VISUAL WALKTHROUGH
Chapter
1
Fundamental
Concepts
INTRODUCTION
Each chapter starts with
an Introduction giving an
overview of the chapter as
well as its scope.
Surveying is one of the oldest arts practised by man. History reveals that the principles
and practices of surveying were used, consciously or unconsciously, even in the primitive
ages, albeit in a crude manner. In the past few decades, however, these have become
more rational and channelised.
The introduction and practice of surveying is indispensable to all branches of engineering. The training that a student receives, irrespective of his branch of engineering,
in the art of observing, recording, and computing data, as well as in the study of errors
their causes and effects, directly contribute to his success in other professional courses.
He develops inter alia such qualities as self-reliance, initiative and the ability to get along
with the others. This also helps an engineer get acquainted with the reasonable limits
of accuracy and the value of significant figures. A knowledge of the limits of accuracy
can best be obtained by making measurements with the surveying equipment employed
in practice, as these measurements provide a true concept of the theory of errors. An
engineer must also know when to work to thousandths, hundredths or tenths of a metre
and what precision in field data is necessary to justify carrying out computations to the
desired number of decimal place. With experience, he learns how the funds, equipments, time, and personnel available will govern the procedure and the results. Taking
field notes under all sorts of field conditions trains a person to become an excellent
engineer, capable of exercising independent judgements.
Surveying is of special importance and interest to a civil engineer. Surveys are
required prior to and during the planning and construction of buildings, dams, highways,
railways, bridges, canals, tunnels, drainage works, water supply and sewerage systems,
etc. They may also be required for planning and construction of factories, assembly
lines, jigs, fabrications, missile ranges, launch sites, and mine shafts. Surveying is the
starting point for any project or constructional scheme under consideration. Details of
the proposed work are plotted from the field notes. The reliability of the estimation of
quantities and the effectiveness of the design depends upon the precision and thoroughness exercised during the survey.
Today, the art of surveying has become an important profession. An introduction to
the principles and practices of surveying is, therefore, desirable as an integral part of
engineering education and training, irrespective of the branch of specialization. A knowledge of surveying trains the ability of engineers to visualize, think logically and pursue
the engineering approach. It promotes a feeling of confidence, a habit of working in
Theodolite 115
4.1.2
Non-transit Theodolite
A theodolite is said to be a non-transit one when its telescope cannot be revolved
through 180° in a vertical plane about its horizontal axis. Such theodolites are
obsolete nowadays. Examples are the Y-theodolite and Everest theodolite.
Point at which observations
are reduced
Vertical axis
Transverse axis
90∞
90∞
Vertical circle rigidly
fixed to the telescope
in face left
Line of sight
Figures have been used to
describe the instrument
details for visualization in
simplified ways.
Alidade
assembly
Horizontal
circle assembly
Levelling head
assembly
Fig. 4.2
The three assemblies of a theodolite
CONSTRUCTION DETAILS
4.2
A line diagram of a transit theodolite is shown in Fig. 4.2. It consists of alidade
assembly at the top, the horizontal circle assembly in the middle and the levelling
head assembly at the bottom.
4.2.1
Alidade Assembly
It is the top-most assembly which includes a telescope supported by two standards of the shape of letter A forming an U-frame (which rests on the horizontal
398 Surveying
a reference station. When the values of the horizontal angles and distances are
known, these are entered into the total-station. The setting-out mode is activated
by pressing the appropriate key. The difference between the entered and measured
horizontal angle values (dHA) is displayed. The telescope is rotated until a difference of zero is displayed. Following this, a pole-mounted prism is located on
the line of sight as near to the required distance as possible. After the alignment,
the prism is sighted and the distance to it is measured by the total-station. The
difference between the measured and entered distances is displayed. By moving
the prism, this difference is reduced to zero to locate the point.
When the coordinates of the point to be set out are known, these coordinates
are entered into the total-station after orientation. The setting-out mode is selected
and the difference between the calculated and measured bearings is displayed. The
telescope is rotated until the difference is zero, such that it points in the required
direction. After the alignment, the prism-mounted pole is used and moved in a
way similar to the previous case for horizontal distance to set out the point.
ELECTRONIC DATA RECORDING
10.4
Initially, plotting detail surveying was done manually. Later, with the advent of
computers, the practice was to key in the data recorded in the field books into the
computer for plotting details. The need for a better method of getting information
from the field to a computer was also accentuated with the introduction of a totalstation. As a result, the conventional method of recording surveys was overtaken
by developments in computer mapping and survey instrumentation which made
electronic data recording and transfer essential.
10.4.1
Data Loggers
Initially, the devices used were simple data loggers. But major advances were
made when it became possible to connect small portable computers to totalstations. These intelligent data loggers could be programmed to ask the surveyor
for information, to record data from an instrument in a suitable format and, if
necessary, to perform calculations using data transmitted to them. The following
are some of the methods of recording data electronically.
10.4.2
Latest development
of electronics based
equipments and electronic
techniques of measurements
are described.
Data Recorders
These are dedicated to a particular instrument and can store and process surveying observations. These are also referred to as electronic field books. They use
solid-state technology enabling them to store large amounts of data in a device
of the size of a pocket calculator.
The angle and distance readings are transmitted from the total-station to a
data recorder and these are stored together with point numbers generated by the
recorder and feature codes which are entered manually on site. Observations are
normally stored as angles and distances, called raw data, but a data recorder can
convert these to three-dimensional coordinates prior to transfer to a microcomputer. All data recorders have some resident programs to collect and process data.
After completion or during the survey, data collected is transferred from a data
recorder to a computer. For faster transmission, a compressed binary form is used.
268 Surveying
Examples help readers to
relate and apply the chapter
content to real life problems.
2. It should be done in summer during the first 4 hours after sunrise and
during the last 4 hours before sunset.
3. Observations should be avoided during high wind, before a thunderstorm,
and in foggy weather.
4. When arriving at the point, it is necessary to wait for 5 – 10 min. for the
aneroid to assume the temperature and pressure of the ambient air.
5. During observations the aneroid should be kept horizontal in a thick
leather casing to avoid jolts and for protecting from direct action of
sunlight.
6. The top of the aneroid should be slightly tapped to overcome the pointer
inertia before reading.
7. Eye should be properly positioned with the pointer to overcome
parallax.
8. Instrument readings should be taken repeatedly, at a number of points, in
intervals of 10 min.
If any one of the above conditions is not observed, it will result in serious
errors. An error in the aneroid of 0.1 mm causes an error in elevation equal to
1 m, and an error in determining the barometer temperature of 1°C causes an
error in height equal to 2 m. Under favourable conditions and by careful work,
the aneroid can be used to determine the elevation of points with a maximum
error of 3 m.
Example 6.26
Find the elevation of a station B from the following data
Time
Barometer reading
Temperature of air
Elevation
Solution
Station A
Station B
9 – 11 a.m.
75 – 75.2 cm
16 – 18°C
50.0 m
10 a.m.
72 cm
8°C
?
Variation of pressure in 2 hours (from 9 to 11 a.m.)
= 75.2 – 75 = 0.2 cm
Variation of pressure in 1 hour
0.2 = 0.1 cm
= ___
2
Hence, probable barometer reading at A at 10 a.m.
= h1 = 75 + 0.1 = 75.1 cm
Observed barometer reading at B at 10 a.m.
= h2 = 72 cm
(18 + 16)
Mean temperature at A = T1 = ________ = 17°C
2
Temperature at B = T2 = 8°C.
108 Surveying
Objective-type Questions
Objective-type questions are
helpful in the preparation
for various competitive
examinations.
3.1 The direction of survey lines may be expressed in terms of
(a) included angles between them
(b) bearings
(c) both of the above
(d) none of the above.
3.2 Which of the following compass can be used without a tripod for observing bearings
(a) trough compass
(b) prismatic compass
(c) surveyor compass
(d) all of the above
3.3 A looking mirror is generally provided on the object vane to
(a) sight on whole circle bearing system
(b) sight the objects too low
(c) sight the objects too high or too low
(d) observe the reading while sighting
3.4 In surveyor compass, the bearings observed
(a) are in whole circle bearing system
(b) are in quadrantal bearing system
(c) can vary from 0 to 90°
(d) both (a) and (c)
(e) both (b) and (c)
3.5 The temporary adjustments of surveyor compass involves
(a) centring only
(b) levelling only
(c) centring and levelling
(d) centring, levelling and focussing the prism
3.6 Bearing of a line is the horizontal angle it makes with
(a) true meridian
(b) magnetic meridian
(c) arbitrary meridian
(d) all of the above
3.7 Which of the following reference direction is used in a geodetic survey?
(a) True
(b) Magnetic
(c) Arbitrary
(d) Any of the above.
3.8 A declination of 3° east means
(a) magnetic north is 3° east of true north
(b) magnetic north is 3° west of true north
(c) true north in 3° east of magnetic north
(d) true south is 3° east of magnetic south
3.9 Dip is defined as
(a) the smaller horizontal angle, a survey line makes with the true
meridian
(b) the angle, which a freely suspended needle makes with the horizontal
plane
Contouring 377
B
90
85
80
C
E
75
70
D
65
62
60
A
(a)
e
b
90
90
85
80
c
75
80
85
Fig. 9.14
70
75
d 65
70
62 60 a
(b)
65 62
Determination of intervisibility
It can be seen that there will be obstruction in the range CD. Similarly, checks
can be made for other points.
Drainage area The extent of a drainage area may be estimated on a contour
map by locating the ridge line around the watershed. The ridge line should be
located in such a position that the ground slopes are down on either side of it.
The area is found by planimeteric measurements.
Capacity of reservoir Reservoirs are made for water supply and for power or
irrigation projects. A contour map is very useful to study the possible location of
a dam and the volume of water to be confined. All the contours are closed lines
within the reservoir area.
The areas A1, A2, A3, ...., An between successive contour lines can be determined
by a planimeter and if h is the contour interval, the capacity of the reservoir can be
estimated by the application of either the trapezoidal or the prismoidal formula.
Trapezoidal formula
Volume,
[
A1 + An
V = h _______ + A2 + A3 + ... + An – 1
2
]
Prismoidal formula
Volume,
h
V = __ [A1 + An + 4 (A2 + A4 + º + An – 1)
3
+ 2 (A3 + A5 + º + An – 2)]
Applications of
the principles are
given to solve field
problems.
Appendices
I.
REFERENCE OF MAPS
The system of reference and numbering map sheets is the reference of a map. To
facilitate the use of a map consisting of many sheets the convention is to give each
map sheet its own designation. The arrangement of the map sheets is indicated in
what is called a reference sheet.
CARTE INTERNATIONALE DU MONDE (CIM) SERIES
These sheets are also known as 1/M sheets or one-in-million maps. To keep
the international agreement, the Survey of India has published maps of India in
sheets on the International Projection. In this series, each sheet extends over 4° of
latitudes and 6° of longitudes. The elevation is shown in metres. The scale used
is 1 : 1 000 000 (1 : 1 M)
Appendices contain
supplementary matter
helpful in enriching
the text.
ARRANGEMENT OF SHEETS ON MAPS OF INDIA
Maps of India and the adjacent series are the basis of numbering topo-sheets in
India. The original sheet bears numbers starting from 1 up to 136, each denoting
a sheet in the series on a scale of 1 : 1 000 000 (one-in-million) and covering
an area within 4° of latitude and 4° of longitude. The one-in-million sheet has
been further subdivided into 16 equal sheets, each of 1° dimension as shown in
Fig. A.1 (a). Each such sheet is known as degree sheet as it represents only one
degree (1°) extent. The 16 degree sheets have been alloted alphabets from A to
P. These are known as Index Numbers.
Sheet No. P 54 denotes P-sheet of the one-in-million map No. 54. They are
also sometimes called quarter-inch maps as they show a scale of 4 miles to an
inch. In general, the contour interval is 250 ft.
The degree sheets have been again subdivide into 16 equal sheets, each representing an extent of 15 minutes (Fig. A.1 (b)). These smaller sheets show a scale
of 1 inch to a mile and are known as one-inch maps. Sheets showing an extent of
136 Surveying
4.24.1
Vertical Axis Error (a)
Let O be the instrument position. Let A0 B 0 C0 be the plane of a truly horizontal
circle, while the plane of the actual circle is A0 B1C0 (Fig. 4.17). The greatest inclination of the vertical axis occurs along OB1, the transverse axis being horizontal.
The maximum inclination of the transverse axis will be when the telescope points
along OA0. This inclination is a cos q when the telescope points in any direction
(say OP) at a horizontal angle q (read as q ¢) from OA0 as reference.
A
Q
P
a cos q
a
P¢
P0
A1
A0
P1
q¢
90∞
Q1
P2
f¢
B1
Q0
w
a
Quantification of
instrumental errors
are explained with
illustrative figures.
O
B0
C0
Fig. 4.17
After sighting a point P when the telescope is depressed, the line of sight
travels down a plane inclined at a cos q with the vertical. The errors in the plane
of the actual circle will be P ¢P1 and on the truly horizontal circle as P0 P2. The
corresponding error being dq, which is practically the same for q and q ¢.
1. Horizontal angles
But
Hence,
P ¢P1 = PP1 tan (a cos q)
PP1 = OP1 tan f¢
OP1 ¥ d q = P ¢P1
dq = tan (a cos q) tan f¢
or
d q = a cos q tan f¢
where q¢ and f¢ are the actual horizontal and vertical circle readings and dq is the
error of sighting. Since in any measurement there will always be two such sights
say q1, f1 and q2, f2 with respective errors of dq1 and dq2, then
D q = dq2 – dq1 = a (cos q 2 tan f 2 – cos q1 tan f 1)
Here, q 1 < q 2 and both the angles are measured clockwise, from zero along
OA0.
The maximum error is when q1 = 0° and q2 = 180° and is equal to a (tan
f1 + tan f 2). Also, if q1 and q 2 are measured from OB1 as zero, sin q must be
written in place of cos q.