surveying -1 short answers i

7/31/2019 SURVEYING -1 Short Answers I

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Surveying (short answers):

Surveying: surveying is the art of determining the relative positions of different objects on the

surface of the earth by measuring the horizontal distances between them, and by preparing a map or

plan to any suitable scale.

Levelling: levelling is the art of determining the relative vertical distances of different points on the

surface of the earth. Therefore in levelling, the measurements are taken only in the vertical plane.

Objectives of surveying: the aim of surveying is to prepare a map or plan to show the relative

positions of the objects on the surface of the earth.

Plan: a plan is the graphically represents the features on or near the earths surface as projected on

horizontal plane. As a plan represents the area on a horizontal plane, the horizontal distances are

measured between various points on the earths surface.

Map: if the scale of the graphical projection on a horizontal plane is small the plan is termed map,

in additional, a map generally shows some additional features such as reliefs, hachure and contour

lines to indicate undulations on the ground.

Accuracy: accuracy denotes the closeness of a measurement to its true value. If the measured value

is very close to its true value, it is very accurate. (or) accuracy indicates nearness to the true value.

Precision: precision of a measurement denotes its closeness to another measurement of the same

quantity. If a quantity is measured several times and the values obtained are very close to another,

precision is high.

Sources of errors: (i) Instrumental errors (ii) personal errors (iii) natural errors.

Types of errors: (i) mistake errors (ii) systematic errors (iii) accidental errors.(Ref: text book: surveying vol:1 page:38-39 for detailed explaination)

Accessible point: accessible points are those points which are intervisible, or the points which can

be measured by direct methods of surveying.

Inaccessible point: inaccessible points are those points which are not intervisible, or the points

which cannot be measured by direct methods of surveying.

Plane surveying: It is the type of surveying in which the curvature of the earth is neglected and it is

assumed to be a flat surface. Plane surveying can be used when one is concered with a small

portions of the earths surface and the areas involved are less than 250 sq.km.

Geodetic surveying: In this type of surveying the curvature of the earth is taken into consideration

and a very high standard of accuracy is maintained. The main object of geodetic surveying is to

determine the precise location of a system of widely spaced points on the surface of the earth.

Representative factor: it is indicated by a ratio. The representative fraction is the ratio of the

distance on the map to the corresponding distance on the ground. Ex: 1cm = 1m.

Line ranger: a line ranger consists of two right angled isosceles triangle prisms placed one above

the other. The diagonals of the two prisms are silvered so as to form the reflecting surfaces. There

by for ranging the points on ground in a fixed straight line.


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General principle of surveying:

The general principles of surveying are:

1. To work from the whole to the part.

2. To locate a new station by at least two measurements

3. To check the work done

a)primary classification

(i)plane surveying: the curvature of earth is not taken into consideration, this is because plane

surveying is carried out over a small area. In such surveying an line joining two points is considered

as straight.

(ii)geodetic surveying: the curvature of the earth is taken into consideration. It is extended over a

large area. The line joining any two points is considered as a curved line.

(b) secondary classification:

(i) based on instruments: chain surveying, compass surveying, plane table surveying, theodolite

surveying.

(ii) based on methods: triangulation surveying and traverse surveying.

(iii) based on object: geological surveying, mine surveying, archaeological surveying, military

surveying(iv) based on nature of field: land surveying, marine surveying, astronomical surveying.

Ranging: the process of establishing intermediate points on a straight line between two end points is

known as ranging. Ranging may be done by direct ranging or by indirect ranging.

Accessories for linear measurements: ranging rods, chains, arrows, tapes.

Reconnaissance: it is the preliminary inspection of the area to be surveyed to have some idea of the

terrain and the principal features of the ground. In reconnaissance the surveyor itself observes the

field by the physical appearance.

Precautions against errors and mistakes:The following precautions should be taken to guard against errors and mistakes:

1. The point where the arrow is fixed on the ground should be marked with a cross.

2. The zero end of the chain or tape should be properly held.

3. During chaining the number of arrows carried by the follower and leader should always tally

with the total numbers of arrows taken.

4. While noting the measurement from the chain, the teeth of the tally should be verified with

respect to the correct end.

5. The chainman should call the measurement loudly and distinctly and the surveyor should repeat

to the correct end.

6. Measurements should not be taken with the tape in suspension in high winds.

7. In stepping operations, horizontality and verticality should be properly maintained.

8. ranging should be done accurately.9. No measurements should be taken with the chain in suspension.

10. Care should be taken so that the chain is properly extended.

Ranging is done by direct method and indirect method.

Direct method: when intermediate ranging rods are fixed on a straight line by direct observation

from end stations the process is known as direct ranging. Direct ranging is possible when the end

stations are intervisible. (see procedure in text for experimental procedure)

Indirect or reciprocal ranging: when the end stations are not intervisible due to there being high

ground between them, intermediate ranging rods are fixed on the line in an indirect way. This

method is known as indirect ranging or reciprocal ranging. (see procedure in the text for

experimental procedure)

Method of chaining on sloping ground: the following are the different methods that are generally

employed: (a) direct method or stepping method (b) indirect method.


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(a) Direct method: this method is applied when the slope of the ground is very steep. In this method

the sloping ground is divided into a number of horizontal and vertical strips like steps. So this

method is also known as the stepping method. The lengths of horizontal portions are measured and

added to get the total horizontal distance between the points. The steps may not be uniform and

would depend on the nature of the ground.

(b) indirect method: When the slope of the ground surface is long and gentle the stepping method

is not suitable. In such a case the horizontal distance may be obtained by the following processes:

1. By measuring the slope with clinometers.

2. By applying hypotenusal allowance and

3. by knowing the difference of level between the points.

CHAIN SURVEYING:

Principle of chain surveying: the principle of chain surveying is triangulation. This means that the

area to be surveyed is divided into a small triangles which should be well conditioned.

Well conditioned triangle: a triangle is said to be well conditioned when no angle in it is less than

300 or greater than 1200. well conditioned triangles are preferred because their apex points are very

sharp and can be located by a single point or dot.

Ill conditioned triangle: ill-conditioned triangles are not used in chain surveying. This is because

their apex points are not sharp and well defined.

DEFINITIONS AND ILLUSTRATIONS OF CHAIN SURVEY:

Main stations: stations taken along the boundary of an area as controlling points are known as

main stations. The lines joining the main stations are called main survey lines. The main survey

lines should cover the whole area to be surveyed.

Subsidiary stations: stations which are on the main survey lines or any other survey lines are

known as subsidiary stations. These stations are taken to run subsidiary lines for dividing the area

into triangles for checking the accuracy of triangles and for locating interior details.

Tie stations: these are also subsidiary stations taken on the main survey lines. Lines joining the tie

stations are known as tie lines. Tie lines are mainly taken to fix the directions of adjacent sides of

the chain survey map.

Base line: the line on which the frame work of the survey is built is known as the base line. It is the

most important line of the survey. Generally the longest of the main survey lines considered the

base line. This line should be taken through fairly level ground, and should be measured verycarefully and accurately.

Check line: the line joining the apex point of a triangle to some fixed point on its base is known as

the check line. It is taken to check the accuracy of the triangle. Some times this line helps to locate

interior details.

Offset: the lateral measurement taken from an object to the chain line is known as offset. Offsets

are of two types: (1) perpendicular offset. (2) oblique offset.

Perpendicular offset: when the lateral measurements are taken perpendicular to the chain line they

are known as perpendicular offsets.


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Oblique offset: any offset not perpendicular to the chain line is said to be oblique. Oblique offset

are taken when the objects are at a long distances from the chain line or when it is not possible to

set up a right angle due to some difficulties.

Selection of survey stations:

1. The stations should be so selected that the general principle of surveying may be strictly

followed.

2. The stations should be intervisible.

3. The stations should be selected in such a way that well conditioned triangles may be formed.

4. The base line should be the longest of the main survey lines.

5. The survey lines should be taken through fairly level ground as far as practicable.

6. The main survey lines should pass close to the boundary line of the area to be surveyed.

7. The survey lines should be taken close to the objects so that they can be located by short offsets.

8. The survey lines should not be very close to main roads, as survey work can be interrupted by

traffic.

9. The tie stations should be suitably selected to fix the directions of adjacent sides.

Procedure of field work:

1. RECONNAISSANCE

2. INDEX SKETCH

3. MARKING THE STATIONS ON THE GROUND.

4. REFERENCE SKETCHES

5. TAKING MEASUREMENTS OF SURVEY LINES AND NOTING THEM IN THE FIELD

BOOK.

CROSS STAFF:

The cross staff is a simple instrument for setting out right angles. The cross staff are open cross

staff (wooden cross staff), French cross staff, adjustable cross staff, optical square.

Compass survey:Definitions:

True meridian: the line or plane passing through the geographical north pole, geographical south

pole and any point on the surface of the earth is known as true meridian or geographical meridian.

Magnetic meridian: when a magnetic needle is suspended freely and balanced properly unaffected

by magnetic substances it indicates direction. This direction is known as the magnetic meridian.

True bearing: the angle between the true meridian and a line is known as true bearing of the line

and is also known as azimuth.

Magnetic bearing: the angle between the magnetic meridian and a line is known as the magnetic

bearing or simply the bearing of the line.

DESIGNATION OF MAGNETIC BEARING:

Magnetic bearing are designated by two systems:

(i) Whole circle bearing (ii) Quadrantal bearing

(i) Whole circle bearing: the magnetic bearing of a line measured clockwise from the north pole

towards the line, is known as the whole circle bearing of that line. Such a bearing may have any

value between 00 and 3600. The whole circle bearing of a line is obtained by prismatic compass.

(ii) Quadrantal bearing: the magnetic bearing of a line measured clock wise or counter clock wise

from the North Pole or South Pole towards the east or west is known as the quadrantal bearing of

the line. This system consists of four quadrants: NORTH-EAST, SOUTH-EAST, SOUTH-WEST

and NORTH-WEST. The value of a quadrantal bearing lies between 00 to 900


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Magnetic declination: the horizontal angle between the magnetic meridian and true meridian is

known as magnetic declination.

When the north end of the magnetic needle is pointed towards the west side of the true meridian the

position is termed declination west

When the north end of the magnetic needle is pointed towards the east side of the true meridian the

position is termed declination east

Isogonic lines: lines passing through points of equal declination are known as isogonic lines.

Agonic lines: The line passing through points of zero declination is said to be the agonic line.

Dip of magnetic needle: if a needle is perfectly balanced before magnetization, it does not remain

in the balanced position after it is magnetized. This is due to the magnetic influence of the earth.

The needle is found to be inclined towards the pole. This inclination of the needle with the

horizontal is known as the dip of the magnetic needle.

Local attraction: a magnetic needle indicates the north direction when freely suspended or pivoted.

But if the needle comes near some magnetic substances such as iron ore, steel structures etc it isfound to be deflected from its true direction, and does not show the actual north. This disturbing

influence of magnetic substances is known as local attraction.

Method of application of correction:

(a) First method: the interior angles of a traverse are calculated from the observed bearings. Then

an angular check is applied. The sum of the interior angles should be equal to (2n-4)900. (Where n

is the number of sides of the traverse). If it is not so, the total error is equally distributed among all

the angles of the traverse.

Then starting from the unaffected line, the bearings of all the lines may be corrected by using the

corrected interior angles. This method is very laborious and is not generally employed.

(b) Second method: in this method the interior angles are not calculated. From the given table the

unaffected line is first detected. Then commencing from the unaffected line the bearings of the

other affected lines are corrected by finding the amount of correction at each station.This is an easy method and one which is generally employed. If all the lines are found to be

affected by local attraction the line with minimum error is identified. The FB and BB of this line are

adjusted by distributing the error equally. Then starting from this adjusted line, the fore and back

bearing of other lines are corrected.

surveying -1 short answers i

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