LEVELLING

LEVELLING

Levelling is the name given to the process of measuring the difference in elevation between two or more points. In engineering surveying, leveling has many applications and is used at all stages in construction projects , from the initial site survey through the final setting-out.

Specialized instruments are required to undertake leveling and traditionally this has been an optical level, with its tripod and staff. However , leveling is now also carried out using digital and laser levels.

Level and Horizontal line :

Ø  A level line is defined as a line along which all points are of the same height. Because the earth is curved, level lines are also curved.

Ø  A horizontal line is one which is normal to the direction of gravity , horizontal lines are therefore , tangential to level lines at individual points.

Datum and Bench marks :

Ø  For all surveys , a level line is chosen to which the elevation of all points is related and is known as a datum or datum surface. This can be any surface but the most commonly used datum is the mean sea level.

     On many construction and civil engineering sites, mean sea level is not often used as datum for leveling, instead a permanent feature of some sort is chosen on which to base all work and this is given an arbitrary height to suit the site conditions. Whatever the chosen datum, the height of a point is relative to a datum is said to be its reduced level.

Ø  Bench Mark : The are permanent reference marks or points, which their reduced levels have been accurately determined by leveling.

Ø  Ordnance Bench Mark (OBM) : these are those bench marks which have been established by the Ordnance Survey throughout the country.

Ø  Temporary or Transferred Bench Mark :  they are marks set-up on stable points near construction sites to which all leveling operations on that particular site will be referred. These are often used when there is no OBM close to the site. They height of a TBM may be accurately established by leveling from the nearest OBM.

AUTOMATIC LEVELS: these instruments establish horizontal lines of sight at each point where they are set-up and consist of a telescope with a compensator. The telescope provides an accurate line of sight and enables the level to be used over distances suitable for surveying purpose. The compensator built into the telescope ensure that the line of sight viewed through the telescope is horizontal even if the optical axis of the telescope itself is not horizontal.

SURVEYING TELESCOPE: since the type of telescope used in levels is also used in theodolite, the method of construction is considered in detail. The surveying telescope is internally focusing. Incorporated in the design of the telescope are special cross lines which when the telescope is adjusted correctly are seen clearly in the field of view. These lines provide a reference against which measurement can be taken. This part of the telescope is called the diaphragm (or graticule) and consist of a circle of plane glass upon which a series of lines is etched. Conventionally the vertical and horizontal lines are called the cross hairs.

The object lens, focusing lens, diaphragm and eyepiece are all mounted on the same optical axis and the imaginary line passing through the centre of the cross hairs and the optical centre of the object lens is called the line of collimation or the line of sight. When using the level all readings are taken using this line.

PARALLAX: it must be realized that for the surveying telescope to operate correctly , the image of a distant point or object must fall exactly in the plane of the diaphragm and the eyepiece must be adjusted so that its focal point is also in the plane of the diaphragm. Failure to achieve either of these settings, results in a condition called parallax and this is  a major cause of error in both levelling and theodolite work. Parallax can be detected by moving the eye to different parts of the eyepiece when viewing a distant object, if different parts of the object appear against the cross hairs then the telescope has not been properly focused and parallax is present.

THE COMPENSATOR:  in an automatic level, the function of the compensator is to deviate the horizontal ray of light at the optical centre of the object lens through the centre of the cross hairs. This ensures that the line of sight (or collimation) viewed through the telescope is horizontal even if the telescope is tilted.

            Whatever type of automatic level is used it must be levelled  within approximately 15-30’  of the vertical to allow the compensator to work. This is usually achieved by using the three foot screw arrangement in conjunction with a small circular level.  The main component of the compensator is a prism which is assumed to be freely suspended within the telescope tube when the instrument has been levelled and which takes up a position according to the angle of tilt of the telescope, provided the tilt is within the working range of the compensator, the prism moves to a position to compensate for this and a horizontal line of sight( collimation) is always observed at the cross hairs.

The wires used to suspend the prism are made of a special alloy, to ensure stability and flexibility under rapid changing atmospheric conditions, vibration and shock. The compensator is also screened against magnetic fields and it uses some form of damping, otherwise the compensator being light in weight , would tend to oscillate for long periods when the telescope is moved or affected by wind and other vibrations.

USES OF THE AUTOMATIC LEVEL: 
 The first part of the levelling process is to set the tripod in position for the initial readings , ensure its top is leveled by eye , after the tripod leg have been pushed firmly into  the ground. Following this the level is attached to the tripod using the clamp provided and the circular bubble is centralized using the three footscrews.

When an automatic level has been roughly levelled, the compensator automatically moves to a position to establish a horizontal line of sight , therefore no further leveling is required after the initial levelling .

As with all type of level, parallax must be removed, a test should be made to see if the compensator is functioning before reading commences.  One of the levelling footscrews should be moved slight off level and if the reading to a leveling staff remains constant, the compensator is working. If the reading changes , it may be necessary to gently tap the telescope to free the compensator. On some automatic levels this procedure is not necessary since a button is attached to the level which is pressed when the staff has been sighted. If the compensator is working, the horizontal hair is seen to move and then return immediately to the horizontal line of sight. Some levels incorporate a warning device that gives a visual indication to an observer, in the field of view of the telescope, when the instrument is not level.