should be in the inverse

ratio to the scale of the map i.e. the smaller

the scale, the greater is the contour interval.


the scale, the greater is the contour interval.

iv) Time and Expense of Field and Office work-

The smaller the interval, the greater is the

amount of field-work and plotting work.


COMMON VALUES OF THE CONTOUR -

INTERVAL

The following are the common values of the

contour interval adopted for various purposes:-

i) For large scale maps of flat country, for

building sites, for detailed design work and


building sites, for detailed design work and

for calculation of quantities of earth work;

0.2 to 0.5 m.


COMMON VALUES OF THE CONTOUR -

INTERVAL

ii) For reservoirs and town planning schemes;

0.5 to 2m.

iii) For location surveys. 2 to 3m.


iii) For location surveys. 2 to 3m.

iv) For small scale maps of broken country and

general topographic work; 3m,5m,10m,or

25m.


METHODS OF CONTOURING (PU 09,10,11,13,14)

There are mainly two methods of locating contours:-

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

Direct Method:

•In this method, the contours to be

located are directly traced out in the

field by locating and marking a

50

48

B.M


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. DIRECT METHOD OF CONTOURING

48

46


METHODS OF CONTOURING (PU 09,10,11,13,14)

TheTheTheThe locationlocationlocationlocation ofofofof aaaa pointpointpointpoint inininin topographictopographictopographictopographic surveysurveysurveysurvey involvesinvolvesinvolvesinvolves bothbothbothbothhorizontalhorizontalhorizontalhorizontal asasasas wellwellwellwell asasasas verticalverticalverticalvertical controlcontrolcontrolcontrol.... TheTheTheThe methodmethodmethodmethod ofofofof contouringcontouringcontouringcontouringdependsdependsdependsdepends uponuponuponupon thethethethe instrumentsinstrumentsinstrumentsinstruments usedusedusedused....

TheTheTheThe methodmethodmethodmethod maymaymaymay bebebebe divideddivideddivideddivided intointointointo twotwotwotwo classesclassesclassesclasses....

� TheTheTheThe DirectDirectDirectDirect MethodMethodMethodMethod

� TheTheTheThe indirectindirectindirectindirect methodmethodmethodmethod

InInInIn thethethethe directdirectdirectdirect methodmethodmethodmethod

� TheTheTheThe contourcontourcontourcontour totototo bebebebe plottedplottedplottedplotted isisisis actuallyactuallyactuallyactually tracedtracedtracedtraced onononon thethethethe groundgroundgroundground.... OnlyOnlyOnlyOnlythosethosethosethose pointspointspointspoints areareareare sursursursurveyedveyedveyedveyed whichwhichwhichwhich happenhappenhappenhappen totototo bebebebe plottedplottedplottedplotted andandandand


TheTheTheThe contourcontourcontourcontour totototo bebebebe plottedplottedplottedplotted isisisis actuallyactuallyactuallyactually tracedtracedtracedtraced onononon thethethethe groundgroundgroundground.... OnlyOnlyOnlyOnlythosethosethosethose pointspointspointspoints areareareare sursursursurveyedveyedveyedveyed whichwhichwhichwhich happenhappenhappenhappen totototo bebebebe plottedplottedplottedplotted andandandandcontourscontourscontourscontours areareareare drawndrawndrawndrawn throughthroughthroughthrough themthemthemthem....

� TheTheTheThe methodmethodmethodmethod isisisis slowslowslowslow andandandand tedioustedioustedioustedious andandandand isisisis applicableapplicableapplicableapplicable forforforfor smallsmallsmallsmall areasareasareasareasandandandand wherewherewherewhere greatgreatgreatgreat accuracyaccuracyaccuracyaccuracy isisisis requiredrequiredrequiredrequired....((((as a lot of time is wastedin searching points of the same elevation for a contour).

InInInIn thethethethe indirectindirectindirectindirect methodmethodmethodmethod

somesomesomesome suitablesuitablesuitablesuitable guideguideguideguide pointspointspointspoints areareareare selectedselectedselectedselected andandandand surveyedsurveyedsurveyedsurveyed(the(the(the(the guideguideguideguidepointspointspointspoints shouldshouldshouldshould notnotnotnot necessarilynecessarilynecessarilynecessarily bebebebe onononon thethethethe contourscontourscontourscontours butbutbutbut serveserveserveserve asasasasbasisbasisbasisbasis forforforfor interpolationinterpolationinterpolationinterpolation ofofofof contours)contours)contours)contours)

45


46

DIRECT METHOD

Each contour is located by Each contour is located by Each contour is located by Each contour is located by determining the positions of a series of points determining the positions of a series of points determining the positions of a series of points determining the positions of a series of points tttthhhhrrrroooouuuugggghhhh wwwwhhhhiiiicccchhhh

the the the the contour passes contour passes contour passes contour passes ....

Vertical Control:Vertical Control:Vertical Control:Vertical Control:

� The points on the contours are traced either with the help of The points on the contours are traced either with the help of The points on the contours are traced either with the help of The points on the contours are traced either with the help of aaaa lllleeeevvvveeeellll aaaannnndddd ssssttttaaaaffffffff oooorrrr hhhhaaaannnndddd

level. level. level. level.

� The level is The level is The level is The level is set at a point to command set at a point to command set at a point to command set at a point to command as much area as is possible and is leveled.as much area as is possible and is leveled.as much area as is possible and is leveled.as much area as is possible and is leveled.

� The staff is kept The staff is kept The staff is kept The staff is kept on BM and the height of the instrument on BM and the height of the instrument on BM and the height of the instrument on BM and the height of the instrument is determined. If BM is not is determined. If BM is not is determined. If BM is not is determined. If BM is not

nearby, nearby, nearby, nearby, fly leveling fly leveling fly leveling fly leveling mmmmaaaayyyy bbbbeeee ccccaaaarrrrrrrriiiieeeedddd oooouuuutttt ttttoooo eeeessssttttaaaabbbblllliiiisssshhhh aaaa tttteeeemmmmppppoooorrrraaaarrrryyyy bbbbeeeennnncccchhhh mmmmaaaarrrrkkkk iiiinnnn tttthhhhaaaatttt aaaarrrreeeeaaaa....

� Having known the Having known the Having known the Having known the height of the instrumentheight of the instrumentheight of the instrumentheight of the instrument, the staff reading is calculated so that the , the staff reading is calculated so that the , the staff reading is calculated so that the , the staff reading is calculated so that the


Having known the Having known the Having known the Having known the height of the instrumentheight of the instrumentheight of the instrumentheight of the instrument, the staff reading is calculated so that the , the staff reading is calculated so that the , the staff reading is calculated so that the , the staff reading is calculated so that the

bottom of the staff is at an elevation equal to the value of contour to be plotted.bottom of the staff is at an elevation equal to the value of contour to be plotted.bottom of the staff is at an elevation equal to the value of contour to be plotted.bottom of the staff is at an elevation equal to the value of contour to be plotted.

EgEgEgEg. If height of instrument is . If height of instrument is . If height of instrument is . If height of instrument is 800.250800.250800.250800.250 meters the staff reading to plot a contour meters the staff reading to plot a contour meters the staff reading to plot a contour meters the staff reading to plot a contour of 800 of 800 of 800 of 800

meters will be 0.250 meters. meters will be 0.250 meters. meters will be 0.250 meters. meters will be 0.250 meters. TTTTaaaakkkkiiiinnnngggg oooonnnneeee ccccoooonnnnttttoooouuuurrrr aaaatttt aaaa ttttiiiimmmmeeee(((( ffffoooorrrr 888800000000mmmm ccccoooonnnnttttoooouuuurrrr)))) tttthhhheeee ssssttttaaaaffffffff

man is directed to keep the staff on those man is directed to keep the staff on those man is directed to keep the staff on those man is directed to keep the staff on those points where staff reading of 0.250 m points where staff reading of 0.250 m points where staff reading of 0.250 m points where staff reading of 0.250 m is is is is

obtained every time. obtained every time. obtained every time. obtained every time.

47

DIRECT METHOD DIRECT METHOD DIRECT METHOD DIRECT METHOD

b) Horizontal Control:b) Horizontal Control:b) Horizontal Control:b) Horizontal Control:

� After having located the After having located the After having located the After having located the points for various contours vertically points for various contours vertically points for various contours vertically points for various contours vertically

they are to be surveyed with a they are to be surveyed with a they are to be surveyed with a they are to be surveyed with a suitable control system for suitable control system for suitable control system for suitable control system for

horizontally located.horizontally located.horizontally located.horizontally located.

� For this, the For this, the For this, the For this, the llllooooccccaaaattttiiiioooonnnn ooooffff ppppooooiiiinnnnttttssss aaaarrrreeee ttttrrrraaaacccceeeedddd bbbbyyyy tttthhhheeee mmmmeeeetttthhhhoooodddd ooooffff ppppllllaaaannnneeee

tabling, chain survey or tabling, chain survey or tabling, chain survey or tabling, chain survey or theodolitetheodolitetheodolitetheodolite....

� Then the Then the Then the Then the contour are drawn contour are drawn contour are drawn contour are drawn through these points.through these points.through these points.through these points.


Then the Then the Then the Then the contour are drawn contour are drawn contour are drawn contour are drawn through these points.through these points.through these points.through these points.

� For accurate For accurate For accurate For accurate contouring sufficient number of the points contouring sufficient number of the points contouring sufficient number of the points contouring sufficient number of the points aaaatttt cccclllloooosssseeee

interval are required.interval are required.interval are required.interval are required.

� The plotted positions of the The plotted positions of the The plotted positions of the The plotted positions of the consecutive points of any contour consecutive points of any contour consecutive points of any contour consecutive points of any contour

are simply joined by straight lines are simply joined by straight lines are simply joined by straight lines are simply joined by straight lines to form contour linesto form contour linesto form contour linesto form contour lines

48


49

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 leveling.

•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.


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.


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.

51

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

Procedure (Contd…..)Er. Pramesh Hada, Assistant Professor, nec

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.


2. Indirect Method:

•In this method some suitable guide points are selected and their

elevations are found. These point may form well shaped geometrical

figures.

•The location of such points are plotted by plane tabling and contours

are drawn by interpolation. These guide points do not fall except by

coincidence, on the contours to be located.

• While interpolating, it is considered that the slope between any two

adjacent guide points is uniform.


adjacent guide points is uniform.

•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.

• Other Guide points are then plotted on the plan and the contours

drawn by interpolation.

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


• 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.

• In this method, the whole area is divided into number of


Indirect 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 corners of the squares are pegged out and the reduced

levels of these points are determined with a level.


SQUARE METHOD



Indirect Method: (Square Method)

13 14 15 16

100.4 97.60 96.05 94.40

98.00 98.00


10 11

6 7

1 2 3 4

5 8

9 12

SQURES LAID ON GROUND

97.45 99.25 98.75 99.40

98.65 99.60 97.80 89.55

100.90 99.70 99.85 98.45

98.00

98.0

99.0 99.0

98,00

CONTOURS INTERPOLATED


2. Indirect Method: (Square Method)

100.4 97.60 96.05 94.40

98.00 98.00

•The important points

within the squares

may be taken when

required and located

by measurements

from the corners.


97.45 99.25 98.75 99.40

98.65 99.60 97.80 89.55

100.90 99.70 99.85 98.45

98.00

98.0

99.0 99.0

98,00

SQUARE METHOD


from the corners.

• The squares are

plotted and the

reduced levels of the

corners are written on

the plan.

57

2. Indirect Method:

By Cross- Sections:

• This method is most suitable for the surveys of long narrow

strips such as a road, railway or canal aignment etc.

• In this method cross sections perpendicular to the centre

line of the area are set out.

• The spacing of the cross-section depends upon the contour

interval, scale of plan and the characteristic of ground.


interval, scale of plan and the characteristic of ground.

• The common value is 10 to 20 m in hilly country and 20-30m in

flat country.

•The centre line and cross sections are plotted along with

important features on the desired scale and their RLs are

entered.

•The contours are then interpolated with respect to these RLs.


Indirect Method:

By Cross- Sections:

70.6 69.1 68.8 69.1 70.8

70.8 70.2 69.1 70.4 70.5

RD 580

RD 560

7070

69 69


•The levels of the points along the section lines are plotted on

the plan and the contours are then interpolated as usual as

shown in the fig.

70.8 70.2 69.1 70.4 70.5

71.2 70.8 66.3 70.6 70.8

71.6 71.2 70.6 72.4 71.7

RD 560

RD 540

RD 520

71

71

70

69

68

67

71

71

Fig. X-Section Method


2. Indirect Method:

(iii) By Tacheometric method:

• A tacheometer is a transit theodolite having a diaphragm

fitted with two stadia wires, one above and other below the

central wire.

Fig. Stadia Wires at Diaphragm


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 tacheometer is used for both the vertical as well as

horizontal measurements.


2. Indirect Method:

iii) By Tacheometric method (contd…):

•This method is most suitable in hilly areas as the number of

stations which can be commanded by a tacheometer 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


•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.



By Tacheometric method

62

DIFFERENCE BETWEEN DIRECT AND INDIRECT METHODS OF DIFFERENCE BETWEEN DIRECT AND INDIRECT METHODS OF DIFFERENCE BETWEEN DIRECT AND INDIRECT METHODS OF DIFFERENCE BETWEEN DIRECT AND INDIRECT METHODS OF

CONTOURINGCONTOURINGCONTOURINGCONTOURING


63

INTERPOLATION OF CONTOURS

• The process of spacing the contours proportionally

between the plotted ground – points is termed as

interpolation of contours .

• This becomes necessary in the case of indirect

contouring as only the spot levels are taken in this

method.


method.

• While interpolation of contours the ground between any

two points is assumed to be uniformly sloping.



There are three main methods of interpolation:

i) By arithmetical calculation:

• This is very tedious & time consuming but accurate

method .

• The positions of contour points between the guide points

are located by arithmetic calculation.

• used for small areas where accurate results are necessary.


• used for small areas where accurate results are necessary.


•Find distance from A for a contourof 99m elevation. (Note – Square = 10*10m)

•Distance from A for contour of 99m = (99—98.75)/ (100.75 —98.75) *10 = 1.25m

65


The contours are interpolated as under:

Example-•A and B are two points at a distance of 30 m and the reduced level of

A and B are 25.45m and 27.54m .

•Taking the contour interval as 1m, 26 and 27 m contours may be

By arithmetical calculation:


A B30 m25.45m 27.54m

26m 27m

Diff (A-B)=2.09m

•Taking the contour interval as 1m, 26 and 27 m contours may be

interpolated in between A and B.

•Difference of level between A and B is 2.09m.the difference of level

between A and 26m,and A and 27m is 0.55mand 1.55 m respectively.

•Horizontal distance between A and 26 m contour =0.55/2.09 x 30m

((Diff of A & 26m / Diff of A & B)* Dist.AB)) and Between A and 27

m contour =1.55/2.09 x 30m.

•These distances are then plotted to scale on the map.



•Graphical method of

interpolation is simpler

as compared to

arithmetical method

and also the results

(ii) By Graphical method:

60

65

B62.5m

60m2


and also the results

obtained are accurate.

• Out of several

graphical methods, the

most common is as

given below:

50

55

A 51.5m

55m1


•As shown in the fig. suppose the

contour interval is 5m, then on a

piece of tracing cloth or tracing

paper, a number of parallel lines

spaced at 0.5 m (usually 1/10th of

the contour interval) are drawn.

Graphical method:



• Every tenth line being made

thick.

•Suppose it is required to

interpolate contours between two

points A and B of elevation 51.5m

and 62.5m respectively.


INTERPOLATION OF CONTOURSBy Graphical method:

• If the bottom line represents an

elevation of 50m. Then the successive

thick lines will represent 55m, 60m and

65m, etc.

•The point A is on the third line from

the bottom, B is on the fifth line above

the 60m thick line.


the 60m thick line.

• The intersection of the thick lines 1

and 2 representing elevations of 55m

and 60 m and the line AB give the

position of the points on the 55m and

60m contours respectively and are

pricked through on the plan with a pin.




iii) By Estimation:

� The position of the contour points between ground

- points are estimated roughly and the contours

are then drawn through these points.

� This is a rough method and is suitable for small

scale maps.

� Accuracy of work depends upon the skill and

experience of surveyor.

70

DRAWING THE CONTOUR LINESContour lines are drawn as fine and smooth free hand

curved lines. Sometimes they are represented by broken

lines .They are inked in either in black or brown colour. A

drawing pen gives a better line than a writing pen and

French curves should be used as much as possible .Every

fifth contour is made thicker than the rest.

The elevation of contours must be written in a uniform


The elevation of contours must be written in a uniform

manner, either on the higher side or in a gap left in the line

.When the contour lines are very long, their elevations are

written at two or three places along the contour .In the case

of small scale maps, it is sufficient to figure every fifth

contour.


CONTOUR GRADIENT� Contour gradient is a line lying throughout on the surface of the

ground and preserving a constant inclination to the horizontal.

� The inclination of a contour gradient is generally given either as

rising gradient or falling gradient, and is expressed as ratio of

the vertical height to a specified horizontal distance.

� If the inclination of a contour gradient is 1 in 50, it means that

for every 50 m horizontal distance, there is a rise (or fall) of 1 m.

� To locate the contour gradient in the field a clinometer, a


� To locate the contour gradient in the field a clinometer, a

theodolite or a level may be used.

� The inclination of a contour gradient is generally given either as

rising or falling gradient and is expressed as ratio of the vertical

height to a specified horizontal distance.

72


73

Contouring - Surveying...for civil Engineering and Architecture..

Engineering

Transcript of Contouring - Surveying...for civil Engineering and Architecture..