Geomatics : Levelling
Topic 2: Levelling
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
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