FUNDAMENTAL SURVEYING : OBJECT Surveying is the art of determining the relative positions of points on, above or beneath the surface of the earth by means of direct or indirect measurements of distance, direction and elevation. It also includes the art of establishing points by predetermined angular and linear measurements. ‘The application of surveying requires skill as well as the knowledge of mathematics, physics, and to some extent, astronomy. Levelling is a branch of surveying the object of which is (i) to find the elevations of points with respect to a given or assumed datum, and (ii) to establish points at a given elevation or at different elevations with respect to a given or assumed datum. The first operation is required to enable the works to be designed while the second operation is required in the setting out of all kinds of engineering works. Levelling deals with measurements in a vertical plane. The knowledge of surveying is advantageous in many phases of engineering. The earliest surveys were made in connection with land surveying. Practically, every engineering project such as water supply and irrigation schemes, railroads and transmission lines, mines, bridges and buildings etc. require surveys. Before plans and estimates are prepared, boundaries should be determined and the topography of the site should be ascertained. After the plans are made, the structures must be staked out on the ground. As the work progresses, lines and grades must be given. In surveying, all measurements of lengths are horizontal, or else are subsequently reduced to horizontal distances. The object of a survey is to prepare plan or map so that it may represent the area on a horizontal plane. A plan or maps the horizontal projection of an area and shows only horizontal distances of the points. Vertical distances between the points are, however, shown by contour lines, hachures or some other methods. Vertical distances are usually represented by means of vertical sections drawn separately. HA PRIMARY DIVISIONS OF SURVEY The earth is an oblate spheroid of revolutions, the length of its polar axis (12,713,800 metres) being somewhat less than that of its equatorial axis (12,756,750 metres). Thus, the polar axis is shorter than the equatorial axis by 42.95 kilometres. Relative to the diameter of the earth this is less than 0.34 percent. If we neglect the irregularities of the earth, the surface of the 12 = SURVEYING—1 imaginary spheroid is a curved surface, every element of which is normal to the plumb line. The intersection of such a surface with a plane passing through the centre of the earth will form a line continuous around the earth, The portion of such a line is known as ‘level line’ and the circle defined by the intersection is known as ‘great circle’. Thus in Fig. 1.1, the distance between two points P and Q is the length of the arc of the great circle passing through these points and is evidently somewhat more than the chord intercepted by the are. Consider three points P, @ and R (Fig. 1. 1) and three level lines passing through these points. The surface within the triangle PQR so formed is a curved surface and the lines Fig. 1.4 forming its sides are arcs of great circles. The figure is a spherical triangle. The angles p, q and r of the spherical triangle fre somewhat more than corresponding angles p' q’ and r' of the plane triangle. If the points are far away, the difference will be considerable. If the points are nearer, the difference will be negligible. . ‘As to whether the surveyor must regard the earth's surface as curved or may regard it is as plane depends upon the character and magnitude ofthe survey, and upon the precision required. ‘Thus, primarily, surveying can be divided into two classes: 1. Plane Surveying 2. Geodetic Surveying. Plane surveying is that type of surveying in which the mean surface of the earth is considered a6 » plane and the spheroidal shape is neglected. All triangles formed by survey. es. The level line is considered as straight and all plumb lines are considered as plane triangles. The : lines are considered parallel. In everyday life we are concerned with small portions of earth’s sind the above assumptions seem to be reasonable in light of the fact that the length of an arc 12 kilometres long lying in the earth's surface is only 1 cm greater than the subtended chord and further that the difference between the sum of the angles in a plane triangle and the sum of those in a spherical triangle is only one second for a triangle at the earth’s surface having an area of 195 sq. km. Geodetic surveying is that type of surveying in which the shape of the earth is taken into account, All lines lying in the surface are curved lines and the triangles are spherical triangles. It, therefore, involves spherical trigonometry. All geodetic surveys include work of larger magnitude and high degree of precision. The object of ‘geodetic survey is to determine the precise position on the surface of the earth, of a system of widely distant points which form control stations to which surveys of less precision may be referred. surfaceEl CLASSIFICATION Surveys may be classified under headings which define the uses or purpose of the resulting maps. (A) CLASSIFICATION BASED UPON THE NATURE OF THE FIELD SURVEY 1. Land Surveying (@®) Topographical Surveys: This consists of horizontal and vertical location of certain points by linear and angular measurements and is made to determine the natural features of a country such as rivers, streams, lakes, woods, hills, etc., and such artificial features as roads, railways, canals, towns and villages. (ii) Cadastral Surveys: Cadastral surveys are made incident to the fixing of property lines, the calculation of land area, or the transfer of land property from one owner to another. They are also made to fix the boundaries of municipalities and of State and Federal jurisdictions. (iii) City Surveying: They are made in connection with the construction of streets, water supply systems, sewers and other works. 2. Marine or Hydrographic Survey. Marine or hydrographic survey deals with bodies of water for purpose of navigation, water supply, harbour works or for the determination of mean sea level. The work consists in measurement of discharge of streams, making topographic survey of shores and banks, taking and locating soundings to determine the depth of water and observing the fluctuations of the ocean tide. . Astronomical Survey. The astronomical survey offers the surveyor means of determining the absolute location of any point or the absolute location and direction of any line on the surface of the earth. This consists in observations to the heavenly bodies such as the sun or any fixed star. (B) CLASSIFICATION BASED ON THE OBJECT OF SURVEY 1. Engineering Survey. This is undertaken for the determination of quantities or to afford sufficient data for the designing of engineering works such as roads and reservoirs, or those connected with sewage disposal or water supply. 2. Military Survey. This is used for determining points of strategic importance. 3. Mine Survey. This is used for the exploring mineral wealth. 4, Geological Survey. This is used for determining different strata in the earth’s crust. 5. Archaeological Survey: This is used for unearthing relics of antiquity. (C) CLASSIFICATION BASED ON INSTRUMENTS USED An alternative classification may be based upon the instruments or methods employed, the chief types being : Chain survey Theodolite survey Traverse survey Triangulation survey Tacheometric survey Plane table survey Photogrammetric survey and Aerial survey. The book mainly deals with the principles and methods of the above types. ~ PASM hepMEGA PRINCIPLES OF SURVEYING The fundamental principles upon which the various methods of plane surveying are based are of very simple nature and can be stated under the following two aspects : 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 measurement from at least two points of reference, the positions of which have already been fixed. Let P and Q be the reference points on the ground. The distance PQ can be measured accurately and the relative positions of P and @ can be plotted on the sheet to some scale. The points P and Q will thus serve as reference points for fixing the relative positions of other points. Any other point, such as R, can be located by any of the following direct methods (Fig. 1.2) P P Ros R a a a a a (a) (b) (o) Fig. 1.2 Location of A point (a) Distances PR and QR can be measured and point R can be plotted by swinging the two arcs to the same scale to which PQ has been plotted. The principle is very much | used in chain surveying. (6) A perpendicular RS can be dropped on the reference line PQ and lengths PS and SR | are measured. The point R can then be plotted using set square. This principle is used for defining details. (©) The distance QR and the angle PQR can be measured and point R is plotted either by means of a protractor or trigonometrically. This principle is used in traversing. (d) In this method, the distances PR and QR are not measured but angle RPQ and angle RQP are measured with an angle-measuring instrument, Knowing the distance PQ, point F is plotted either by means of a protractor or by solution of triangle PQR. This ote is very much used in triangulation and the method is used for very extensive work. ©) Angle RQP and distance PR are measured and point R is plotted either by protracting an angle and swinging an arc from P or plotted trigonometrically. This principle, used | in traversing, is of minor utility. Figs. 1.2 (b), (c) and (d) can also be used to illustrate the principles of determining relative elevations of points. Considering these diagrams to be in vertical plane, with PQ as horizontal. Fig.1.2 (b) represents the principle of ordinary spirit levelling. A horizontal line PQ is instrumentally established through P and the vertical height of R is measured by taking staff reading. Similarly, Fig. 1.2 (c) and (d) represent the principles of trigonometrical levelling.