Leveling

LECTURE NOTES:

LEVELLING

PREPARED BY:
NAFISAH HARUN
FAUZIAH MOHD YUSOF
NORAYAHATI NGAGIMAN

LEVELLING

LECTURE NOTES - LEVELLING

TERBITAN EDISI 2020

BUKU “LECTURE NOTES – LEVELLING” ADALAH SEBAGAI RUJUKAN DAN
BACAAN UMUM TERUTAMA KEPADA PENSYARAH DAN P E L A JA R
P O L I T E K N I K D A N K O L E J K O M U N I T I M A L AY S I A B A G I
MENGAPLIKASIKAN AMALAN TERBAIK DALAM PERLAKSANAAN KAEDAH
P E N G A JA R A N DA N P E M B E L A JA R A N B E R KO N S E P K A N T E K N O LO G I
AUGMENTED REALITY.

EDITOR
MOHD ROZAIMIN ABDUL HAMID

PENULIS LIYANA AHMAD BAZULI
NAFISAH HARUN
FAUZIAH MOHD YUSOF
NORAYAHATI NGAGIMAN

DITERBITKAN OLEH

UNIT PEMBELAJARAN DIGITAL
BAHAGIAN INSTRUKSIONAL DAN PEMBELAJARAN DIGITAL
JABATAN PENDIDIKAN POLITEKNIK DAN KOLEJ KOMUNITI ARAS 6, GALERIA PJH,
JALAN P4W, PERSIARAN PERDANA, PRESINT 4,
62100 PUTRAJAYA

Website : www.celt.edu.my
E- mail : [email protected]



CONTENT

1.0 LEVELING
1.1 PRINCIPLE AND USE LEVEL SURVEY
1.1.1 PRINCIPLE OF LEVELING
1.1.2 SURVEY LEVEL USE
1.2 TECHNICAL TERMS USED IN LEVELLING
1.2.1 LEVEL LINE
1.2.2 REDUCED LEVEL
1.2.3 BENCH MARK
1.2.4 TEMPORARY BENCH MARK
1.2.5 BACK SIGHT
1.2.6 FORE SIGHT
1.2.7 INTERMEDIATE SIGHT
1.2.8 CHANGE POINT

1.3 INSTRUMENT USED IN LEVELING
1.3.1 LEVEL INSTRUMENT
1.3.2 TRIPOD
1.3.3 MEASURING TAPE
1.3.4 LEVELING STAFF
1.3.5 CIRCULAR ROD LEVEL
1.4 TYPES OF SURVEYING LEVELS
1.4.1 DUMPY LEVEL
1.4.2 TILTING LEVEL
1.4.4 DIGITAL LEVEL
1.5 RECITATION METHOD STAFF
1.6 ADJUSTMENT OF A LEVEL
1.6.1 TEMPORARY ADJUSTMENT
1.6.2 PERMANENT ADJUSTMENT
1.6.2.1 TWO PEG TEST
1.7 LEVELLING OR FIELD PROCEDURES

1.8 BOOKING AND REDUCTIONS
1.8.1 RISE AND FALL METHOD
1.8.2 HEIGHT OF INSTRUMENTS (COLLIMATION) METHOD
1.8.3 COMPARISON OF THE TWO METHODS

1.9 PROFILE LEVELLING
1.9.1 LONGITUDINAL SECTIONS
1.9.2 CROSS SECTIONS

1.10 CONTURING
1.10.1 CHARACTERISTICS OF CONTOUR
1.10.2 METHOD OF CONTOURING
1.10.3 INTERPOLATION OF CONTOUR

LEVELLING

1.0 LEVELLING

2.1 PRINCIPLE AND USE LEVEL SURVEY

Levelling is the art of determining relative altitudes of points on the surface of the
earth of beneath the surface of the earth. This branch of surveying deals with
measurements in vertical planes.

For the execution of engineering projects, such as railways, highways, canals, dams,
water supply and sanitary schemes, it is very necessary to determine elevations
of different points along the alignments of the proposed projects. Success of such
projects, depends upon accurate determination of elevations. Levelling is employed
to provide an accurate network to heights, covering the entire area of the project.
Levelling is of prime importance to the engineers, both in acquiring necessary data
for the design of the project and also during its execution.

2.1.1 Principle of Levelling

To determine the difference in height between different points it is necessary to produce
a horizontal line of sight. This requires the use of an instrument. In surveying, this
instrument is known as a surveyor's level, often generically known as a dumpy level
although this is not strictly correct. All levels consist of these Features: a telescope
with a cross hair, mounted on a device that enables the line of sight to be oriented in
a horizontal plane or line.

Figure 2.1.1 Measurement Level / Difference High Drop between two
points

1

LEVELLING

Let the respective reading on staff A and staff B be 1.353 m and 2.887 m respectively.
The difference of level between A and B = 1.353 - 2.887 = -1.543 m

If Reduce level of A is 100.000 m, the reduce level of B, may be calculated as under:
Reduce level of the point A = 100.00 m
Reduce of the line of sight = 100.00 – 1.357 = 98.446 m
Reduce level of the point B = 98.446 – 2.887 = 95.559 m

2.1.2 Survey Level Use

Levels can be used for:

1. Determining the height of a particular point
2. Determining differences in height between points
3. Determining the contours of a land profile
4. Providing data for road cross-sections
5. Providing data to calculate volumes for earthworks
6. Setting out level surfaces for construction
7. Setting out inclined surfaces for construction

2.2 TECHNICAL TERMS USED IN LEVELLING

2.2.1 Level Line

A line lying on the level surface is known as a level line. Every point of a level line is equidistant
from then centre of the earth. The cross section of still water of a lake represents a level line.

2.2.2 Reduced Level

The height or depth of a point above or below the assumed datum is called reduced level. It
is also known as elevation of the point. Elevation of the points below the datum surface, are
known as negative elevation.

2.2.3 Bench Mark

A benchmark (B.M.) is a definite point on a permanent object which has a known elevation
and a known location. Temporary benchmarks, (T.B.M.) are used many times to supplement
permanent benchmarks. The elevation of a location of these points is also known but is
not intended to be permanent. A benchmark is a point of reference which is convenient for
leveling in a given locality. The relation to sea-level is very precise and obtained by running a
level circuit such that the elevation of the beginning and the end of the circuit are known and
tied together.

2

LEVELLING

2.2.4 Temporary Bench Mark
Temporary benchmark is fixed dots but behave less permanent and established nearby to
site survey to save works reference to benchmark which may too long.
2.2.5 Back sight
The first sight taken on a leveling staff held at point of known elevation is called back sight. It
ascertains the amount by which the line of sight is above or below the elevation of the point.
Back sight enables the surveyor to obtain the height of the instrument.
2.2.6 Fore sight
The sight taken on a leveling staff held at point of unknown elevation to ascertain the amount
by which point is above or below the line of sight is called a fore sight. Fore sight enables
the surveyor to obtain the elevation of the point. It is also generally known as minus sight as
the foresight reading is always subtracted from the height of the instrument (except when the
staff is held inverted) to obtain the elevation.

Figure 2.2.6 Back Sight and Fore Sight

3

LEVELLING

2.2.7 Intermediate sight
The fore sight taken on a levelling staff held at a point between two turning points, to
determine the elevation of that point, is known as intermediate sight. It may be noted that for
one setting of a level, there will be only a back sight and fore sight but there can be a number
of intermediate sight.

Figure 2.2.7 Intermediate Sight
2.2.8 Change Point
The point at which both the fore sight and back sight are taken during the operation of leveling,
is called a change point. Two sight, are taken from two different instrument stations, a fore
sight to ascertain the elevation of the point while a back sight is taken on the same point to
establish the height of the instrument of the new setting of the level. The change point is
always selected on a relatively permanent point.

Figure 2.2.8 Change Point Position

4

LEVELLING

2.3 INSTRUMENT USED IN LEVELLING


Levelling is carried out by the use of:
2.3.1 Level Instrument
Levelling instrument selection is dependent to cost factor, time and measurement accuracy.
It is also consisting essentially of the following parts:
a. A telescope to provide a line of sight
b. A level tube to make the line of sight horizontal
c. A levelling head to bring the bubble of the tube level at the centre of its run
However, generally levels could be divided into four special types, which are: -
a. Dumpy Level
b. Tilting Level
c. Automatic Level
d. Digital Electronic Level

Figure 2.3.1 Fundamental Building Cross Section Levelling Instrument

5

LEVELLING

2.3.2 Tripod

A sturdy tripod in good condition is essential for obtaining accurate measurements. The
legs of the tripod are adjustable and are made of wood, fiberglass or aluminum and are
adjustable for use with many different pieces of surveying equipment. Tripods made of wood
or aluminum can influence readings in certain weather conditions, fiberglass can be heavy
to carry when surveying over various terrains and distances. Tripods come with two different
styles of heads: flat or dome. Dome heads allow for more adjustment suitable for automatic
level. While flat head tripods have less play suitable for dumpy and tilting levels, but are
sometimes easier to fit.

Figure 2.3.2 Types of Tripod

2.3.4 Measuring Tape
Using a tape or a graduated rod to measure the vertical distance between two points. Only
applicable when an unobstructed vertical line between the two points exists.

Figure 2.3.4 Measuring Tape

6

LEVELLING

2.3.5 Levelling Staff
Leveling rods are manufactured from wood, metal, or fiberglass and are graduated in feet or
meters.
The length of rod is 4 m or 5 m, the rod continusly each other.

Figure 2.3.5 Metric rods
2.3.6 Circular Rod Level
The rectangular sectioned rods are either the folding (hinged) or the sliding variety. Newer
fiberglass rods have oval or circular cross section and fit telescopically together for heights of
3, 5 and 7 m. Benchmark levelling utilizes folding (one-piece) rods or invar rods, both of which
have built-in-handles and rod levels. When the bubble is centered, the rod is plumb. All other
rods can plumb by using rod level.

Figure 2.3.6 Types of Circular Rod Level

7

LEVELLING

2.4 TYPES OF SURVEYING LEVELS

2.4.1 Dumpy Level
The essential features are:

a. The telescope barrel and vertical spindle are cast in one piece and the telescope is
of the internal focusing type.

b. The bubble tube is mounted on the left-hand side of the telescope. An inclined mirror
enables the bubble to be viewed from the eyepiece end of the telescope and this.
When folded down, protects the bubble.

c. A circular bubble mounted on the upper plate of the leveling head enables
preliminary leveling up of the instrument to be made.

d. The leveling head consists essentially of two plates, the telescope being mounted on
the upper plate while the lower plate screws directly on to a tripod. The two plates
are held apart by three leveling screws or foot screws, and adjustments to these
enable accurate leveling of the instrument to be carried out.

e. By means of a clamping screw, the telescope may be fixed along any particular line
of sight. And slight lateral deviations may be made to left or right of this line with the
tangent screw.
This type of level is mainly confined to construction sister or other cases where large numbers
of level sights are required from a single instrument position.

Figure 2.4.1Dumpy Level Wild N01/NK01
(Source : Wild Heerbrugg (Switzerland) Ltd)

8

LEVELLING

2.4.2 Tilting Level

The tilting level is roughly leveled by observing the buble in the circular spirit level. Just before
each rod reading is to be taken, and while the telescope is pointing at the rod, the telescope
is precisely leveled by manipulating a tilting screw, which effectively raises or lowers the
eyepiece end of the telescope. The level is equipped with a tube level that is precisely leveled
by operating the tilting screw. The buble is viewed through a separate eyepeice or, as is case
in Figure 2.4.2.1, through the telescope. The image of the buble is longitudinally split in two
and viewed with aid of prisms. One – half of each end of the buble can be seen, and after
adjustment, two half – end are brought to concidence and appear as a continuous curve.
When coincidence has been achieved, the telescope has been precisely leveled.

Figure 2.4.2.2

Figure 2.4.2.1 Tilting Level Wild N05/NK05 Continuous curve bubble

(Sumber : Wild Heerbrugg (Switzerland) Ltd

9

LEVELLING

2.4.3 Automatic Level
The automatic level (see Figure 2.4.3) employs a gravity – referenced prism or mirror
compensator to orient the line of sight (line of collimation) automatically. The instrument is
quickly leveled when a circular spirit level used. When the bubble has been centered (or
nearly so), the compensator takes over and maintains a horizontal line of sight, even if the
telescope is slightly tilted. Automatic levels are extremely popular in present-day surveying
operations and are available from most survey instrument manufacturers. They are easy
to set up and use. Many automatic levels utilize a concave base that, when attached to its
domed-head tripod top, can be roughly leveled by sliding the instrument on the tripod top.
This rough leveling can be accomplished in a few second. If the bull’ eye bubble is nearly
centered by this maneuver and the compensator is activated, the leveling screw may not be
needed at all to level the instrument.

The bubble must nearly at the
center of the tube. Easy to
setting up level

Figure 2.4.3 Automatic Level Topcon AT-22A
(Source: Topcon Instruments (Malaysia) Sdn. Bhd.)

10

LEVELLING

2.4.4 Digital Level

Figure 2.4.4 Digital Level Topcon DL-101C
(Source: Topcon Instruments (Malaysia) Sdn. Bhd.)

Figure 2.4.4 shows a digital level which features a digital electronic image-processor that uses
a charge-coupled device (CCD) for determining heights and distances, with the automatic
recording of data for later transfer to a computer. Data can be stored in internal on-board
memory or on easily transferring the PCMCIA Memory Card System and then transferred to
a computer. Manufacturers report that use of the digital level increases productivity by about
50 percent, with the added bonus of the elimination of field- book entry mistake. The more
precise digital levels can be used in first-and second-order leveling, whereas the less precise
digital levels can be used in third-order leveling and construction survey.

11

LEVELLING

2.5 RECITATION METHOD STAFF
Readable staff by upright (normal) or hold by upside-down (invert). It is customary staff hold
by upright, yet to get reduced level under floor something object, staff would be held by
upside-down.

Figure 2.5 Upright Staff And Upside-down Staff
Reading method is the same, only reading figures to staff will see upside-down. Therefore,
recitation staff must do by careful. To facilitate in work recording and calculation, reading take
by staff upside-down add on to with negative signal (-). Base in figure 4.10, under reduced
level bridge is = AL BA PB PH.

2.6 ADJUSTMENT OF A LEVEL

There are two types of setting up the level:
a) Temporary Adjustment
b) Permanent Adjustment
2.6.1 Temporary Adjustment
It involving works to install, levelling instrument and focus telescope. This adjustment should
be done every time levelling instrument in construct. Measures to do adjustment while are as
follows.

12

LEVELLING

Step 1 Step 2

These screws are used in pairs to center The telescope is then rotated
the bubble in the tube below the telescope 90° in either direction so that
when the telescope or sighting tube is the telescope is directly over the
aligned over a pair of leveling screws. other pair of leveling. The bubble
Using either or both of these screws (2 & is brought to the center of the
3), bring the bubble into the center of tube. tube by loosening one screw
while tightening the other with
the thumb and first finger of each
hand.

Step 3 Step 4

Care is taken to not over tighten the screws. The telescope is turned back
The telescope is then rotated 90° in either 90° so that the level is over the
direction so that the telescope is directly original pair of leveling screws.
over the other pair of leveling screws. The The bubble is checked to make
process is repeated to bring the bubble to sure that the level is centered.
the center of the tube.

13

LEVELLING

Step 5 (Check on) Step 5

The telescope is turned back 90° so If the bubble is no longer
that the level is over the original pair of centered, then the leveling
leveling screws If the instrument is in good screws are adjusted to bring
adjustment, the bubble remains centered. the bubble to center and the
telescope rotated back over the
pair of leveling screws to check
the bubble again.

Figure 2.6.1 Direction of thumb and bubble
movement

14

LEVELLING

2.6.2 Permanent Adjustment
All instruments are subject to errors of calibration and adjustment. In the case of levelling
instruments the main source of instrumental error is where the line of sight, produced by the
cross hairs in the telescope, is not parallel to the horizontal line of collimation produced by the
manufacture of the instrument. This error is known as collimation error as it effects the line of
collimation. The test that we undertake in order to determine the amount of errror and then
eliminate it from our measurement is known as the two peg test.
2.6.2.1 Two Peg Test

Figure 2.6.2 Two Peg Test
The two peg test is a very simple test which is used in the field to determine if the line of
sight of the telescope is exactly parallel to the bubble tube. This is one of the most important
properties of a level and is required to be checked periodically.
The steps below are required to be followed to check this adjustment in a level.

1. Set two hubs 40 m apart
2. Set up the level half way between the two hubs and level the instrument properly.
Figure 2.6.2 illustrates a sketch of the level properly established between the two
stakes represented by points A and B. The location is selected so that the tops of the
stakes are obviously different in elevation.
3. Sets the rod on stake A and rocks the rod on this point. The instrument operator
reads the rod (a1), making sure that the rod is at the center of the cross hairs
and that the level bubble is truly centered when the reading is taken. moves up to
point B, sets the rod on the stake, and rocks the rod as the instrument operator turns
the instrument to obtain a precise reading of the rod at point B (b1). Once again, the
bubble level is centered when the reading is taken. This is the first set-up.

15

LEVELLING

4. The level is then moved to the highest stake, in this case stake B, and set so that the
eye piece of the level is just a few inches from the rod when the level rod is plumbed
on the stake.

5. The rod reading is taken on stake B by sighting backwards through the telescope
(b2).

6. Takes the rod to stake A and a precise reading is taken on stake A (a1).
7. The true difference in elevation between point A and B is computed from the two-
readings obtained from the first setup (a1 – b1).
8. The elevation difference of the second set-up is computed (a2 – b2). If the
plane of sight is truly horizontal, the elevation difference of the second set is equal
to the true difference in elevation (a1 – b1) = (a2 – b2). If this is not so, the line
of sight is not parallel to 2-5 the bubble tube on the telescope and adjustments to
the instrument are required. Adjustments vary from instrument to instrument and the
proper procedure is different for a transit and a level.

• Set the distance of AC=CB=BD
• If the difference between AC=e, so that difference of CB = BD =e.
• Thus, the distance of DA=3e

16

LEVELLING

• Refer the following table:

• If (a2 – b2) ≠ (a1 – b1), so adjustment must be set up.
• With instrument at D, Actual reading a2= A + 3e

Thus reading at A = (a2 –3e),

Actual reading b2 = B + e

Thus reading at B = (b2 – e)

Hence,

Actual correction = Actual reading at A – Actual reading at B

= (a2 – 3e) – (b2 – e)

= (7.86 – 3e) – (6.36 –e)

= 1.5 – 2e

But true difference = a1 – b1

= 3.75 – 4.25

= - 0.5

So - 0.5 = 1.5 – 2e

2e = 2

e = 1m/distance

With instrument at D,

Actual reading at A (a2) = 7.86 – 3 = 4.86

Actual reading at B (b2) = 6.36 – 1 = 5.36

17

LEVELLING

2.7 Levelling or Field Procedures

To determine the difference in level between points on the surface of the ground, a series of
levels will need to be carried out. This is called a level traverse or level run. The leveling or
field procedure that should be followed is shown in Figure 2.7.1 below. While Figure 2.7.2
shows the plan view of levelling survey work.

1. Set up the levelling instrument at position L1 ensuring that the line of sight intersects
the staff held at Bench Mark (BM 49.87m). Level it correctly.

2. With the bubble central, take the back staff reading at the staff held vertically on the
BM 49.87m. This will be a backsight because it is the first staff reading after the
levelling instrument has been set up. The backsight reading is 2.191m.

3. Move the staff to the next point A and B. Take the staff reading for both point. The
both point will be an intermediate sight. The reading for point A and B are 2.505m
and 2.325m respectively.

4. Move the staff to the point C and take a reading. This will be a foresight, because
after this reading the level will be moved. The reading at point C is 1.496m
(A change plate should be placed on the ground to maintain the same level.)

5. The distance between the stations should be measured and recorded in the field
book.

18

LEVELLING

6. Set up the level at position L2 and leave the staff at C on the change plate. Point C
will be a change point for both level position. Turn the staff so that it faces the level
and take a reading. This will be a back sight and the reading is 3.019m.

7. Move the staff to D and take a reading. This will be an intermediate sight with the
staff reading is 2.513m.

8. Move the staff to E and take a reading. This will be a foresight; because after taking
this reading the level will be moved. The foresight reading is 2.811m.

9. Now move the level to the position L3 and leave the staff at E on the change plate.
The back sight reading is 1.752m.

10. Lastly, take the reading on the staff vertically on Temporary Bench Mark (TBM
48.71m). This will be the fore sight and the reading is 3.824m.

11. All staff readings should be recorded in the field book. To eliminate errors resulting
from any line of sight (or collimation) back sights and foresights should be equal in
distance. Length of sight should be kept less than 100 meters. Always commence
and finish a level run on a known datum or benchmark and close the level traverse;
this enables the level run to be checked.

2.8 Booking and Reductions

2.8.1 Rise and Fall Method

19

LEVELLING

In this method, the reduced level of each point is deduced from that of the one immediately
preceding it. If the staff reading on the first point is more than the staff reading on the next
point there is a Rise and if the staff reading on the first point is less than that on the next point
there is a fall.

There are three Arithmetic checks in this method:

Σ (Back sight) – Σ (Foresights) = Σ (Rises) - Σ (Falls) = Last R.L - First R.L.

6.962 - 8.131 = 1.515 - 2.684 = 49.701 - 49.87

1.169 = 1.169 = 1.169

This method affords a complete check on all reduced levels including the intermediate points
although it is more tedious than H.I. method. Another disadvantage of this method is that if
there is any mistake in calculating The R.L. of any one point. The R.L. of all the subsequent
points are also in error.

This method is preferred in differential leveling and for reduction to levels on important works.

Allowable closing error is normally expressed in the form:
E= (0.012 √ K) m
Where K is the length of the circuit covered in km.

And alternative method for short lines-
E = ± 5√n mm
Where n is the number of instrument stations.

The permissible closing error is distributed pro-rationally over the R.L-'s of the change points,
the R.L.'s of the intermediate sights being corrected by the same amount as the change point
immediately preceding it.

2.8.2 Height of Instrument (Collimation) Method.

20

LEVELLING

In this method, the reduced levels of points are obtained by calculating the reduced levels of
the plane of collimation for each set up of the instrument. The height of collimation is obtained
by adding the staff reading, which must be a Backsight, to the known R.L. of The point on
which the staff stands. All other readings are deducted from the height of collimation, until the
instrument setting is changed. Where upon the new height of collimation is determined by
adding the backsight to the R-L. at the change point.

There are two Arithmetic checks in H.l. method:

Σ(Back sights) - Σ (Foresights) = LastR.L. - FirstR.L.

6.962 - 8.131 = 48.701 - 49.87

1.169 = 1.169

2.8.3 Comparison of the Two Methods

21

LEVELLING

2.9 PROFILE LEVELLING
Profile leveling is one of the most common applications of running levels and vertical
distance measurement for the surveyor. The results are plotted in the form of a profile, which
is a drawing that shows a vertical cross section. Profiles are required for the design and
construction of roads, curbs, sidewalks, pipelines, etc. In short, profile leveling refers to the
process of determining the elevation of points on the ground at mostly uniform intervals along
a continuous line. Profile levelling divided into two section: -
a. Longitudinal sections
b. Cross sections
2.9.1 Longitudinal Sections
Longitudinal sections are sections which follow some pre-determined line defining a part of
a new construction and are usually run along the center lines of the proposed work such as
new roads, canals, railways, pipe-lines, etc.
The aim of longitudinal section leveling is to provide data for the following:

a. Deciding the most suitable and economic levels and gradients, in its longitudinal
section;

b. Locating the places of cut or fills;
c. Locating the place where neither cut nor fill occurs.

Figure 2.9.1 Longitudinal Cross Section Plan view

C1,C2…..C6 is point along the longitudinal section and
a,b,d and d is point along the cross section

22

LEVELLING

2.9.2 Cross Section

Cross sections are sections set out normal to the longitudinal section. The aim of cross section
leveling is the reproduction of an accurate section of the ground which is to be covered by the
construction works.

Cross sections provide data for the following:

a. Deciding the most suitable and economic levels in the traverse direction;
b. Supplying details for locating the position, height and slope of embankments or
cuttings;
c. Earthwork quantities for costing purposes (Mass Haul Diagrams)

Figure 2.9.2 Profile Plotting

2.10 CONTOURING

A contour or a contour line may be defining as a line joint the points having the same elevation
above the datum surface. The process of tracing contour lines on the surface of the earth is
called contouring and the maps upon which these line are drawn are called contour maps.

The constant vertical distance between any two consecutive contours is called the contour
interval and the horizontal distance between any two adjacent counters is termed as the
horizontal equivalent and depends upon the slope of the ground. The contour interval
depends upon the nature of the ground, scale of the map, purpose and extent of the survey.
The contour interval of any survey in inversely proportional to the scale of the map and it
should be constant throughout the survey.

23

LEVELLING

2.10.1 Characteristics of Contour
The following are important characteristics of contour:

1. All points in a contour line have the same elevation
2. When the contour lines are widely divide, it indicates a flat ground and when they run
close together, it indicates a steep ground.
3. When the contour lines are uniformly spaced, it indicated a uniform slope and when
they are straight, parallel and equally spaced, it indicated a plane surface.
4. A series of closer contour lines on the map indicates a hill, if the higher values are
inside.
5. A series of closer contour lines on the map indicates a depression, if the higher
values are outside.
6. The contour lines cross ridge or valley lines at the right angles. If the higher values
are inside the bend or loop in the contour, it indicates a ridge and if the higher values
are outside the bend, it indicates a valley.
7. When the contour lines merge or cross one another on map, it indicates an
overhanging cliff.
8. When several contours concede and the horizontal equivalent becomes zero, in
indicates a vertical cliff.

24

LEVELLING

2.10.2 Method of Contouring

The commonly used methods for contouring are:

a. Direct Contouring.

The contours to be located are directly traced out in the field by locating and making a number
of points on each contour. The direct method by radial lines is suitable for small areas where
a single point in the center can command the whole area.

It consists in finding vertical and horizontal controls of the points which lie on the selected
contour line. For vertical control levelling instrument is commonly used. A level is set on a
commanding position in the area after taking fly levels from the nearby bench mark. The
plane of collimation/height of instrument is found and the required staff reading for a contour
line is calculated.

The instrument man asks staff man to move up and down in the area till the required
staff reading is found. A surveyor establishes the horizontal control of that point using his
instruments.

After that instrument man directs the staff man to another point where the same staff reading
can be found. It is followed by establishing horizontal control. Thus, several points are
established on a contour line on one or two contour lines and suitably noted down. Plane
table survey is ideally suited for this work.

After required points are established from the instrument setting, the instrument is shifted to
another point to cover more area. The level and survey instrument need not be shifted at the
same time. It is better if both are nearby to communicate easily.For getting speed in levelling
sometimes hand level and Abney levels are also used. This method is slow, tedious but
accurate. It is suitable for small areas

b. Indirect Contouring: Gridding. Spot Heights, Cross Sections and Radiating
lines.

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. This method of contouring is also
known as contouring by spot levels. This method is cheaper, quicker and less tedious as
compare with direct method.

In this method, levels are taken at some selected points and their levels are reduced. Thus
in this method horizontal control is established first and then the levels of those points found.
After locating the points on the plan, reduced levels are marked and contour lines are
interpolated between the selected points.

For selecting points any of the following methods can be used:

a. Method of squares

25

LEVELLING

b. Method of cross-section
c. Radial line method
a. Method of Squares
In this method area is divided into a number of squares and all grid points are marked (Ref.
Figure 2.10.1).

Figure 2.10.1
Commonly used size of square varies from 5 m × 5 m to 20 m × 20 m. Levels of all grid points
are established by levelling. Then grid square is plotted on the drawing sheet. Reduced levels
of grid points marked and contour lines are drawn by interpolation [Ref. Figure 2.10.1].

b. Method of Cross-Section
In this method cross-sectional points are taken at regular interval. By levelling the reduced
level of all those points are established. The points are marked on the drawing sheets, their
reduced levels (RL) are marked and contour lines interpolated.

Figure 2.10.2

26

LEVELLING

Figure 2.10.2 shows a typical planning of this work. The spacing of cross-section depends
upon the nature of the ground, scale of the map and the contour interval required. It varies
from 20 m to 100 m. Closer intervals are required if ground level varies abruptly.
The cross- sectional line need not be always being at right angles to the main line. This
method is ideally suited for road and railway projects.

c. Radial Line Method
[Figure 2.10.3]. In this method several radial lines are taken from a point in the area. The
direction of each line is noted. On these lines at selected distances points are marked and
levels determined. This method is ideally suited for hilly areas. In this survey theodolite with
tachometry facility is commonly used.

Figure 2.10.3
2.10.3 Interpolation of Contour
The process of spacing the contours proportionally between the plotted ground points is
termed as interpolation of contours. There are three method of interpolation by mathematical,
by graphical and by scale method.

a. Mathematical Interpolation Method
In this method, positions of contours between two known points are located by making accurate
calculations. Hence, the method, though very accurate is time consuming and laborious. it is
generally adopted when higher accuracy is demanded for a limited area.

27

LEVELLING

Figure 2.10.4 Mathematical Interpolation Method
A and B are two plotted point at 20 m apart and their ground reduced levels are 37.2 and
35.8m respectively. It is required to draw contours at 36m

Similarly, the distance of 36 m from the point B will be 2.86 m. to achieve better accuracy,
interpolation along the diagonals AB may also be done. Then the contours are drawn through
the point 36m.

b. Graphical Interpolation Method
In this method, actual calculation for interpolation of contours between known heights, is not
done but location of the contours is obtained graphically with the help of a tracing paper or
tracing cloth.

28

LEVELLING

Figure 2.10.5 Graphical Interpolation Method
c. Scale Interpolation Method
The scale position is shown in figure 2.10.6 with the unit 1.70 placed next to the 1.70m high
point. While a line is made from unit 3.6 to the height of 3.60m. In order to get this line, the
scale needs to change so that unit 3.6 in the scale represents the 3.60m high point. Mark the
contours of 2m and 3m by using one set-square and making a parallel motion from the first
line. Positions 2 and 3m indicate the required contour point position.

Figure 2.10.6 Scale Interpolation Method

29