It is the method of surveying used for determination of the differences of elevations or levels of various points on the surface of the earth. The elevation of a point is its vertical distance above or below the reference level called datum. The leveling deals with distances in a vertical plane.

uses of leveling

          Design highway, railroads, canals, sewers, water supply systems.

             Layout construction projects according to planned elevations.

          Calculate volume of earthwork, or other materials by taking longitudinal and cross sectioning.

             Develop topographical maps showing general ground configuration.

               For fixing precise vertical and control positions.

          Investigate drainage characteristics of an area by preparing contour map.

          Study earth subsidence and crustal motion.

Some Basics terms

Ø   Vertical line – It is a line from any point on the earth surface to the center of the earth . It is commonly considered to be the line defined by a plumb line.

Ø   Level surface - A curved surface that, at every point is perpendicular to the local plumb line (the direction in which gravity acts).

Ø   Level line - A line in a level is perpendicular to plumb line at all points.

Ø   Horizontal plane – It is a plane tangential to the level surface at the point under consideration. It is perpendicular to plumb line.

Ø   Horizontal line - A line in a horizontal plane. It is a straight line tangential to level line .

Ø   Datum - Any level surface to which elevations are referenced.

( for example , mean sea level.)

Ø   Elevation - The distance measured along a vertical line from a vertical datum to a point or object.

Ø   Mean Sea level: It is the average height of sea for all stages of is derived by averaging the hourly tide height over a period of 19 years.

Benchmark (BM) - A relatively permanent object, natural or artificial, having a marked point whose elevation above or below a reference datum is known or assumed.

i)                          GTS Bench mark (Geodetic Triangulation Survey) : These Bench marks are established by national agency like Survey of Nepal. They are established with highest precision. Their position and elevation above MSL is given in a special catalogue known as GTS Maps ( 100 km. interval).

ii)                        Permanent Bench Mark : They are fixed points of reference establish with reference to GTS Bench mark (10 km. interval).

iii)                      Arbitrary Bench mark : These are reference points whose elevations are arbitrarily assumed for small levelling operations. Their elevation don't refer to any fixed datum.

iv)                      Temporary bench mark: These are the reference point on which a days work is closed and from where levelling is continued to the next day. Such a BM is carefully established or permanent object like kilometer stones, parapets etc

Ø   Station: A point where the levelling staff is kept.
Ø   Axis of telescope : it is a line joining the optical center of the objective to the center of eyepiece.

Ø   Line of sight: It is a line joining the intersection of the cross –hairs to the optical center of the objective and its continuation. Since in levelling the line of sight should remain horizontal while making the sights, the line of sight when is called the line of collimation.

Ø   Height of instrument: It is the elevation of the plane of sight with respect to assumed datum. It is also known as plane of collimation.

Ø   Back sight(BS): It is a staff reading taken on a point of known elevation, e.g. a sight on a bench mark (station A) or an a change point i.e. station C . In fig, a & c are back sight. It is the first staff reading taken after level is set up. It is called PLUS sight because it is added to elevation of that point to get height of instrument or plane of collimation.

Ø   Fore sight(FS): It is a staff reading taken on a point whose elevation is to determined, e.g. a sight on a change point i.e. station C & D .In figure c & d are the fore sight . This is also a MINUS sight. It is the last staff reading and denotes the shifting of the level.

Ø   Intermediate sights(IS): It is a staff reading taken on a point of unknown elevation between back sight and fore sighting .e.g. a sight on a station fig. b is the intermediate sight. These are called MINUS sights. These are subtracted from plane of collimation to find the reduced level of different points.

Ø   Change point(CP) or turning point(TP): It is a point denoting the shifting of the levels. Both F.S. & B.S. are taken on this point e.g. point C
Ø   Reduced level(RL): The elevations of the points with respect to assumed datum.

Ø   Parallax: it is the apparent movement of the image relative to the cross –hairs when the image formed by the objective does not fall in the plane of diaphragm.

Basic principle
The principle of leveling lies in furnishing a horizontal line of sight and finding the vertical distances of the points above or below the line of sight. The line of sight is provided with a level and a graduated levelling staff is used for measuring the height of line of sight above the staff positions.

In figure , let O represents center of the earth A & A’ are the positions whose elevation difference in elevation is required. C is the position of the level. The line CO is the direction of plumb line. BB’represents the line of Sight which is

perpendicular to CO. AB& A’B’ are the reading on the staff vertically held at A & A’ respectively.

OA+ OB=OA’+A”A’+A’B’ AB-A’B’=A’’A’=δh (δh= the difference of the staff reading)