In Chapters 2 and 9 we looked at different tidal levels and their use as zero or Datum Levels [1]. Tidal datums are also used to define shorelines, adopted as the state, national and international boundaries shown on maps. Generally the important levels are some form of High Tide, or some form of Low Tide, depending on locally adopted definitions. The determination of these levels, and their projection to mapped shorelines may require long records of sea-level measurements, often a complete nodal cycle of 18.61 years. Inevitably, knowledge of tidal principles has an important role to play in the development and interpretation of legal rules, but the technical aspects of defining tidal boundaries have sometimes been underemphasised [2].

The three components of a robust boundary definition are:

• clear and measurable tidal level parameters,

• a means of converting these levels into map coordinates,

• a legal system for interpreting and enforcing the boundaries.

To these we might add a process for adjusting to tidal and coastal changes over the long-term.