# levelling surveying part 2

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Comparison of the two systems

The collimation system is less tedious more rapid and involves less calculations. However, mistakes made in reduction of levels of the intermediate points remain undetected, while in the rise and fall system, there is a complete check on each of the intermediate reductions. The first system is generally used in profile leveling and in setting out levels for constructional work, while the second one is preferred for differential leveling, and other important work.

PROFILE LEVELLING

It is the operation in which the object is to determine the elevations of points at a known distances apart along a given line, and thus to obtain the accurate outline of the surface of the ground. It is also called the longitudinal leveling or sectioning.”
The leveling operations always start from a bench mark and end on a bench mark. If the permanent bench mark is not near the line of section, flying levels may be run from the permanent bench mark to establish a bench mark near the line of section.

Effect of Curvature: 1. In the case of long sights, the horizontal line is not a level line, i.e. a line of equal altitude due to the curvature of the earth. The level line falls away from the horizontal line , and the vertical distance between the horizontal line and the level line represents the effect of the curvature of the earth.   2. In fig, A is the station at which the level is set up; P the point at which a staff is held. 3. A’B and A’C represent the horizontal and level lines passing through the axis of the telescope and intersecting the staff at B and C respectively. On looking through the telescope, we sight in the direction of A’B and obtain the staff reading PB. The point B thus appears to be at the same level A’ as but actually the points A’ and C are at the same level. The true reading is, therefore PC. Thus, there is an apparent loss of elevation of the staff station P. The difference BC between the observer (or apparent) reading PB and the true reading PC represent the error due to the curvature of the earth. The true reading may be obtained by applying a correction equal to BC to the observed reading PB.

Contouring:“An imaginary line, on the ground, joining the points of equal elevation above the assumed datum is called a contour”. It is a plan projection of the plane passing through the points of equal height on the surface of the Earth. The elevations and depressions (the undulations) of the surface of the ground are shown on a map by means of contour lines. For example, suppose a depression in the surface of the ground is partially filled with water, and the elevation of the water surface is, say 80m. The shore line of this body of water will then represent the 80m contour. If the level of the water raised successively by 1m, the successive shore lines will represent 81, 82, 83m contours and so on.

Contour interval:   The vertical distance between any two consecutive contours is called the contour interval

.Horizontal equivalent:   The least horizontal distance between any two consecutive contours is known as the horizontal equivalent.

For a given contour interval, the horizontal equivalent depends upon the steepness The nature of the ground. The purpose and extent of the survey.  The scale of the map.
(iv) The time and expense required in its determination.

Characteristics of Contour Lines: (i) Contour lines run close together near the top and wide apart at the foot of a hill indicating flat ground. (ii) A uniform slope is indicated when contour lines are uniformly spaced, while a plane surface is indicated when they are straight and equally spaced. (iii) Contour lines, cross ridge lines or valley lines at right angles. A ridge line is shown when the higher values are inside the loop or bend in the contour, while in the case of a valley line, the lower values are inside the loop. The same contour appears on either side of a ridge or valley. (iv) Contour lines cannot merge or cross one another on the map, except in the case of an overhanging cliff. A vertical cliff is indicated when several contours coincide, the horizontal equivalent being zero. (V) Contour lines cannot end anywhere, but close on themselves either within or without the limits of the map.A series of closed contours on the map indicates a pond/depression or a hill/summit, according as the lower or higher values are inside them. Depressions between summits are called saddles. Line passing through the saddles and summits gives the divide line or watershed line. The line becomes the boundary line of the catchments area of a nalla or a river.(vii) Contours do not have sharp turnings.Uses of Contours:-

(i) By Inspection of a contour map, information regarding the genral    character of the tract of the country is obtained, whether it is flat, undulating(ii) The most economical or suitable site for engineering works (iii) Quantities of earthwork may be computed from the contour maps. (iv) Contours may be used to determine the area of the drainage basin and the capacity of the reservoir. (v)  The sections may be easily drawn in any direction from the contours. (vi) Intervisibility of two given points can be ascertained from the map. (vii)  A route of a given grade line can be traced on the map.(viii) To decide the best positions of guns, the line of march and camping grounds by the army commanders during wars.

Locating Contours:- The various methods of locating contours may be classified as:

(1) direct and (2) indirect

.Direct Method:In this method, contours to be plotted are actually traced out in the field. This method is comparatively slow and is generally not adopted on largesurveys unless a superior accuracy is demanded. It is suitable for contouring of small areas where better accuracy is required. The whole field work may be divided into two steps:a). Vertical Control b). Horizontal Control

Indirect Methods: Indirect method of contouring is commonly employed in small scale surveys of extensive areas. This method is cheaper, quicker and less tedious as compared with direct method of contouring. Indirect method of contouring can be employed in three different ways detailed below:

a. By Squares Method

b. By Cross Sections Method

By Square Method:  This method is suitable if the area is not very extensive. In this method the area is divided into a series of squares, and the corners of the squares are , depending upon the nature of the ground and the contour interval. The size of the squares need not to be the same throughout. Each separate square is pegged out. The elevations of the ground at the corners of the squares are determined with a level. The system of the square is plotted and near each corner is written its elevation. The contour lines are then interpolated in the usual way.By Cross Section Method: This method is most commonly used in a route surveys of a road, railway or canal alignment. Cross-sections are run transverse to the centre line of a road, railway, or canal, and the points of change in slope are located. The cross-section line may be inclined at an angle to the centre line  depends upon the character , and 100m in a flat country. The reduced levels of the various points along the section lines are plotted on the plan and the contours are then interpolated.
By Radiation Method: This method is particularly suitable when a contoured map of a hill is required. A number of lines are set out, radiating at a given angular interval from each of the traverse stations, and the representative points on these lines are located in the field by observing, (i) The horizontal distances, and (ii) the staff readings of the bottom, middle, and top wire.The elevations and the distances of these points are then calculated. The survey is plotted and the contour lines are then interpolated.