cve345 levelling and cve 345 12 (2).pptx

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1. LEVELLING IN SURVEYING Levelling is a process of determining the height of one level relative to another. It is used in surveying to establish the elevation of a point relative to a datum, or to establish a point at a given elevation relative to a datum. The process of measuring vertical distances in surveying is called levelling THE BASIC CONCEPT Principle of Levelling • The principle of levelling 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.

2. DEFINITIONS OF IMPORTANT TERMS IN LEVELING Level Surface: This is a surface which is normal to the direction of gravity at all points as indicated by a plumb line. Level Line : A line lying throughout on a level surface is a level line. This is normal to the plumb line at all points. Horizontal Plane: A horizontal plane through a point is a plane tangential to the level surface at the point. This is normal to the direction of gravity at that point. Horizontal Line: Any line lying throughout in a horizontal plane is termed as “horizontal line .” Datum Surface: A level datum is an arbitrary level surface to which elevations are referred. The most common surveying datum is mean sea-level (MSL), but as hydrological work is usually just concerned with levels in a local area, we often use: Elevation of a Point: is the vertical distance above or below the datum. This is usually called the reduced level (R.L) of the point. Objective of levelling To find the elevations of given point with respect to some assumed reference line called datum To establish point of required elevation with respect to datum

3. Objective of levelling To find the elevations of given point with respect to some assumed reference line called datum To establish point of required elevation with respect to datum Dumpy level:   Tilting level: Automatic level Types of Levels LEVEL This are Levelling instruments employed to determining the relative heights of different points on the earth's surface. A level essentially consists of a telescope to which a bubble tube is attached such that the axis of the bubble tube and the line of collimation of the telescope are parallel to each other Line of Collimation: The line of collimation or the line of sight is the line joining the intersection of cross-hairs to the optical center of the object glass and its continuation. Axis of Telescope: The axis of a telescope is the line joining the optical center of the object glass to the center of the eye piece.

4. Dumpy level Dumpy level ii a simple, compact and stable instrument. The telescope is rigidly fixed to its supports. Hence, it can not be rotated about horizontal axis

5. Tilting Level Tilting level is known as Indian office Pattern (I.O.P). In this level, the telescope tilts about its horizontal axis and hence ,it is called tilting level.

6. Automatic Level It is known as self aligning level. The fundamental difference between automatic level and other levels is that the level is not manually levelled but automatic levelled. It is achieved by inclination compensating devices

7. Automatic level

8. Levelling Staff | Procedure of Taking Staff Reading. Levelling Staffs: is an instrument used to determine the difference in height between two points. Levelling staff are scales in which these distances are measured. There are two types of levelling staff Self reading staff Target staff Self Reading Staff The self reading staff can be read directly by the level man looking through the telescope. The common types of Self reading staffs are; Ordinary staff Sop- with telescope staff Folding staff

9. METHODS OF LEVELLING Direct leveling Trigonometric leveling Barometric leveling Stadia leveling Direct leveling: , measurements are observed directly from leveling instrument . Direct leveling is divided into different types as follows : Simple leveling Differential leveling Fly leveling Profile leveling Precise leveling Reciprocal leveling

10. Benchmark Benchmark is the point laid above or below the datum line with a known elevation. Bench mark is considered as check for the other level points . A mark created at a recognized height which is used as the basis for measuring different altitude of topographical point.

11. . Errors in Levelling Following are some of the errors encountered in levelling survey Instrumental Errors Collimation Error Error due to curvature and refraction Other Errors such as Incorrect setting-up of instrument, Movement of staff from position when changing level station . Staff not held vertically.

12. Under sensitive bubble. Errors in staff graduation Loose tripod head. Telescope not parallel to bubble tube Telescope not at right angles to the vertical axis Collimation error . Error of Collimation : Instrumental error and Correction Collimation error occurs when the collimation axis is not truly horizontal when the instrument is level. The effect is illustrated in the sketch below, where the collimation axis is tilted with respect to the horizontal by an angle a:

13.   Methods of reduction in Levelling Collimation Method Rise and Fall Method Arithmetical Check: The difference between the sum of the back sights and the sum of the fore sights should be equal to the difference of the first and the last R.Ls i.e. Σ B.S. – Σ F.S. = Last R.L. -First R.L. This check verifies the calculation of R.Ls. of the planes of collimation and of the change points only. There is no check on the reduction of R.Ls. of the intermediate points. This method may more clearly be understood by the following example

14. Station B.S I.S F.S RISE FALL I C RL(m) Remark 2.191         52.064 49.873   2.505     0.314   49.559   2.325   0.180     48.739 3.019   1.496 0.829   53.587 50.568   2.513   0.506     51.074 1.752   2.811   0.298 52.528 50.776     3.824   2.072   48.704 6.962   1.515 8.131   2.684     Σ𝐵⋅𝑠−Σ𝐹∙𝑆= Σ R −Σ F=Last RL - First st RL −8.131+6.962= −1.169𝑚 Last RL – 𝐹𝑖𝑟𝑠𝑡 𝑅𝐿 = −1.169𝑚 Ie : 48.704 – 49.875 Σ𝑅𝑖𝑠𝑒−Σ𝐹𝑎𝑙𝑙 = 1.515 – 2.684= −1.169𝑚

15. Assignment one Data from a differential leveling have been found in the order of B.S., F.S..... etc. starting with the initial reading on B.M. (elevation 150.485 m) are as follows : 1.205, 1.860, 0.125, 1.915, 0.395, 2.615, 0.880, 1.760, 1.960, 0.920, 2.595, 0.915, 2.255, 0.515, 2.305, 1.170. The final reading closes on B.M.. Put the data in a complete field note form and carry out reduction of level by Rise and Fall method. All units are in meters. STATION B.S I.S F.S RISE FALL REDUCE LEVEL REMARKS B.M 761 2.11     22.13 Bench mark 10 1.14 20 0.95 30 0.84 40 1.55 50 1.88 Last reading Assignment two