Reduced Levels

Reduced levels indicate heights on a drawing. They are usually shown to two decimal places only – that is, to the nearest 10 mm.

Reduced levels may have ‘RL’ in front of them to show that the figure represents a reduced level, for example RL 12.65. Another draftsperson might put them in a little box instead, like this:

A reduced level on a drawing indicates the height of that point relative to a given reference point. This reference point is called the job datum (or sometimes the temporary benchmark or TBM) and is a fixed, unmovable point for the project. It may be marked by a nail driven into a metal plate on the kerb at the front of the block, or even a nail in the road. It will be given a height value by the architect and all points on the site can be related in height to this point (i.e. above or below).

The RL of the job datum is often an arbitrary number decided by the architect or surveyor and is usually a nice round number such as 10.00 or 20.00.

The job datum may sometimes be related to the national survey scheme (the Australian Height Datum) and have an RL that reflects this, such as 18.635.