LEVELLING METHODS AND INSTRUMENTS IN SURVEYING

LEVELLING AND INSTRUMENTS USED FOR LEVELLING. TERMINOLOGY IN LEVELLING. TYPES OF LEVELLING. METHODS OF LEVELLING. LEVELLING STAFF

1. LEVELLING METHODS AND INSTRUMENTS

2. What is “Leveling?” Levelling is the process by which differences in height between two or more points can be determined. Leveling is a branch of surveying, the object of which is to find or establish the elevation of a given point with respect to the given or assumed Datum (reference point).

3. TERMS USED IN LEVELLING 1.   Level Surface :  S urface parallel to the mean spheroid of the earth is called a level surface  and the line drawn on the level surface is known as a level line . Hence all points lying on a level surface are equidistant from the centre of the earth. Figure shows a typical level surface. Curved surface E ach point is perpendicular to the direction of gravity at the point Level line is normal to plumb line

4. 2 .  Horizontal Surface: A surface tangential to level surface at a given point is called horizontal surface at that point. Hence a horizontal line is at right angles to the plumb line at that point 3. Vertical Line:   A vertical line at a point is the line connecting the point to the centre of the earth. It is the plumb line at that point. Vertical and horizontal lines at a point are at right angles to each other.  

5. 4.   Datum:    The level of a point or the surface with respect to which levels of other points or planes are calculated, is called a datum or datum surface. 5.   Mean Sea Level (MSL):     MSL is the average height of the sea for all stages of the tides. At  any particular place MSL is established by finding the mean sea level (free of tides) after averaging tide heights over a long period of at least 19 years. In India MSL used is that established at Bombay. In all important surveys this is used as datum.

6. 6.   Reduced Levels (RL):  The level of a point taken as height above the datum surface is known as RL of that point. 7.  Benchmarks:   A benchmark is a relatively permanent reference point, the elevation of which is known with respect to datum  (assumed or known w.r.t. MSL) . It is used as a starting point for levelling or as a point upon which to close for a check.

7. Principle of Leveling: Spirit Levelling (Direct Levelling) It is the most commonly used method of leveling., in which measurements are observed directly from leveling instrument. leveling instrument is placed between the points which elevation is to be find. Leveling rods are placed at that points and sighted them through leveling instrument . It is performed only when the points are nearer to each other without any obstacles.

8. The vertical distances with respect to a horizontal line (perpendicular to the direction of gravity) may be used to determine the relative difference in elevation between two adjacent points. In spirit levelling, a spirit level and a sighting device (telescope) are combined and vertical distances are measured by observing on graduated rods placed on the points.

9. Barometric levelling Barometer is an instrument used to measure atmosphere at any altitude. Atmospheric pressure at two different points is observed, based on which the vertical difference between two points is determined . It is a rough estimation and used rarely. Biggest drawback is that air pressure is affected by other things besides elevation such as changes in temperature or weather.

10. Trigonometric Levelling (Indirect Levelling) The process of leveling in which the elevation of point or the difference between points is measured from the observed horizontal distances and vertical angles in the field is called trigonometric leveling.

11. INSTRUMENTS LEVEL Used to furnish horizontal line of sight for observing staff readings and determining reduced levels The levelling instruments essentially consist of the following: A levelling head with three foot screws which enables to bring the bubble at its centre . Telescope that provides line of sight to bisect distinct objects. A bubble tube to make the line of sight horizontal either mounted on top or side of the telescope. A tripod for supporting the levelling instrument.

12. The different types of levels are : 1. Dumpy level 2. Wye (Y) level 3. Reversible level 4. Tilting level DUMPY LEVEL Simple Compact Stable Used for rough work Telescope is rigidly fixed Gives fairly accurate work

13. WYE (Y) LEVEL In wye level the telescope is carried in two vertical Y supports. The wye level has an advantage over the dumpy level that the adjustments can be easily tested Telescope can be rotated and moved

14. REVERSIBLE LEVEL

15. TILTING LEVEL Telescope tilt about its horizontal axis

16. LEVELLING STAFF Levelling staff is a rectangular rod having graduations . The staff is provided with a metal shoes at its bottom to resist wear and tear . The foot of the shoe represents zero reading. Levelling staff may be divided into two groups: ( i ) Self reading staff (ii) Target staff.

17. SELF READING STAFF Staff reading is directly read by the instrument man through telescope. One metre length is divided into 200 subdivisions Each of uniform thickness of 5 mm. All divisions are marked with black in a white background . Metres are written in red colour Usually 3 to 4m long Thickness 1.8 cm and width 7.5 cm Background is always white Metre numerals in right and decimeter numerals in left hand side

18. 1. SOLID STAFF It is a single piece of 3 m.

19. A staff of two pieces each of 2 m which can be folded one over the other. 2. FOLDING STAFF

20. A staff of 3 pieces with upper one solid and lower two hollow . The upper part can slide into the central one and the central part can go into the lower part. 3. TELESCOPIC STAFF

21. TARGET STAFF If the sighting distance is more, instrument man finds it difficult to read self reading staff. Target staff is provided with a movable target. Target is a circular or oval shape, painted red and white in alternate quadrant . It is fitted with a vernier at the centre . The instrument man directs the person holding target staff to move the target , till its centre is in the horizontal line of sight.

22. Method of Leveling: When the level is set up and correctly leveled, the lines of collimation will be horizontal. When the telescope is rotated about its vertical axis, it will revolve in a horizontal plane known as the plane of collimation. Therefore all staff readings taken with the telescope will be vertical measurements made downwards from this plane. There are two essentials steps in leveling. 1. Height of Collimation Method 2. Rise and Fall Method

23. To find the elevation or R.L. or (H.I) of the level by taking a back sight on a bench mark. To find the levitation of R.L. of any other point by taking a reading on the staff held at the point. Height of Instrument (H. I.)  = R.L. of the plane of collimation                         = R.L. of B.M. + B.S.                 R. L. of point    = H.I.-F.S.                                          = H. I. – I.S.

24. It consist of finding the elevation of the plane of collimation ( H.I.) for every set up of the instrument, and then obtaining the reduced level of point with reference to the respective plane of collimation. 1. Elevation of plane of collimation for the first set of the level determined by adding back side to R.L. of B.M. 2. The R.L. of intermediate point and first change point are then obtained by starching the staff reading taken on respective point (IS & FS) from the elation of the plane collimation. [H.I.] 3. When the instrument is shifted to the second position a new plane collimation is set up. The elevation of this plane is obtained by adding B.S. taken on the C.P .

25. BS FS BS HI HI FS BS FS HI RL RL BM

26. From the second position of the level to the R.L. C.P. The R.L. of successive point and second C.P. are found by subtract these staff reading from the elevation of second plane of collimation Arithmetical check Sum of B.S. – sum of F.S. = last R.L. – First R.L. This method is simple and easy. Reduction of levels is easy. Visualization is not necessary regarding the nature of the ground. There is no check for intermediate sight readings