UNIT 2
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.
Elements of distance and angular measurements
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.
Chain surveying
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.
Ranging is of two types
Direct Ranging
Indirect Ranging
Compass surveying
Compass surveying is the branch of surveying in which the position of an object is located using angular measurements determined by a compass and linear measurements using a chain or tape. Compass surveying is used in following circumstances:
If the surveying area is large, chain surveying is not adopted for surveying rather compass surveying is employed.
If the plot for surveying has numerous obstacles and undulations which prevents chaining.
If there is a time limit for surveying, compass surveying is usually adopted.
Compass surveying is not used in places which contain iron core, power lines etc which usually attracts magnets due to their natural properties and electromagnetic properties respectively. Compass surveying is done by using traversing. A traverse is formed by connecting the points in the plot by means of a series of straight lines.
Prismatic compass
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.
The bearings are expressed in the following two ways:
the horizontal angle which a line makes with the north direction of the meridian measured in the clockwise direction and can value upto 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.
B. Quadrantal Bearings:
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.
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.
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:
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.
Electronic Distance Measurement
Electronic distance measurement (EDM) is a method of determining the length between two points using electromagnetic waves. EDM is commonly carried out with digital instruments called theodolites.
EDM instruments are highly reliable and convenient pieces of surveying equipment and can be used to measure distances of up to 100 kilometres. Each piece of EDM equipment available at Engineer Supply provides dependably accurate distance measurements displayed on an easy-to-read digital screen.
Devices known as total stations share similarities with theodolites and can be used to measure distances as well as angles.
Theodolites
A Theodolite is a instrument for measuring both horizontal and vertical angles, as used in triangulation networks. It is a tool used in the surveying and engineering industry, but theodolites have been adapted for other specialized purposes as well. A theodolite consists of a telescope mounted movably within two perpendicular axes, the horizontal or trunnion axis, and the vertical axis. When the telescope is pointed at a desired object, the angle of each of these axes can be measured with great precision, typically on the scale of arc seconds. The measurements are typically recorded by hand as they are not recorded by a computer or data collector.
Reference Books
