UNIT 3
SURVEYING
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.
CLASSIFICATION OF SURVEYING
Generally, surveying is divided into two major categories: plane and geodetic surveying
- 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 reestablished 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.
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.
Angular Measurements
There are two methods for angular measurement:
1) Triangulation Survey
2) Traverse Survey
Triangulation Survey
In the past it was difficult to accurately measure very long distances, but it was possible to accurately measure the angles between points many kilometres apart, limited only by being able to see the distant . This could be anywhere from a few kilometres, to 50 kilometres or more. Triangulation is a surveying method that measures the angles in a triangle formed by three survey control points. Using trigonometry and the measured length of just one side, the other distances in the triangle are calculated.
Traverse Survey
Traversing is that type of survey in which a number of connected survey lines form the framework and the directions and lengths of the survey lines are measured with the help of an angle measuring instrument and a tape or chain respectively.
Leveling is defined as “an art of determining the relative height of different points on, above or below the surface
Principle of Leveling
The principle of leveling is to obtain horizontal line of sight with respect to which vertical distances of the points above or below this line of sight are found.
Object of leveling
- To Find the elevation of given point with respect to some assumed reference line called datum.
- To establish point at required elevation respect to datum.
Types of Leveling Instruments
According to the general arrangement of various parts, the levels may be classified as:
- Dumpy level,
- Wye level,
- Reversible level such as Cooke’s reversible level and Cushing’s level, and
- Tilting level.
1. Dumpy level.
- In the modern form of Dumpy level also called “solid Dumpy level”, the vertical spindle and the telescope are rigidly fixed so that the telescope can neither be rotated about its longitudinal axis nor removed from the supports.
- This leveling instrument is more stable when compared to others and retains its permanent adjustment for a long time.
2. Wye or Y level.
- In this instrument, the stage carries two “wye” supports in which the telescope is fixed.
- To the body of the telescope, two hanged collars of equal diameters are fixed. These collars rest on the “Wyes.”
- The telescope can be rotated about its longitudinal axis, or it can be taken out and placed end-for-end in the wyes.
- A clamp and a tangent screw are provided to facilitate accurate sighting of the objects. This is a very delicate instrument and consists of a large number of loose and open parts.
- Due to the reversibility of the telescope, the instrument may be more easily tested for permanent adjustments.
3. Cooke’s reversible level.
- This Levelling instrument combines the good features of both the Dumpy and the Wye levels.
- By loosening the screw, the telescope can be rotated about its longitudinal axis and can also be withdrawn from the sockets and placed end-for-end.
Cushing’s level.
- Cushing’s level is in that type of leveling instruments in which, the telescope is rigidly fixed in the collar as in a Dumpy Level.
- The two ends of the telescope barrel have equal sockets which can either received the objective or the eye-piece and diaphragm.
- Reversal of the line of collimation may be established by interchanging the objective and the eyepiece. The eye-piece can be rotated in its fitting.
4. Tilting Level.
- In the above four types of leveling instruments, the line of collimation is at the right angle to the vertical axis, if the instrument is in the permanent adjustment.
- Therefore, when the bubble is centered the line of collimation is made horizontal and the vertical axis is made truly vertical.
- In the tilting level, the telescope along with its bubble tube can be leveled by a micrometer screw without using the foot screws of the instrument, i.e., the line of collimation may be made horizontal independent of the vertical axis.
Calculation of reduced level by Height of instrument and Rise & Fall method
Reduced level of the line of sight, RL= 0 + h1 =h1
Reduced level of the point B = h1-h2
With a leveling instrument, it is possible to establish only a horizontal line not a level line. However, for the small distances involved in ordinary leveling, the distinction between level line and horizontal line is negligible. The errors due to instrumental faults can be eliminated if the points are equidistant from the leveling instrument. If the point is lower, then the staff reading will be higher.
Arithmetic Check
(i) For rise and fall method
∑B.S - ∑F.S = ∑Rise - ∑Fall = Last R.L – First R.L
(ii) Height of instrument method
∑ B.S - ∑ F.S = Last R.L – First R.L
Total Station
Total Station is a lightweight, compact and fully integrated electronic instrument combining the capability of an EDM and an angular measuring instrument such as wild theodolite. Total Station can perform the following functions:
- Distance measurement
- Angular measurement
- Data processing
- Digital display of point details
- Storing data is an electronic field book
The important features of total station are,
1. Keyboard-control – all the functions are controlled by operating key board.
2. Digital panel – the panel displays the values of distance, angle, height and the coordinates of the observed point, where the reflector (target) is kept.
3. Remote height object – the heights of some inaccessible objects such as towers can be read directly. The microprocessor provided in the instrument applies the correction for earth’s curvature and mean refraction, automatically.
4. Traversing program – the coordinates of the reflector and the angle or bearing on the reflector can be stored and can be recalled for next set up of instrument.
5. Setting out for distance direction and height -whenever a particular direction and horizontal distance is to be entered for the purpose of locating the point on the ground using a target, then the instrument displays the angle through which the theodolite has to be turned and the distance by which the reflector should move.
