LEVELING

lLeveling is the general term applied to any of the various processes by which elevations of  points or differences in elevation are determined

lLeveling results are used

1- design highways, railroads, canals, sewers, water supply systems,

2- layout construction projects according to planned elevations

3- calculate volumes of earth or other materials

4- investigate drainage characteristics of an area

5- develop maps showing general ground configurations

6- study earth subsidence and crustal motion

LEVELING TERMS

lLevel line : A line in a level surface (a curved line)

lVertical line : A line that follows the direction of gravity as indicated by a plumb line

lHorizontal line : A line in horizontal plane. In plane surveying, a line perpendicular to the vertical

lLevel surface : A curved surface that every point is perpendicular to the local plumb line (the direction in which gravity acts) e.g. a still lake

lHorizontal plane : A plane perpendicular to the direction of gravity. In plane surveying, a plane perpendicular to the vertical line

lVertical datum : Any level surface to which elevations are referred. This is the surface that is arbitrarily assigned an elevation of zero

 

 

 

lElevation : The vertical distance above or below the datum.It is also known as reduced level (RL)

lBench Mark (BM) 

   A permanent object that has a known elevation.

lTemporary BenchMark  (TBM) 

    A moveable object that has a known elevation. 

lTurning Point (TP) 

    A fixed object used when determining the elevation of other points

lHeight of Instrument (HI) 

     The elevation of the line of sight established by the instrument.

lBacksight (BS) 

    The reading on the rod when held on a known or assumed elevation.

    Backsights are used to establish the height of instrument.

lForesight (FS) 

     The reading on the rod when held at a location where the elevation is to be determined.

     Foresights are used to establish the elevation at another location, often a turning point.

lIntermediate Foresight (IFS)

      The reading on the rod when held at a location where the elevation is to be determined but not used as a turning point.

DIFFERENTIAL LEVELING

From Fig. 4-5 ;

HI (Height of Instrument) = Elevation + BS

                    828. 42       = 820.00 + 8.42

Elevation = HI - FS

        827.22       = 828.42 - 1.20

EQUIPMENT FOR DIFFERENTIAL LEVELING

ERRORS IN LEVELLING

lIncorrect setting-up of instrument

lMovement of staff from position when changing level station.

lStaff not held vertically

lParallax: Instrument knocked or moved during backsight-foresight reading

lTripod or rod settles between measurements e.g Bubble off center

lCollimation error

 

CONTOURING

  lA contour is a line drawn on a plan joining all points of the same height above or below a datum.

lContours cannot  cross, split or join other contours, except in the case of an overhang. e.g. a cliff.

lThe height between successive contours is called the vertical interval or the contour interval.

  Its value depends on the variation in height of the area being contoured.

  The contour interval is kept constant for a plan or map.

lThe plan spacing between contour line indicates the steepness of slopes.

  Closely spaced lines indicates a steep gradient.

  Widely spaced lines indicate a flatter gradient.

PRODUCTION OF CONTOURS

lDirect contouring

•This is the only method that produces true contours.

•It involves finding points on the ground with the value of the required contour.

•By establishing a series of such points the contour lines are effectively marked out on the ground.

•It is then possible to plot them by conventional detail surveying.

 

 

lIndirect contouring