GUJARAT TECHNOLOGICAL UNIVERSITY

Unit – 4

Introduction to surveying and levelling

Question Bank

1. Explain  the Fundamental principles of surveying.

 Answer:

Principles of survey

The two principles are:

1. LOCATION OF A POINT BY MEASUREMENT FROM TWO POINTS OF REFERENCE:

The relative positions of the points to be surveyed should be located by taking measurement from at least two points of reference. Points of reference are points whose position is already fixed

2. SECOND PRINCIPLE IS TO WORK FROM WHOLE TO PART:

It is very essential to establish first a system of control points and to fix them with higher precision. Minor control points can be established by less precise methods and the details can be located using these minor control points by running minor traverses.

2. What are the Classification of surveying?

Answer:

CLASSIFICATION OF SURVEYING

Generally, surveying is divided into two major categories: plane and geodetic surveying

1. PLANE SURVEYING

PLANE SURVEYING is a process of surveying in which the portion of the earth being surveyed is considered a plane. The term is used to designate survey work in which the distances or areas involved are small enough that the curvature of the earth can be disregarded without significant error. In general, the term of limited extent. For small areas, precise results may be obtained with plane surveying methods, but the accuracy and precision of such results will decrease as the area surveyed increases in size. To make computations in plane surveying, you will use formulas of plane trigonometry, algebra, and analytical geometry.

2.     GEODETIC SURVEYING

GEODETIC SURVEYING is a process of surveying in which the shape and size of the earth are considered. This type of survey is suited for large areas and long lines and is used to find the precise location of basic points needed for establishing control for other surveys. In geodetic surveys, the stations are normally long distances apart, and more precise instruments and surveying methods are required for this type of surveying than for plane surveying.

• TOPOGRAPHIC SURVEYS

The purpose of a TOPOGRAPHIC SURVEY is to gather survey data about the natural and man-made features of the land, as well as its elevations. From this information a three-dimensional map may be prepared. You may prepare the topographic map in the office after collecting the field data or prepare it right away in the field by plane table. The work usually consists of the following:

1. Establishing horizontal and vertical control that will serve as the framework of the survey

2. Determining enough horizontal location and elevation (usually called side shots) of ground points to provide enough data for plotting when the map is prepared

3. Locating natural and man-made features that may be required by the purpose of the survey

4. Computing distances, angles, and elevations

5. Drawing the topographic map

Topographic surveys are commonly identified with horizontal and/or vertical control of third-and lower-order accuracies.

ROUTE SURVEYS

The term route survey refers to surveys necessary for the location and construction of lines of transportation or communication that continue across country for some distance, such as highways, railroads, open-conduit systems, pipelines, and power lines. Generally, the preliminary survey for this work takes the form of a topographic survey. In the final stage, the work may consist of the following:

1. Locating the center line, usually marked by stakes at 100-ft intervals called stations

2.  Determining elevations along and across the center line for plotting profile and cross sections

3. Plotting the profile and cross sections and fixing the grades

4. Computing the volumes of earthwork and preparing a mass diagram

5. Staking out the extremities for cuts and fills

6. Determining drainage areas to be used in the design of ditches and culverts

7. Laying out structures, such as bridges and culvert

8. Locating right-of-way boundaries, as well as staking out fence lines, if necessary

SPECIAL SURVEYS

SPECIAL SURVEYS are conducted for a specific purpose and with a special type of surveying equipment and methods. A brief discussion of some of the special surveys familiar to you follows.

• Land Surveys

LAND SURVEYS (sometimes called cadastral or property surveys) are conducted to establish the exact location, boundaries, or subdivision of a tract of land in any specified area. This type of survey requires professional registration in all states. Presently, land surveys generally consist of the following chores:

1. Establishing markers or monuments to define and thereby preserve the boundaries of land belonging to a private concern, a corporation, or the government.

2.  Relocating markers or monuments legally established by original surveys. This requires examining previous survey records and retracing what was done. When some markers or monuments are missing, they are re-established following recognized procedures, using whatever information is available.

3. Rerunning old land survey lines to determine their lengths and directions. As a result of the high cost of land, old lines are re-measured to get more precise measurements.

4. Subdividing landed estates into parcels of predetermined sizes and shapes.

5.   Calculating areas, distances, and directions and preparing the land map to portray the survey data so that it can be used as a permanent record. 6. Writing a technical description for deeds.

• Control Surveys

CONTROL SURVEYS provide "basic control" or horizontal and vertical positions of points to which supplementary surveys are adjusted. These types of surveys (sometimes termed and traverse stations and the elevations of bench marks. These control points are further used as References for hydrographic surveys of the coastal waters; for topographic control; and for the control of many state, city, and private surveys.

Horizontal and vertical controls generated by land (geodetic) surveys provide coordinated position data for all surveyors. It is therefore necessary that these types of surveys use first-order and second-order accuracies.

• Hydrographic Surveys

HYDROGRAPHIC SURVEYS are made to acquire data required to chart and/or map shorelines and bottom depths of streams, rivers, lakes, reservoirs, and other larger bodies of water. This type of survey is also of general importance to navigation and to development of water resources for flood control, irrigation, electrical power, and water supply.

3.     Explain Linear measurement and the methods of linear measurements.

Answer:

Linear Measurements

The determination of the distance between two points on the surface of the earth is one of the basic operation of surveying. Measurement of horizontal distances or measuring linear measurement is required in chain surveying, traverse surveying and other types of surveying.

Methods of making linear measurements

Direct methods

In the direct method, the distance is actually measured during field work using a chain or a tape. This is the most commonly used method for linear measurements. 

Optical methods

In the optical methods, principles of optics are used. The distance is not actually measured in field but it is computed indirectly. The instrument used for making observations is called tacheometer.

E.D.M methods

Electronic Distance Measuring (E.D.M) instruments have been developed quite recently.

These are practically replacing the measurement of distances using chains or tapes. There is a large variety of such instruments and depending upon the precision required the instruments should be used.

Instruments used

Methods of making linear measurements

Direct methods

In the direct method, the distance is actually measured during field work using a chain or a tape. This is the most commonly used method for linear measurements. 

Optical methods

In the optical methods, principles of optics are used. The distance is not actually measured in field but it is computed indirectly. The instrument used for making observations is called tacheometer.

E.D.M methods

Electronic Distance Measuring (E.D.M) instruments have been developed quite recently.

These are practically replacing the measurement of distances using chains or tapes. There is a large variety of such instruments and depending upon the precision required the instruments should be used.

Answer:

Chain surveying is that type of surveying in which only linear measurements made in the field.

Surveying is suitable for surveys of the small extent on open ground to secure data for an exact description of the boundaries of a piece of land or to take simple details.

The principle of chain survey or Chain Triangulation, as is sometimes called, is to provide a skeleton or framework consists of a number of connected triangles, as the triangle is the only simple draw that can be plotted from the lengths of its sides measured in the field book.

To good results in plotting, the framework should be consist of triangles which are as nearly equilateral as possible.

Principle of Chain Surveying

The principle of chain surveying is triangulation. This means that the area to survey is spilled into a number of small triangles which should be well-conditioned.

In chain surveying, the side of the triangles are measured directly from the field by chain or tape, and no angular measurements are used. Here, the check lines and tie lines control the accuracy of the given work.

It is noted that plotting triangles requires no angular measurements to be made if the three sides are known.

5.    What are the Error in chaining?

Answer:

Errors in chaining may be classified as:

(i) Personal errors

(ii) Compensating errors, and

(iii) Cumulating errors.

Personal Errors

Wrong reading, wrong recording, reading from wrong end of chain etc., are personal errors. These errors are serious errors and cannot be detected easily. Care should be taken to avoid such errors.

Compensating Errors

These errors may be sometimes positive and sometimes negative. Hence they are likely to get compensated when large number of readings are taken. The magnitude of such errors can be estimated by theory of probability. The following are the examples of such errors:

(i) Incorrect marking of the end of a chain.

(ii) Fractional part of chain may not be correct though total length is corrected.

(iii) Graduations in tape may not be exactly same throughout.

(iv) In the method of stepping while measuring sloping ground, plumbing may be crude.

Cumulative Errors

The errors, that occur always in the same direction are called cumulative errors. In each reading the error may be small, but when large number of measurements are made they may be considerable, since the error is always on one side. Examples of such errors are:

(i) Bad ranging

(ii) Bad straightening

(iii) Erroneous length of chain

(iv) Temperature variation

(v) Variation in applied pull

(vi) Non-horizontality

(vii) Sag in the chain, if suspended for measuring horizontal distance on a sloping ground.

Errors (i), (ii), (vi) and (vii) are always +ve since they make measured length more than actual.

Errors (iii), (iv) and (v) may be +ve or –ve.

6.    Define Offset.

Answer:

The lateral measurement taken from an object to the chain line is known as ‘offset’. Offsets are taken to locate objects with reference to the chain line. They may be of two kinds.

1. Perpendicular offset and

2. Oblique offset.

Perpendicular offset: When the lateral measurements are taken perpendicular to the chain line, they are known as perpendicular offsets

Oblique offset: Any offset not perpendicular to the chain line is said to be oblique offset. Oblique offset taken when the objects are at the long distance from the chain line or when it is not possible to set up a right angle.

7.    Define R.

Answer:

Ranging is of two types

Direct Ranging

• The ranging in which intermediate ranging rods are placed in a straight line by direct observation from either end.
• Direct ranging is possible only when the end stations are inter visible.

Indirect Ranging

• The ranging in which intermediate points are interpolated by reciprocal ranging or running an auxiliary line.
• Indirect ranging is done where end points are not visible and the ground is high .

8.    Explain Prismatic compass?

Answer:

Prismatic compass is a portable magnetic compass which can be either used as a hand instrument or can be fitted on a tripod. It contains a prism which is used for accurate measurement of readings. The greatest advantage of this compass is both sighting and reading can be done simultaneously without changing the position.

9.    Explain the bearings.

Answer:

1. Whole circle bearings
2. Quadrantal bearings

1. Whole Circle Bearings:

The horizontal angle which a line makes with the north direction of the meridian measured in the clockwise direc­tion and can value up to 360° i.e. the whole circle, is known as whole circle bearing (W.C.B.) of the line.

The prismatic compass measures the bearings of lines in the whole circle system.

The horizontal angle which a line makes with the north or south direction of the meridian whichever is nearer the line measured in the clockwise or counter clockwise direction towards east or west and can value up to 90° i.e. one quadrant of a circle is known as quadrantal bearing of the line.

The surveyor’s compass measures the bearings of lines in the quadran­tal system.

10.                         Explain system of bearing and local attraction.

Answer:

System of Bearing

Bearings of lines may be calculated if bearing of one of the lines and the included angles measured clockwise between the various lines are given.

• Bearing of a line = given bearing + included angle.

The bearings of lines may be given in:

(i) The whole circle system, or

(ii) The quadrantal system.

Local attraction

Local attraction is the phenomenon by which the magnetic needle is constantly prevented to point towards the magnetic north at a place. This is because that these magnetic compass is influenced by other magnetic objects at that locality such as wires carrying electric current, rails, steel and iron structures, steel tapes etc.

The occurrence of local attraction can be detected by observing the difference between the fore and back bearings. If there is no influence of local attraction and other error, this difference will be 180. So we can then conclude that both stations are free from local attraction.