Basic_Surveying.pdf

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UNESCO-NIGERIA TECHNICAL &

VOCATIONAL EDUCATION

REVITALISATION PROJECT-PHASE II

YEAR I- SEMESTER 2

THEORY

Version 1: December 2008

NATIONAL DIPLOMA IN

BUILDING TECHNOLOGY

BASIC PRINCIPLES IN SURVEYING II

COURSE CODE: SUG102


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BASIC PRINCIPLES IN SURVEYING II (TSL 102)

THEORETICAL COURSE INDEX

WEEK 1. 1.0 TACHEOMETRIC SURVEYING

1.1 Introduction to Tacheometry.1

WEEK 2. 1.2 Calibration of Instrument...7

WEEK 3. 1.3 Sources of error in Stadia

Tacheometry....11

1.4 Application of Stadia

Tacheometry....12

WEEK 4. 2.0 THE THEODOLITE

2.1 Theodolites and Uses... .17

2.2 Theodolites Resolutions.. 17

2.3 Basic Components of an optical

Theodolite18

WEEK 5. 3.0 GLOBAL POSITIONNING SYSTEM

3.1 Introduction G.P.S....23

3.2 Space Segment of the G.P.S...............243.3 G.P.S. Positionning Methods.26

WEEK 6. 3.4 How G.P.S. Works..28

WEEK 7. 3.5 G.P.S. Instrumentation..30

3.6 Application of the G.P.S. .32

WEEK 8. 4.0 THE TOTAL STATION

4.1 Introduction to Total Station.394.2 Angle Measurement ..43

4.3 Distance Measurement.43

4.4 Power Supply.45

WEEK 9. 5.0 THEODOLITE TRAVERSING.

5.1 Theodolite Traversing.................46

5.2 Types of Traversing.............46


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5.3 Traverse Specification

and Accuracy .......48

WEEK 10. 5.4 Traverse Field Work ...50

5.5 Station Markings 51

WEEK 11. 6.0 SETTING OUT IN CIVIL ENGINEERING

6.1 Introduction to Setting Out53

6.2 Aims of Setting Out......53

6.3 Stages in Setting Out54

WEEK 12. 6.4 Equipment for Setting

Out Buildings....56

WEEK 13. 6.5 Setting Out a Simple Building

Plan626.6 Setting Out Subsidiary Lines...63

WEEK 14. 7.0 TRIANGULATION AND TRILATRATION

7.1 Triangulation and Trilatration65

7.2 Triangulation and Trilatration Field

Work....67

WEEK 15. 7.3 Distance Measurement..70

7.4 Angle Measurement..70


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WEEK ONE

Introdution toTachoemetry.

1.1 Tacheometry is a branch of surveying where heights and distances are determined

from the instrumental readings alone; these readings are usually taken with a specially

adopted theodolite known as a Tacheometer. Chaining operation is eliminated and

tacheometry is therefore very useful in broken terrain e.g. land cut by ravines, river valley,

over standing crops etc. where direct linear measurement would be difficult and inaccurate.

All that is needed is that the surveying assistant, who carries a levelling staff on which the

tacheometer is sighted shall be able to reach the various points to be surveyed and levelled

and that a clear line of sight exists between the levelling staff and the tacheometer must not

exceed a maximum, beyond which error due to inaccurate reading becomes excessive,

normally, 50m.

The field work in tachometry is rapid compared with direct levelling and measurement and it

is widely used therefore to give contoured plans of areas, especially for reservoir and hydro

electric project tipping site, road and railway reconnaissance, housing sites etc. With

reasonable precautions, the results of tachometry obtained can be of the same order of

accuracy as, or even better than the results obtained by direct measurement in some cases.

In stadia tachometry, a levelling staff is held vertically at one end of the line being measured

and a level or theodolite is set up above or below the other. The staff sighted and readings

taken using lines engraved on the telescope diaphram as shown in figure below. The vertical

angle along the line of sight can be either horizontal or inclined as shown in the below figure.

The vertical compensating system of the theodolite must be in correct adjustment since

vertical angles are read on face only.


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RLx

Figure1.0 Inclined line of sight in stadia tacheometry.

With reference to the above figure;

Horizontal distance P x = D = KS cos2 + C cos (i)

Vertical distance V = K S sin 2 + C sin (ii)

Reduced level of X = RLX= RLP+ hi V m (iii)

Where;

K = the multiplying constant of instrument, usually 100

C = the additive constant of the instrument, usually 0

S = the difference between the two stadia readings

= the vertical angle along the line of sight

hi = the height of trunnion axis above point p

m = the middle staff reading at X

Stadia linesM

X

hi

D

P

S

V


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+v = used if there is an angle of elevation

- v = used if there is an angle of depression

Example 1.

A theodolite having a multiplying constant of 100 and additive constant of 0.00was centred

and levelled at a height of 1.48m above a point C, of reduced level 46.87m.

A levelling staff was held vertically at points D and L in turn and the readings shown in table

1.1 below were taken.

Required:

calculate;

a. The reduced levels of points D and L

b.

The horizontal distances; DCDand DCL

Table 1.0 : Stadia readings.

Staffposition

Staff readings(m)

Vertical circlereadings

Horizontal circlereadings

D 3.240, 3.047, 2.853 02021

07

56

049

31

L 2.458, 2.230, 2.002 020

2136

98

007

18


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SOLUTION

Figure 1.2 tacheomeyric Exercise.

(a) The reduced levels of points D and L are obtained as follows;

From equation (ii)

VCD = KS sin 2 + C sin

= (100) (3.240 2.853) sin 2(02021

07

) + 0

= 50 (0.387) sin (4.703890)

= 1.587m

VCL = KS sin 2 + C sin

= (100) (2.458 2.002) sin 2(02021

36

) + 0

= 50(0.456) sin (4.7200)

= 1.876m

Using equation (iii)

Reduced level at point D = RLD.

RLD = RLC+ hi + VCD m

= 46.87 + 1.48 + 1.587 3.047= 46.89m

RLL = RLC+ hi VCD m

= 46.87 + 1.48 1.876 2.230

= 44.24m

(b) The horizontal distance DL = KS cos2 + C cos

VCL

m C

D

VCD

m

1.48m

L

02021

36

020

21

07

DCL DCD


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From equation (i)

Horizontal distance DCD= 100 (3.240 2.853) cos2(02

021

07

) +0

= 38.7 (0.998315911)

= 38.635m

Horizontal distance = 100 (2.458 2.002) cos2(0202136)

= 45.60 (0.998304362)

= 45.523m

Slope = Difference in RL = 46.89 44.24

Total distance 38.635 + 45.523

= 2.650 = 0.0315

84.158

= tan-10.0315 = 1.8040


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WEEK TWO

1.2 CALIBRATION OF INSTRUMENT.

Figure 2.1 A Theodolite.


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Figure 2.2 Observing through the Instrument.

The tacheometer at every time has two constants namely;

The multiplying constant, denoted by K.

Additive constant denoted by C.

These constant can be determined for any instrument whose either are not known or need to

be recalibrated.

Finding K and C for an instrument

The instrument to be calibrated or recalibrated is centred at a point on a horizontal ground;

the eyepiece focusing is used for stadia measurements of two staff positions placed at

different distances away from the instrument say 100m and 200m with the staff held


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vertically upright. The staff is sighted with the telescope at a horizontal line of sight. The

lower and upper staff readings are carefully read and recorded as illustrated in the figure

below.

Fiigure: 2.3 Calibration of Equipment

We know that horizontal distance = KS cos2 + C cos

Since the line of sight is horizontal, it follows that = 0.This implies that horizontal distance,

D = KS + C . (iv)

Equation (iv) is used to determine the constants for any instrument whose readings are taken

as explained previously.

Example 2.

The table below shows the observations (readings) taken to determine the two constants for

an instrument.

TheodoliteUpper Stadia

reading

Lower Stadia

reading

Upper Stadia

reading

Lower Stadia

reading

100m

200m

Staff


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Table 2.0 : Stadia readings

Required:

Find the values of these constants

Using equation (iv)

D = KS + C

For staff 1, D1= KS1+ C

=> 30 = K (0.3) + C .. (1)

For staff 2, D2= KS2+ C

=> 90 = K (0.9) + C .. (2)

Subtracting equation (1) from (2)

90 = K (0.9) + C

- 30 = K (0.3) + C

60 = 0.6K + 0

K =60

/0.6= 100

K = 100

Horizontal distance

(m)

Reading on staff.

( m)

Lower wire Upper wire

30 1.133 1.433

90 1.452 2.352

Theodolite 1.433

1.13

2.352

1.452

30m

90m

Staff 1Staff 2


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Substituting the value of K into equation (1)

30 = 100 (0.3) + C

30 30 = C

=> C = 0

Hence, the values of these constants are;

K = 100 and C = 0

Questions:

A theodolite is to be used in tacheometric survey to pick the details of up and down stream

features of a new dam. It was tested on known bases as follows;

Table 2.2 Stadia readings.

Horizontal distance

(m)

Vertical angle Staff reading

(m)

Upper Lower

30.00 90000

00

1.433 1.183

89.98

90000

00

2.247 1.542

Required: Find the multiplier and additive constants of the instrumentThe instrument above was used to generate the following data;

Upper stadia reading = 1.330m

Lower stadia reading = 1.100m

What is the horizontal distance between the instrument and the staff?

The instrument height = 1.210m and the reduced level at the instrument position = 39.47m


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WEEK THREE

1.3 SOURCES OF ERROR IN STADIA TACHEOMETRY

The accuracy of basic stadia tacheometry depends on two categories of error; instrumental

and field errors.

(A) INSTRUMENTAL ERRORS

An incorrectly assumed value for K, the multiplying constant, caused by an error in the

construction of the diaphragm of the theodolite or level used.

Errors arising out of the assumption that K and C are fixed when strictly, both K and C arevariable.

The possible errors due to 1 and 2 above limit the overall accuracy to distance measurement

by stadia tachometry to 1 in 1000.

(B) FIELD ERRORS

These can occur from the following sources.

when observing the staff, incorrect readings may be recorded which result in an error in the

staff intercept S. Assuming K = 100, an error of 1mm in the value of S results in an error of

10mm in D.

Since the staff reading accuracy decreases as D increases, the maximum length of a

tachometric sight should be + 50mm.

2. Non verticality of the levelling staff can be a serious source of error. This and

poor accuracy of staff readings form the worst two sources of error.

The error in distance due to the non verticality of the staff is proportional to both the angle

of elevation of the sighting and the length of the sighting. Hence, a large error can be caused

by steep sightings, long sighting or a combination of both. It is therefore advisable not to

exceed = 100for all stadia tachometry.

3. A further source of error is in reading the vertical circle of the theodolite. If the line of

sight is limited to 100, errors arising from this source will be small. Usually, it is

sufficiently accurate to measure the vertical angle 1and although it is possible to improve


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this reading accuracy, it is seldom worth doing so due to the magnitude of all the other errors

previously discussed.

Considering all the sources of error, the overall accuracy expected for distance measurement

is 1 in 500 and the best possible accuracy is only 1 in 1000.

The vertical component V is subject to the same sources of error described above for

distances and the accuracy expected is approximately 50mm.

The precision of stadia tachometry is of paramount importance for best results.

1.4 APPLICATIONS OF STADIA TACHEOMETRY

Vertical staff tacheometry is ideally suited for detail surveying by radiation techniques. This

method of survey is best restricted to the production of contoured site plans since the bestpossible accuracy obtainable is only 1 in 1000 and should not be used to measure distances

where precision better than this are required.

Example 3.

The following readings were taken on a vertical staff with a tacheometer fitted with an

anallatic lens and having a constant of 100.

Table 3.0 : Stadia readings.

Staff

station

Bearing Stadia Reading (m)

Lower middle Upper

Vertical angle

A.

B.

27030

00

20703000

1.000, 1.515, 2.025

1.000, 2.055, 3.110

+80000

00

- 500000

Required: Calculate the relative levels of the ground at A and B, and the mean slope between

the two points (A & B).


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Solution

Figure 3.0 Tacheo metric Exercise.

From Equation (ii)VA= KS Sin 2 + C sin

= (100) (2.025-1.000) Sin 2(8000

00

) + 0

= 50(1.025) sin (16000

00

)

= 14.13m

VB= KS sin 2 + C Sin

= (100) (3.110-1.000) sin 2(50 00I00II)

= 50 (2.110) Sin (100

0000

)

= 18.32m

For horizontal distances HAand HB, equation (i) is applicable.

Hence, HA= Ks cos2 + C cos

= 100 (1.025) cos2(8

000

00

) + 0

= 100.51m

HB = Ks cos2 + C cos

= (2.110) cos2(5

000

00

)

V2

m C

A

V1

m

hi

B

-500000

+800000

H2 H1


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= 209.40m

Let x be the height of instrument above datum;

Level of A = 14.12 + x 1.515

= x + 12.61

Level of B = 18.32 x 2.055

= x 16.27

The difference in level from A and B;

Level of A level of B

= (x + 12.61) (x 16.27)

= x + 12.61 x + 16.27

= 12.61 + 16.27

= 28.89m

The bearings slow that A, B and the instrument lie on a straight line

(207030

00

27

030

00

=18

000

00)

=> Mean slope = Difference in level

HA+ HB

= 28.89

(100.15 + 209.40)

= 28.8

309.91

= 0.093220

= tan-10.093220 = 5.33

0

ASSIGNMENT 1

Q1. Tachometric survey conducted to pick the details of the up and down stream features

of a proposed new dam generated the following data;

Table 3.1 :Stadia readings.

Levelling staff

station

Upper stadia

reading (m)

Middle stadia

reading(m)

Lower stadia

reading (m)

Vertical angle


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A 1.433 1.308 1.183 20015

00

B 2.247 1.849 1.452 - 12030

00

The instrument has a multiplying constant of 100 and an additive constant of 0, whose height

is 1.47m and centred at point x of reduced level 39.85m.

Required:

Calculate the reduced levels of stations A and B.

Determine the horizontal distance from station A to B.

Q2. A theodolite whose height of instrument level is 182.56m has a multiplier constant of

100 and an additive constant of 1.00

If the angle of elevation is 10000

00

and upper, middle and lower stadia readings

are 5.00, 3.50 and 2.00 respectively.

What is the horizontal distance of the staff from the station of instrument if the instrument is

levelled at 1.42m height?

What is the reduced level at the staff?

Q3. Given the following data, determine the two constants for the instrument used and

distance x

Station of

Instrument

Staff distance

(m)

Staff reading (m)

Lower

Stadia

Upper stadia

A 100 3.620 4.610

200 2.980 4.970

X 1.830 2.100


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WEEK FOUR

2.1 THEODOLITES AND USES.

Theodolites are telescopic instruments used basically for measuring both vertical and

horizontal angles. They are also useful in determining horizontal and vertical distances by

stadia prolonging straight lines and low order differential levelling.

Figure 4.0 : Detailed sketch of a Theodolite.

Theodolites are precision instruments used extensively in construction work for measuring

angles in the horizontal and vertical planes.

Many different theodolites are available for measuring angles and they are often classified

according to the smallest reading that can be taken with the instrument known as the

Theodolite Resolution


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2.2 TheodoliteResolution.

This can vary from 1to 0.1and for example, a 1theodolite is one which can be used read

to 1directly without any estimation.

At this point, it is worth noting that a full circle is 3600and a reading system capable of

resolving to 1directly shows the degree of precision in the manufacture of theodolites.

In order to measure horizontal and vertical angles, the theodolite must be centred over a point

using a plumbing device and must be levelled to bring the angle reading systems of the

instrument into appropriate planes.

All types of optical theodolites are similar in construction and the general features of the

SOKKIA TM20H are shown in figures below.

The various parts of a theodolite and their functions are given as follows;

Figure 4.2 : Parts of a Theodolite.

2.3 BASIC COMPONENTS OF AN OPTICAL THEODOLITE

Alidade level


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Transparent tube that contains liquid and an air bubble; it serves as a guide for positioning the

alidade on the vertical axis.

Illumination mirror

Adjustable polished glass surface that reflects light onto the circles so that the angles can be

read.

Leveling head

Platform serving as a support for the theolodite.

Horizontal clamp

Knob that locks the alidade to prevent it from rotating.

Leveling head locking knob

Knob that locks the alidade to the leveling head.

Leveling head level

Transparent tube that contains liquid and an air bubble; it serves as a guide for positioning the

leveling head on the horizontal axis.

Base plate

Plate to which the leveling head is attached by means of three leveling screws.

Leveling screw

Screw that adjusts the theodolites leveling head level on the horizontal plane.

Telescope

Optical instrument composed of several lenses; it can be adjusted in the horizontal and

vertical planes and is used to observe distant objects.

Optical sight


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Device with an eyepiece that precisely aims the telescope at the target whose angles are to be

measured.

Adjustment for horizontal-circle image

Knob that adjusts the sharpness of the image of the horizontal circle (graduated from 0 to

360) in order to read the angles on the horizontal axis.

Micrometer screw

Knob that adjusts the micrometer to give a very precise reading of the circles measurements.

Adjustment for vertical-circle image

Knob that adjusts the sharpness of the image of the vertical circle (graduated from 0 to 360)

in order to read the angles on the vertical axis.

Alidade

Part of the theodolite that rotates on a vertical axle to measure angles by means of the

telescope.

The trivets stage

This forms the base of the instrument and in order to be able to attach the theodolite to the

tripod, most tripods have a clamping screw which locates into a5/8inch threaded centre on

the trivet. This enables the instrument to move on the tripod head and allows the theodolite to

be centred. The trivet also carries the feet of three threaded levelling foot screws.

The tripod

This is used to provide support for the theodolite, the tripod may be telescopic i.e. it has

sliding legs or may have legs of fixed lengths.


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Figure 4.3 A tripod stand.

The Tribrach

This is the body of the instrument carrying all other parts. It has a hollow slightly conical

shape socket into which fits the reminder of the instrument. The tribrach can be levelled

independently of the trivet stage.

The lower plate

This carries the horizontal circle. The term glass arc has been used to describe optical

theodolites because the horizontal and vertical circles on which the angle graduations are

photographically etched are made of glass. Many types of optical theodolite are available,

varying in reading precision from 1to 0.1although 20and 6reading theodolites are most

commonly used in engineering surveying.


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The focusing screw

This is fitted concentrically with the barel of the telescope and diaphram can be illuminated

for night or tunnel wok. When the main telescope is rotated in altitude about the trunnion axis

from one direction to face in the opposite direction, it has been transmitted. The side of the

main telescope, viewed from the eyepiece, containing the vertical circle is called the face.

Standards

This is the frame mounted directly on the cover plate carrying the telescope.

Transit axis or trunnion axis

This axis rests on the limbs of the standard and is securely held in position by a lock nut.

Attached to the transit are the telescope and the vertical circle.

Plate Bubble

When this is levelled, that is at the centre of its run, the line of sight is horizontal.

Optical plummet

This assists the centering of the instrument particularly in windy weather.


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WEEK FIVE

3.1 GLOBAL POSITIONING SYSTEM (G.P.S.)

The global positioning system is one of the available systems used for satellite positioning. It

is also another method of determining horizontal and three dimensional positions for

engineering surveys by processing measurements from artificial Earth satellites.

The development of GPS (also known as NAVSTAR for navigation system using timing and

ranging) began in 1973. Designed primarily for military users, GPS is managed and is under

the control of the U.S. department of Defence. It is developed so that a user at any point on

near the Earth can obtain three dimensional coordinates instantaneously. These fixes can be

taken at any time of the day or night and in the any weather conditions.

The GPS consists of three segments called the space segment, the control segment and user

segment.


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Figure 5.1 : A hand held skygolf-slycaddie-sg4-gps-system


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3.2 Space segment

Figure 52 :SPACE_GPS_NAVSTAR_IIA_IIR_IIF_Constellation.


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Figure 5.3 : A 748px-GPS_Satellite_NASA,

When fully operational, the space segment will consist of 24 satellites all of which will be it

orbits at an altitude of 20200km. at this altitude, each satellite will orbit the Earth every 12

hours and this, together with a suitable choice of orbital plane for each satellite, ensure that at

least four (the minimum needed for a position fix) will be in view at any time.

All satellites in the constellation transmit two L band signals known as L1and L2of

frequencies 1575.42MHZand 1227.60MHZrespectively. L1is modulated with two binary

codes known as the C/A (course acquisition) and p (precise) codes and a data message.

The data message consists of an almanac giving the approximate position of all the satellites

in the system, the satellite ephemeris which contains; precise information about the position

of the host satellite and subsidiary information such as clock corrections and the status of the

system. The L2carrier is modulated with the p code and data message only.

The coarse acquisition code allows access to the standard position service which has an

intended accuracy for single point positioning of the order of 150m and the precise code

allows the precise positioning service which has an intended accuracy of about 15m. By

giving an approximate position, the C/A code is able to help a receiver acquire code for more

precise measurement of position.

Control and user segments


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Five monitor stations form the control segment of global positioning system; a master control

station situated at Colorado Springs and four other stations at Hawaii, Kwajalein, Diego

Garcia, and Ascensio Island. Each station tracks all the GPS satellites and this information is

relayed to the master control station (at Colorado) where it is used to predict future orbits for

all satellites. In addition, the clock on board each satellite is monitored and comparison is

made with the GPS clock at Colorado Springs to enable corrections to be computed to keep

satellite clocks in step with the GPS time. The ephemeris predictions and clock corrections

are loaded to the satellites regularly and the data message transmitted by each satellite

changes every hour. In case problems arise with the tracking network, each satellite stores

sufficient data to be able to predict and transmit orbit data 14days without any update.

The GPS user segment consists of all civil and military users of the system. In addition to

land surveyors, the number of other civilian users of GPS is considerable since it is capable

of dynamic positioning and has applications in hydro graphic surveying, setting out, civil

engineering construction, vehicle navigation and all forms of navigations.

3.3 GPS POSITIONING METHODS

Pseudo ranging: by this method, GPS positioning is carried out with a single receiver

determining Pseudo ranges

Both the C/A and P codes transmitted by each GPS satellites are digital Pseudo random

timing codes. The C/A code has a frequency of 1.023MHZand repeats every 1ms, whereasthe p code has a frequency of 1.023HMZ, but is 38 weeks long. At a frequency of

1.023HMZ, the spacing between binary digits on the C/A code is about 300m and for the p

code, the spacing is about 30m. These spacing (or the frequency of the codes) dictate what

accuracy is possible from Pseudo range measurements with GPS. Since the C/A code

repeats every 1ms, it is easy for a ground receiver to acquire without knowing the pseudo

random sequence.

However, unless a user has prior knowledge of the p code, it is impossible to decode

because of the long pseudo random sequence involved.

When a receiver locks into a satellite, the incoming signal triggers the receiver to generate a

C/A code identical to that produced by the satellite. The replica code generated by the

receiver is then compared with the satellite code in a process known as cross correlation in

which the receiver code is shifted until it is phase with the satellite code. The amount by

which the receiver generated code is shifted is equal to the transit time of the signal between

satellite and receiver, multiplied by the speed of light; this gives the distance between the


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satellite and receiver. If measurements are to be taken using the p code, the receiver repeats

all cross correlation process and the so called hand over word instructs the receiver

which portion of the p code to generate.

All GPS satellites are fitted with very accurate caesium clocks which are all kept

synchronised in GPS time by applying frequent corrections. On the other hand, receiver

clocks are of power quality and are not usually synchronised with GPS time.

Consequently, the receiver generated codes contain a clock error which affects transit times.

Because of this, all ranges measured by receiver will be biased and are called pseudo

ranges.

As well as pseudo range measurement, a receiver will also decode the data message and

from this computes the position of the satellite at the time of measurement.

ii. Carrier phase measurement: Although the equipment and methods used in EDM are

very different to GPS, the GPS receivers also use phase comparison to measure distances to

satellites but this is carried out by comparing the phase of an incoming satellite signal with a

similar signal generated by the receiver. These phase measurements are taken on the L1and

L2carrier waves with a resolution of about 10and because the carriers have very short wave

lengths of 0.19m (L1signal) and 0.24m (L2signal) it is possible to observe a range with

millimetre precision.

At the start of a measurement, a GPS receiver must first remove the codes from the L1

and L2signals so that it can access the carrierwave.


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WEEK SIX

3.4 How GPS Works

Figure 6.1 G.PS. Configuration.

When people talk about "a GPS," they usually mean a GPS receiver. The Global Positioning

System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and

three extras in case one fails). The U.S. military developed and implemented this satellite

network as a military navigation system, but soon opened it up to everybody else.

Each of these 3,000- to 4,000-pound solar-powered satellites circles the globe at about 12,000

miles (19,300 km), making two complete rotations every day. The orbits are arranged so that

at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.,


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A GPS receiver's job is to locate four or more of these satellites, figure out the distance to

each, and use this information to deduce its own location. This operation is based on a simple

mathematical principle called trilateration. GPS receiver calculates its position on earth based

on the information it receives from four located satellites. This system works pretty well, but

inaccuracies do pop up. For one thing, this method assumes the radio signals will make their

way through the atmosphere at a consistent speed (the speed of light). In fact, the Earth's

atmosphere slows the electromagnetic energy down somewhat, particularly as it goes through

the ionosphere and troposphere. The delay varies depending on where you are on Earth,

which means it's difficult to accurately factor this into the distance calculations. Problems can

also occur when radio signals bounce off large objects, such as skyscrapers, giving a receiver

the impression that a satellite is farther away than it actually is. On top of all that, satellites

sometimes just send out bad almanac data, misreporting their own position.

Differential GPS (DGPS) helps correct these errors. The basic idea is to gauge GPS

inaccuracy at a stationary receiver station with a known location. Since the DGPS hardware

at the station already knows its own position, it can easily calculate its receiver's inaccuracy.

The station then broadcasts a radio signal to all DGPS-equipped receivers in the area,

providing signal correction information for that area. In general, access to this correction

information makes DGPS receivers much more accurate than ordinary receivers.


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WEEK SEVEN

3.5 GPS INSTRUMENTATION

Figure 7.1 G.P.S. Instrumentation.

*** The weld GPS system 200 from Leica has a 9 - channel dual frequency

receiver which means that it can track 9 satellites simultaneously and can take measurements

on both L1and L2signals. It uses a reconstructed carrier in phase measurements but should

the p code becomes encrypted, it can switch to the signal squaring method. The system 200

supports all the measurement modes used for precise GPS surveying and with their SKI post

processing soft ware, the accuracy quoted by Leica for baseline measurements is 5mm x

1ppm of the baseline length. For single point positioning with pseudo ranges the accuracy

is 15m subject to SA.


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*** The 400 SSE Geodetic surveyors from Trimble navigation is also a dual

frequency of 9 - channel receiver. Normally, it uses p codes measurements on both L1and

L2frequencies ambiguity resolution but during periods of p code encryption the receiver

measures the cross correlation of the encrypted by either reconstructing the original carrier

or by using a signal squaring technique. In order to be able to reconstruct the original

carrier, an exact knowledge of pseudo random binary codes (usually the p code) is

required.

Squaring techniques, on the other hand, require no knowledge of codes (this is known as the

codeless approach) and give a carrier with codes eliminated at twice the original frequency.

Because of this, the squaring technique is capable of being more accurate since phase

measurements are taken at half the original wavelength. Unfortunately this method suffers the

disadvantage that the squaring process destroys the data message and an external ephemeris

must be used to obtain satellite positions.

P Codes in conjunction with the C/A codes instead. This combination of observables

according to the manufacturer, provides faster ambiguity resolution than squaring techniques

when used for static positioning, the 4000 SSE has a quoted accuracy of 5mm + 1ppm times

the baseline length and when used in the various kinematic surveying mode sit has a quoted

accuracy of 200mm + 1ppm of the base-line length.Data processing for the 400 SSE is

carried out with a software package known as GPSurvey.

*** The Ashtech Z 12 is a 12 channel GPS receiver that uses the p code on both L1

and L2frequencies and the C/A code to obtain carrier phase and pseudo range

measurements. These are all combined to resolve carrier phase ambiguities when anti

spoofing (AS) is turned on, the instrument automatically activates its Z Tracking mode

which enable the cancellation of the effects of AS. The Z 12 has an accuracy quoted in

millimetres, the exact figure depending on observation times and operating mode.


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3.6 APPLICATIONS OF GPS

The GPS is a rather complex system that can be used in so many ways. For basic point

positioning and navigation, hand held receivers with an accuracy at the 100m level have

found wide spread use while at the other end of the GPS spectrum, geodetic receivers with a

computer and post processing software are now starting to be used for routine survey work

at the centimetre level.

Although the accuracy of GPS is important, some surveyors feel that the main advantage of

the instrument compared with conventional surveys is that it can be used in any weather

condition day or night. This enables GPS surveying to be carried out over extended periods at

any time of the year without restrictions such as rain, fog and poor visibility delaying work.

Another advantage when surveying with GPS is that inter visibility between stations orpoints surveyed is not necessary. This allows control stations to be placed where convenient

and not at locations which may be difficult to get to in order to establish lines of sight.

At the moment, the full potentials of GPS has not been realised even though the accuracy

required for engineering surveys can be achieved. One of the reasons for this is the cost of

GPS surveying which can be uneconomical compared with conventional surveying. These

high costs are caused by firstly the receivers which are between five and ten times more

expensive than total stations and secondly the fact that GPS is not fully kinematics and there

are problems with satellite coverage, both of which can result in long occupation times.

Added to these, there are difficulties in defining heights above survey datums such as mean

sea level and with real time data processing and control.

Despite these draw backs, GPS has been very successfully used for control surveys where it

has joined traversing triangulation and trilateration as a method for coordinating stations in a

network.

The best application identified so far for GPS have been for improving existing national

control networks and for surveys in remote areas. GPS is also used on engineering projects

that extend over large areas, especially where a high degree of precision is required, e.g. in a

tunnel network surveying.

Another application where GPS has been successfully utilised in engineering surveying is in

providing control for a number of major route location and highway maintenance schemes. In

these examples, GPS provided what is known as the primary control or points with height

precision spread out over relatively long distances. These were used as reference points for


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providing further control, for example link traversing was carried out between the GPS

reference points using total station or combined theodolite and EDM systems. This may well

be the best use for GPS in future where it is integrated with other methods of surveying rather

than trying to compare with them.

As far as detail survey and setting out are concerned, GPS is not used extensively in Civil

Engineering and construction as it can not compete with conventional large scale surveying

systems at present, particularly regarding costs. However, the possible applications in

engineering surveying f0r low cost, small size GPS black box capable of high precision,

real time surveys are enormous. Such surveying system would be integrated with or even

replace existing methods for control surveys, detail surveys and setting out and would

completely change surveying as it is known today. Much research is being carried out to

achieve this and developments in receiver technology and associated software.


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WEEK EIGHT

4.1 INTRODUCTION TOTOTAL STATIONS

The total station otherwise known as electronic tachometer is an instrument used in surveying

which is capable of measuring angles and distances electronically. Just as is common withother electronic surveying instruments, the total station is operated using a multi function

key board which is connected to a micro processor built into the instrument. The micro

processor not only controls both the angle and distance measuring systems but also used as a

small computer that can electronically calculate slope corrections, vertical components, and

rectangular co ordinates and in some cases, can also store observations directly using an

internal memory.

Figure 8.1: A Detailed sketch of the Total Station.

Below is a figure showing the NIKON DTM A 5LG, SOKKIA SET3C and the Zeiss Elta 5,

a sample of total station from the extensive range now available.


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Figure 8.2 : A KTS-442R-445R-Total-Station.


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Figure 8.3 : A 76e%20TotalStation


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Figure 8.4 : A TOTAL STATION

The table below shows the technical specifications in summary of the types given

Table 8.1: Specifications of some examples of total station.


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Instrument Type. Nikkon DMT

A5LG

SOKKIA SET 3C ZEISS ELTA 5

Angle measurement

- H accuracy

- V accuracy

2

2

3

3

5

5

Distance measurement

- to one prism

- to three prisms

Accuracy

2.3km

3.1km

(2mm + 2ppm)

2.2km

2.9km

(3mm + 3ppm)

1.0km

1.5km

(5mm + 3ppm)

Measurement time 3.0 seconds 3.2 seconds 3.4 seconds

Data displayed

- H and V angles

- SD, HD and VD

- X, Y and Z co ords

- setting out data

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Data recording Data recorder field

computer

Data recorder field

computer memory

card

Data recorder field

computer

Compensator battery Singleaxis Ni

cad 7.2v

Dual axis Ni cad

6.0v

Single axis Ni cad

4.8v

4.2 Angle measurement

This exercise is done using an electronic theodolite .All features associated with the

electronic theodolite (a theodolite that produces a digital output of direction or angle) is

appliance to all total stations.

Typically, a total station can record angles with resolution of between 1and 20

and all

instruments incorporate some form of compensator, the more expensive using dual axis and

the less sophisticated, single axis compensator.


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4.3 Distance measurement

Currently, most total stations use a Ga As infra red carrier source and phase comparison

techniques in order to measure distances. However, compared to theodolite mounted systems

nearly all total stations use coaxial optics in which the EDM transmitter and receiver are

combined with the theodolite telescope. This makes the instrument much more compact and

easier to use on site. Normally a total station will measure a slope distance and the micro

processor uses the vertical angle recorded by the theodolite along the line of sight (line of

distance measurement) to calculate the horizontal distance. In addition, the height difference

between the trunnion axis and prism centre is also calculated and displayed. All instruments

use some form of signal attenuations to protect the receiver.

Three modes are usually available for distance measurement namely;

Standard (or coarse) mode: This has a resolution of 1mm and a measurement time of 1 2

seconds.

Precise (or refine) mode: This has a resolution of 1mm but a measurement time of 3 4

seconds. This is more accurate than the standard mode since the instrument repeats the

measurement and refines the arithmetic mean value.

Tracking (or fast): Mode in which the distance measurement is automatically replaced at

intervals of less than one second. Normally, this mode has a resolution of 10m and is used

extensively when setting out since readings are updated very quickly and vary in response to

movements of the prism which is usually pole mounted.The range of a total station is typically 1 3km to a single prism assuming visibility is

good and up to a range of 500m which covers 90 percents of the distance measured on site,

the precision of a typical total station is about 5mm. Most instruments allow for the input of

temperature and pressure which enables the distance readings to be automatically corrected

for atmospheric effects. Also, any value of prism constant can be entered into the instrument

via the alpha numeric key board.

If a code is entered from the key board to define the feature being observed, the data can be

processed much more quickly by downloading it into approximate software.

On numeric key boards codes are represented by numbers and/or letters which give greater

versatility and scope. The alpha- numeric control panel of the Topcon GTS 6 is shown in

figure below.


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Figure 8.5 A sketch of the Alpha Numeric key board of the Total Station.

4.4 Power supply

Rechargeable nickel cadmium (Nicad) batteries are now standard for surveying instruments

and these are connected directly to the total station without using cables. For angle and

distance measurements, between two and ten hours use can be obtained from a battery,

depending on the instrument. Most total stations are capable of giving a battery power

indication and some have an auto save feature which switches the instrument off or into

some standby mode after it has not been used for a specified time. It is a good practice, no

matter what assurances a manufacturer may give about the life of a battery to have a fully

charged spare with the instrument at all times.

The micro processor of the total station apart from controlling the angle and distance

function, it is also programmed to perform coordinate and other calculations.

Even though a total station can perform many of the calculations often done manually on site,

this does not mean that the surveyor or engineer should lose this ability. Thus, this

opportunity should be seen as a method and not a substitute for other surveying principles.

GTS-6

REC BAT MENU

F1 F2 F3 F4 F5 F6

DISPLY SREEN


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WEEK NINE

5.1 THEODOLITE TRAVERSING

A traverse is a continuous framework of lines connecting a number of points, lengths of the

lines and their angular relationship to each other being measured. The lines are known as legs

and the points as stations.

A traverse is a means of providing horizontal control in which rectangular coordinates are

determined from a combination of angle and distance measurements along lines joining

adjacent stations.

USES

Traverse surveys are used where site conditions make the chain triangulation method

impossible, i.e. a wood, built up factor blocks, long winding river or where the survey is of

large area and details are required.

The main purpose of theodolite traversing is to establish the bearing and lengths of a series of

adjoining lines which together form the framework for the survey of a particular area. The

bearings and distances are then plotted with protractor or by triangular co ordinate.

5.2 TYPES OF TRAVERSING

We have two major types;Open traverse

Close traverse

OPEN TRAVERSE

A traverse whose starting and finishing stations do not coincide or are not both fixed or

known is called an open traverse. This type of traverse is used to survey rivers, roads or

railway routes.

Open traverse commences at a known point and finishes at unknown point or station and

therefore are not close. Since open traverse are used only in exceptional circumstances, there

is no external check on the measurement.


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CLOSE TRAVERSE

When a framework form a close figure (or when the traverse connects station which position

are known) it is known as a closed traverse, such a traverse is easily checked, as a surveying

start and finishes at a fixed point or points.

In the figure below, a traverse has been run from station A (of known position) to stations 1,

2, 3 and another known position B. Traverse A ,1, 2, 3,B is therefore; closed at B. This type

of traverse is called a link, connecting or close route traverse.

Figure 9.1 :Closed Traverse Network.

The figure shown below is a framework of a closed traverse known as a polygon traverse it

started and ended at a common point x.

Figure 9.2 : Closed Traverse Network.

A

1

2

3

B

N

X


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5.3 TRAVERSE SPECIFICATION AND ACCURACY

The accuracy of a traverse is governed largely by the type of equipment used and the

observing and measuring techniques employed. These are basically dictated by the purpose of

the survey work.

Many types of traverse are possible but three broad groups can be defined and are given in

table 2.1 below.

The most common type of traverse for general engineering work and site surveys would be of

typical accuracy 1 in 10, 000.

An important factor when selecting traverse equipment is that the various instruments should

produce roughly the same order to precision, that is, it is pointless using a 1theodolite to

measure traverse angles if the lengths are being measured with a synthetic tape.

Table 9.1 : General traverse specifications

Type Typical Purpose Angular

measurement

Distance

measurement

Geodetic or

precise

1 in 50, 000 or

better

1. Major control

for mapping

large circles

2. Provision of

very accurate

reference points

for engineering

surveys.

0.1 EDM

General 1 in 5, 000 to 1

in 50, 000

1. General

engineering

survey that is,

setting out and

site surveys

2. Secondary

control for

mapping large

areas

1or 2

theodolite

EDM, steel taps


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Low accuracy 1 in 500 to 1 in

5, 000

1. Small scale

detail surveys.

2. Rough large

scale detail

surveys.

3. preliminary

or

reconnaissance

surveys

20or 1

theodolite

Synthetic tapes,

stadia

tacheometry


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WEEK TEN

5.4 Traversing field work: Reconnaissance

This is one of the most important aspects of any survey and must always be undertaken

before any angles or lengths are measured. The main aim of the reconnaissance is to locate

suitable positions for traverse stations and a poorly executed reconnaissance can result in

difficulties at later stages in a survey leading to waste of time and inaccurate work.

To start a reconnaissance, an over all picture of the area us obtained by walking all over the

site keeping in mind the requirements of the survey. If an existing map or plan of the area is

available, this is a useful aid at this stage.

When sitting station, an attempt should be made to keep the number of stations to a minimum

and the lengths of traverse legs should be kept as long as possible to minimise the effect of

any centring errors.

If the traverse is being run for a detail survey then the method which is to be used for the

subsequent operation must be considered. For most sites a polygon traverse is usually sited

around the area at points of maximum visibility. It should be possible to observe across

checks or lines across the area to enable other points inside the area to be fixed and also to

assist in the location for angular errors. Traverse for read works and pipelines generally

require a link traverse. Since these sites tend to be long and narrow. The shape of the read or

pipe line dictation the shape of the traverse.

If distance measurements are to be carried out using tapes the ground conditions between

stations should be suitable for this purpose, steep slopes or badly broken ground along the

traverse lines should be avoided and it is better if there are as few changes of slopes as

possible. Roads and paths that have been surfaced are usually good for ground measurements.

Stations should be located such that they are clearly inter visible, preferably at ground level,

that is, with a theodolite set up at one point, it should be possible to see the ground marks at

adjacent stations and as many others as possible. This eases the angular measurement process

and enhances its accuracy.

Stations should be placed in firm level ground so that the theodolite and tripod are supported

adequately when observing angles at the stations. Very often stations are used for a sited

survey and at later stage for setting out. Since some time elapse between the site survey and

the start of the construction, the choice of firm ground in order to prevent the stations moving

in any way becomes even more important. It is sometimes necessary to install semi

permanent stations.


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Owing to the effects of lateral refraction and shimmer traverse ,lines of sight should be well

above ground level (greater than 1m) for most of their length to avoid any possible angular

errors due to rays passing close to ground level (grazing rays). These effects are serious in hot

weather.

When are stations have been sited a sketch of the traverse should be prepared approximately

to scale. The stations are given reference letters or numbers. This greatly assists in the

planning and checking of fieldwork.

5.5 STATION MARKING

When a reconnaissance is completed, the stations have to be marked for the duration or

longer of the survey. Station markers must be permanent, not easily disturbed and they should

be clearly visible. The construction and type of station depends on the requirements of the

survey.

For general purpose traverse, wooden pegs are used which are hammered into the ground

until the top of the peg is almost flushed with the ground level (see figure below).

300-500mm

Figure 10.1 ; station peg.

If it is not possible to drive the whole length of the peg into hard ground the excess above the

ground should be sawn off. This is necessary since a long length of peg left above the ground

is liable to be knocked down. A nail should be tapped into the top of the peg to define the

exact position of the station as shown in figure above.

Stations in roadways can be marked with 75mm pipe nail driven flush with the surface. The

nail surround should be painted for easy identification. These marks are fairly permanent, but

it is usually prudent to enquire if the road is to be resurfaced in the near future.

A more permanent station would be normally required on marks set in concrete, typical

station designs are shown in the figure below.

50mm square

wooden peg

Nail


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A reference or witnessing sketch of the features surrounding each stations should be

prepared, especially if the stations are to be left for any time before being used, or if they will

be required again at a much later stage.

Measurements are taken from the station to nearby permanent features to enable it to be

relocated. A typical sketch is shown in figure below.

Metal

post

Station

14

(Iron Bar in Concrete)

Building

Man hole

3.4m 3.8m

N

Figure 10.2: Witnessing Sketch


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WEEK ELEVEN

6.1 SETTING OUT

The International Organization for Standardization (ISO) in their publication ISO/DP 7078

Building construction defines setting out as the establishment of the marks and lines to define

the positions and levels of the elements for the construction work so that works may proceed

with reference of them.

Setting out can also be defined as the process whereby the positions and levels of new works

already recorded on a working plan are transferred to the ground.

A definition often used for setting out is that, it is the reverse of surveying. This definition

means that whereas surveying is the process of producing a plan or map of a particular area

or site, setting out begins with the plan and ends with the various elements of a particular

engineering project correctly positioned on the ground in the area.

However, as in surveying, setting out must be arranged so that the work at hand can be

properly checked. Every peg placed must be proved to be in its correct position as provided

in the plan within allowable limits.

6.2 AIMS OF SETTING OUT

There are two main aims when undertaking setting out operations;

The various elements of the scheme or work must be correct in all three dimensions both

relatively and absolutely, that is each element must be in its correct size, in its correct

position and at its correct deduced level.

Once setting out begins, it must proceed quickly and with little or no delay in order that the

works can proceed smoothly and the costs can be minimized.

In practice, there are many techniques, which can be used to achieve these two aims.

However, they are all based on three general principles

Points of knownE, Ncoordinates must be established within or near the site from which thedesign point can be set out in their correct plan positions. This involves horizontal control

techniques

Points of known elevation relative to an agreed datum are required within or near the site

from which the design points can be set out at their correct reduced levels. This involves

vertical control technique.


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Accurate methods must be adopted to establish design points from these horizontal and

vertical controls. This involves positioning techniques

6.3 STAGES IN SETTING OUT.

As the work proceeds, the setting out falls into two broad stages;

Initially, techniques are required to define the site, to set out the foundations and to monitor

their construction. Once this has been done, emphasis changes to the above ground

elements of the scheme and methods must be adopted which will ensure that they are fixed at

their correct levels and positions. These two stages are explained as follows

FIRST STAGE SETTING OUT.

The first stage when setting out any work is to locate the boundaries of the works in their

correct position on the ground surface and to define the major elements. In order to achieve

this, horizontal and vertical control points must be established on or near the site as explained

earlier. These are then used not only to define the perimeter of the site which enables fences

to be erected and site clearance to begin but also to set out critical design points on the

scheme and to define slopes, directions and so on. For example; in a structural project, the

main corners and sites of the building will be located and the required depths of dig to

foundation level will be defined. In a road project, the centre line and the extent of the

embankments and cuttings will be established together with their required slopes.

When the boundaries and major elements have been pegged out, the top soil is stripped off

and excavation work begins. During this period, it may be necessary to relocate any peg(s)

that are accidentally disturbed by the plant and equipment. Once the formation level is

reached, the foundations are laid in accordance with the drawings and the critical design

points located earlier.

Setting out techniques is used to check that the foundations are in their correct three

dimensional positions. The first stage ends once construction to ground floor level, sub base

level or similar levels has been completed.


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WEEK TWELVE

SECOND STAGE SETTING OUT.

This continues from the first stage, beginning at the ground floor slab, road sub base level

or similar levels. Up to this point, all the controls will still be outside the main construction,

for example, the pegs defining building corners, centre lines and so on will have been

knocked out during the earth moving work and only the original control will be

undisturbed. Some off set pegs may remain but these too will be set back from the actual

construction itself.

The purpose of second stage setting out therefore is to transfer the horizontal and vertical

controls used in the first stage into the various elements of the scheme.

6.4 EQUIPMENT FOR SETTING OUT OF BUILDING.

The nature and complexity of the building or any engineering work like; bridges, dams, roads

etc. determine the accuracy that need to be achieved, which in turns, defines or determines

which types of equipment will be selected for the task e.g. dumpy level or theodolite, fibre

glass tape or steel tape, plumb bob and line or optical plummet etc.


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Figure 12.1 : A MEASURING TAPE.

Figure 12.2 : A STEEL MEASURING TAPE.

Figure 12.3 : A DIGITAL MEASURING TAPE.

PEGS

:These are usually made of two materials;


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Figure 12.4 : Wooden Pegs.

i.

Timber pegs 50mm section of variable length but having a pointed end to

facilitated driving into the ground by hammering. A timber peg may have a nail

fixed to its top at the centre to locate exactly the station point. All setting out pegs

should be clearly marked with a 50mm deep and red paint and should have a

board of blue paint.

ii.

Steel pegs they are usually formed from lengths of steel reinforcement rods, cut to

a suitable lengths and may have one edge sharpened to facilitate careful driving

positions have been checked, they are normally surrounded by concrete.

Identification works may be made into the surface of the concrete before it sets

hard

Lines:

They are strings, wire, nylon etc. the weather condition plays a very vital role in selecting

which material to be used so that the line is safe from damage, stretch, sag in prevailing

working conditions of the weather. The lines provides straight out lines from a peg to

another. They define straight lines from points or stations.

PROFILE BOARDS:

These are used in conjunction with pegs so that extended lines positions may be marked by

using profile boards, the string or wire lines can be removed in the knowledge that when they

are required again, they can be positioned exactly as they were originally. Normally, a profile

boards is erected near each off set peg and used in exactly the same way as a sight rail, a

traveler are being used between profile boards to monitor excavation


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FIGURE 12.5 : SHOWING PROFILE BOARD

SITE SQUARE:

This is an optical device used for setting out right angles whereby unskilled labour can attain

an accuracy of 5mm in 30m.

The instrument is basically of two telescopes mounted one above the other and with their

lines of sight set at 900to each other. The site square is supported on a tripod stand, which

can be set over a fixed mark on the ground. The lower telescope is aimed along the line from

which the right angle is to be established being brought to bear on any site mark in the line bymoving the telescope:

In the vertical plane

Laterally by means of a fine turning screw. Once the adjustment of the lower telescope is

complete, the upper telescope will trace out a line at right angles to the original line and a

further site mark can be positioned as required by moving this telescope in the vertical plane

only.


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Figure 12.6 : Travelers used in setting out slopping ground


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WEEK THIRTEEN

6.5 SETTING OUT OF BUILDING

Setting out the base line: The base line adopted in setting out a building is usually the

building line, although on extensive factory layouts are centre lines of buildings are

sometimes runs of machinery. In either case, the location of such lines is reacted to the

physical features of the site. The building line is the line of the front face of the building as

indicated in figure below (line AB).

Figure 13.1 : Setting out a simple building plan.

The position of the building line may be defined on the working plan by measurements from

any of the following;

The property boundary

The edge of the road kerb

The centre line of the road.

It is important to note that

Proposed building (structure)EXISTING

BUILDING

EXISTING

BUILDING


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Where there is no indication of the building line, its position must be agreed on site with the

local authority-building inspectors.

Where there is an obvious line of existing building frontages, this line is usually adopted as

the building line.

The building line is first ranged by eye and pegs are placed at the two front corners of the

outer face of the proposed building. Critical measurements are made from the boundary to the

building corners as shown in figure above or defined by local regulations and along the face

of the proposed building by nails hammed into the pegs.

6.6 B. Setting out the subsidiary lines:

From the two front pegs, A and B, angles are set out in accordance with the building plan to

follow the outer face of the flank walls. This could be done with a theodolite, setting up over

each peg in turn and turning off the required angle from the building line in each case. As the

angle of the flank wall is most often 900this could be set out without a theodolite using the

following;

a. A 3:4:5 taped triangle

b. A builders square, which is a 3:4:5-ratio triangle made out of timber

c. An optical square

d. A site square, which is a proprietary instrument consisting of two small telescopes

fixed rigidly at right angles on a small stand.

e. A level incorporating a horizontal circle like a theodolite, but reading by venire toabout 5only. When the two rear pegs; C and D are placed and nail marked, they are

checked by measuring between them and by measuring the diagonals.

In a rectangle building, the two diagonals must be equal to prove the positioning of

the pegs.

After the main outline has been pegged ,any minor extensions or returns from the

main figure are pegged and checked, such as the pegs at e, f, g, h, j and k, when the complete

outline of the outer face of the building has been pegged and checked.

Setting out the reference marks: The pegs now placed will be destroyed as the

foundations are excavated and the reference system must be adopted. This can be achieved by

the use of profile boards, illustrated in the above figure.

Profile boards are constructed of 150 or 200mm by 25mm, boards supported on 50mm

square posts hammered firmly into the ground, well clear of the working area. On well

organized sites, the boards are placed at one level, usually finished floor level or dam proof

course level. The advantages of these are as follows;


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They help to keep the tape horizontal when making measurements.

They provide a level datum around the site so that less check leveling is needed subsequently.

Disturbance of the boards can easily be noted visually.

Levels of work below ground can be controlled by travelers using the profile boards as sight

rails.

Approximate levels can be obtained by direct vertical measurements up or down from lines

strung between the profile boards.

Once all boards have been placed in position, all at one level, lines are strung between

them and positioned vertically above the nail markers defining the building outline. If the line

is some distance above the peg, the peg position must be plumbed upward, using plumb line

in reference to the less accurate brick layers spirit level. When the lines have been accurately

strung across the profile boards, positions are marked with a nail or saw cut so that they

may be replaced at any time. The intersections of the various strung lines will then define the

peg positions when they are removed for excavations. Profile boards for minor buildings or

projections are not always erected or needed. Full foundation width is marked on the boards

and two lines strung between these points to define the width of the foundation trench to be

dug. Once the trench has been started, the lines are removed.

Alternatively, the lines may be temporarily defined along the ground by means of

strips of lime or sand to guide excavation of the trench.


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WEEK FOURTEEN

7.1 TRIANGULATION AND TRILATERATION.

Like traversing, triangulation and trilateration are surveying methods used to locate

control points or stations which form a network.

A triangulation network consists of a series of single or overlapping triangles as

shown below, the points (vertices) of each triangle forming control stations. Position is

determined by measuring all the angles in the network and by measuring the length of one or

more base lines such as XY or IB, with the base line, application of the Sine Rule in each

triangle throughout the network enables the lengths of all triangle sides to be calculated.

These lengths when combined with the measured angles enable the coordinates of the stations

to be computed.

Figure 14.1 : Triangulation Network.


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Figure 14.2 A map.

7.2 TRIANGULATION AND TRILATERATION: FIELDWORK.

The methods that can be used to establish and observe a combined network vary

considerably with its size and it is emphasized the following sections are concerned solely

with civil engineering and construction sites where distances between control stations seldom

exceed 1km.


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Figure 14 3:TRIANGULATION NETWORK

A trilateration network is a series of singles or overlapping triangles but in this case,

position is determined by measuring all the distances in the network instead of the angles.

However, to enable station coordinate to be computed, the measured distances are combined

with angle values derived from the side length s of each triangle.

Until the advent of EDM, the measurement of distances in a trilateration scheme with

sufficient accuracy was a very difficult and time consuming process and because of this

trilateration techniques were seldom used for establishing horizontal control. Traversing

techniques were also limited since it was not possible

to maintain a uniformly high accuracy when traversing over long distances. As a result

of that, triangulation was used extensively in the past to provide control for survey covering

large areas.

However, nowadays, because of the high precision, and accuracy of modern

equipment, traversing, triangulation and trilateration can all be used as methods of

establishing horizontal control. Although traversing is the most popular method for providing

control on site, combined triangulation and trilateration is often used; this involves the


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measurements of angles and distances through out a network rather than between selected

stations as in traversing. On construction sites, combined network are used where horizontal

control is required to be spread over large areas and they are also used to provide reference

points for control extension, for monitoring and for precise engineering work.

The reconnaissance for a network is the most important aspect of the survey and is

carried out to determine the positions of the control stations. Since this is linked to the size

and shape of the figure to be used in the scheme and to the number of measurements to be

taken, the reconnaissance will determine the amount of fieldwork that will have to be

undertaken.

To start the reconnaissance, information relevant to the survey area should e gathered,

especially that relating to any previous survey. Such information may include existing maps,

aerial photographs and any site surveys already prepared for the construction project.

From this information, a network diagram should be prepared, approximately to scale,

showing proposed locations for the stations. It is also important that the survey area is visited,

at which time the final positions for the stations are chosen.

Many guidelines for reconnaissance when traversing are also applicable here, but

particular attention must be paid to the establishing the station points, the layout of stations in

relation to the survey work and the precision and reliability of the network must be assessed.

Based on the reconnaissance, decisions regarding the measurements to be

taken are made and the instruments to be used for the survey are specified. Most

importantly, a check should be made to ensure that the survey meets its specifications


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WEEK FIFTEEN

7.3 DISTANCE MEASUREMENT.

During the observation of a network, the lengths of as of the triangles sides as possible are

measured using some sort of EDM equipment. When using the EDM equipment, the

meteorological conditions at the time of measurements must be monitored carefully and

suitable corrections made; also, any systematic instrument errors present in the equipment

must be allowed for by careful calibration of the of the equipment. For National Grid based

surveys, the scale factor is applied to each measured distance and, if the distance has been

measured at an appreciable elevation, a height correction must be applied since mean sea

level is the datum height for the National Grid.

7.4 ANGLE MEASUREMENT.

The instrument normally required for measurement of the angles in networks is a 0.1/0.2

or 1 double reading optical micrometer theodolite or an electronic theodolite of similar

precision.

The theodolite is set up and angles are observed and booked. Very often, a total station or

theodolite mounted EDM system is used to observe a network and distances and angles are

measured simultaneously at each station.


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Figure 15.1 :Distance measuring Instrument.

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