ABSTRACT
It is necessary to have a fixed point on site to which all levels can be related. This is the site datum and is fixed at a convenient height, usually ground floor level. The datum itself must be related to some fixed point, preferably an ordnance bench mark (O.B.M), or some other clearly def fined point, e.g. a manhole cover. The level of an O.B.M. some distance away may be transferred to the site by means of a series of levels (known as flying levels) using a levelling instrument and staff. The site datum is marked by a peg or steel angle, concreted in, fenced off to protect it, and conveniently situated, preferably near the site office. Fig. 1.
