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 leve(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 heigh
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 benchmark. 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.