Surveying : Levelling

Levelling

  • Levelling- The art of determining relative heights of different points on or below the surface of the earth is called LEVELLING
  • Deals with measurements in the vertical plane. For execution of Engineering Projects it is very necessary to determine elevations of different points along the alignment of proposed project.

Uses or purpose of Levelling

  • To prepare a contour map for fixing sites for reservoirs, dams etc. & to fix the alignment of roads, railways, irrigation canals etc.
  • To determine the altitudes of different important points on a hill or to reduced levels of different points on or below the surface of the earth.
  •  To prepare a longitudinal & cross section of a project (roads, railways, irrigation canals etc) in order to determine the volume of earth work.
  • To prepare a layout map for water supply , sanitary or drainage schemes.

Terms used in Levelling

  •  A level surface is the equipotential surface of the earth's gravity field. It is a curved surface and every element of which is normal to plumb line. A body of still water provides the best example of a level surface
  •  Level line -A line lying in a level surface is a level line. It is thus a curved line normal to the plumb at all points.
  • Datum - A level surface taken as reference for the determination of elevations of points.
  • Mean Sea Level (M.S.L.) : M.S.L. is obtained by making hourly observations of the tides at any place over a period of 19 years. MSL adopted by Survey of India is now Bombay which was Karachi earlier
  • Reduced Level (RL) : The vertical distances or Height or depth of a point above or below the assumed datum is called Reduced level. It may be positive or negative.
  • Line of Collimation : It is the imaginary line passing through the intersection of the cross hairs at the diaphragm and the optical center of the object glass and its extensions, it is also called line of sight or collimation.
  • Bench Mark (BM) :These are fixed reference point or marks of known elevation or RL determined with reference to the datum line.
  • . Back sight (B.S.) : This is the first staff reading taken in any setup of the instrument after the levelling has been perfectly done. This reading is always taken on a point of known RL, i.e. on a bench mark or change point.
  •  Permanent Bench Mark : These are fixed points or marks established by Government departments like PWD, Railway, Irrigation, etc. RLs of these points are determined with reference to GTS Bench mark (10 km. interval) & are kept on permanent points like the plinth of building, parapet of bridge or culvert etc.
  • Back sight (B.S.) : This is the first staff reading taken in any setup of the instrument after the levelling has been perfectly done. This reading is always taken on a point of known RL, i.e. on a bench mark or change point.
  • Fore Sight (F.S.) : It is the last staff reading in any setup of the instrument, and indicates the shifting of the instrument
  • Intermediate Sight (IS) : It is any other staff reading between the BS & FS in the same setup of the instrument.
  • Change Point (CP) : The point indicates the shifting of the instrument. OR The point on which both the foresight and back sight are taken during the operation of levelling is called change point.

Types of Levelling Equipments

  i) Dumpy level

 ii) Tilting level

 iii) Automatic level

 iv)  Digital Auto level

Types of Levelling operation

1. Simple leveling

 2. Differential Levelling

 3. Fly Levelling

4. Longitudinal or Profile Levelling

5. Cross-sectional Levelling

6. Check Levelling

Simple levelling -When the difference of level between two points is determined by setting the levelling instrument midway between the points, the process is known as simple levelling.

Differential levelling

-i) If the difference in elevation between them is too great.

 ii) If there are obstacles intervening. In such case it is necessary to set up the level in several positions and to work in series of stages.

Reciprocal Levelling  In this levelling set up on both banks of the river or valley and two set of staff reading are taken by holding the staff on the both banks

Sources of Errors in levelling

  • Instrumental errors -The permanent adjustment of the instrument may not be perfect. i.e. the line of collimation may not be parallel to the axis of the bubble tube.
  • Personal errors

a) The instrument may not be levelled perfectly.

        b) The focusing of the eye-piece & object glass may not be perfect

 c) The position of the staff may be displaced at the change point at the time of taking FS & BS readings.

  • Errors due to natural causes-

a)  When the distance of sight is long, the curvature of the earth may affect the staff reading.

b) The effect of refraction may cause a wrong staff reading to be taken.

 c) The effect of high winds & a shining sun may result in a wrong staff reading

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