Aims -Heights, datums and bench marks -Levelling equipment -Field procedure for levelling -Calculating reduced levels -Sources of error in levelling -Other levelling methods
Levelling – how heights are defined Engineering surveying involves the measurement of three quantities; heights, angles and distances. Levelling it the process of measuring heights. It is possible when levelling to measure heights with an accuracy of millimetres Heights can also be measured using total stations, handheld lasers and GPS devices. However, levelling offers an inexpensive, simple and accurate method for measuring heights, and it is widely used in construction sites. Any method of measuring the heights of points above or below the ground using an agreed datum. These datum's or reference points are present in all construction sites and has an arbitrary height assigned to the point. Most construction sites will have several of these benchmarks, and if they have heights based on an arbitrary datum, they are known as Temporary Bench Marks.
Heights Heights are defined using horizontal and vertical lines. The figure below shows a plumbbob suspended at point P, the direction of gravity along the plumb-line defines the vertical at point P. A horizontal or level line is any line at right angles to this
For site work, any horizontal line can be chosen as a datum for heights and for levelling. The height of a point is measured along the vertical above or below the chosen datum. The height of a point relative to a datum is known as its reduced level (RL). On most construction sites there is a permanent datum. The horizontal line or surface passing through this, with its height, becomes the levelling datum. The height of the datum can be arbitrary, a value often used for this is 100.000m. This is chosen to avoid any negative heights occurring. Any reference point on site which has had a height assigned to it is known as a bench mark. For most surveys and construction work, several bench marks would normally be established by levelling from the datum. If heights are based on an arbitrary datum these are known as Temporary Bench Marks or TBMs.
Curved Surfaces Level (or horizontal) lines are always at right angles to the direction of gravity. The direction of gravity is generally towards the centre of the earth. Over large areas, as the Earth is curved, level surfaces will also be curved. For these, a height difference is measured along a vertical between two curved level surfaces.
When surveying over a large area, a curved level surface of zero height has to be defined. This has been established by the Ordnance Survey, this is called the Ordnance Datum (OD). This corresponds to the average sea level measured Poolbeg or Malin Head. Heights based on these are know as OD heights.
The levelling staff Levelling involves measuring vertical distances with reference to a horizontal plane or surface. To do this, a levelling staff is needed to measure vertical distances and an instrument known as a level is required to define the horizontal plane.
Many types of staff are used with varying lengths and different markings. The E-type face is commonly used in the UK and Ireland. This can be read directly to 0.01m and by estimation to the nearest mm. The staff must be held vertically – a circular bubble is sometimes fitted to help this.
1. Focusing screw 2. Eyepiece 3. Foot screw 6. Tangent screw 7. Circular bubble
4. Horizontal circle 5. Base plate 8. Collimator (sight) 9. Object lens
The main features of the telescope
1.Object lens 2.Focusing screw 3.Focusing lens 4.Diaphragm 5.Eyepiece
The object lens, focusing lens, diaphragm and eye piece are all mounted on an optical axis called the line of collimation or the line of sight. This is an imaginary line which joins the optical centre of the object lens to the centre of the cross hairs. When looking through the...