Tidal Interface Compendium of Related Terms

Tidal Interface Compendium of Terms

The ICSM Tidal Interface Working Group (TIWG) has compiled a compendium of Australian and New Zealand terms which are used in the inter-tidal zome.  This compendium has two parts:

  • The Full Compendium.
  • The Compendium Summary provides a more succinct overview.
    The compendium summary also attempts to map the different terms used to the ‘key terms’ given below.

In compiling the Compendium, members of TIWG have attempted to identify terms used in a legal context within each jurisdiction.  While the Compendium is far from definitive, it does highlight that there are numerous terms used to describe the tidal interface and that these terms frequently have varying definitions or are not defined at all.  It should also be noted that the complexity and significance of this issue varies between jurisdictions, depending on the nature of the coastal area and tidal variations experienced and its impact on activity in the tidal interface area.

In an attempt to rationalise the number of terms and add greater certainty to definitions, the TIWG has developed a list of key terms as described below.

Key Terms for Tidal Definitions

The TIWG have identified the following list of key terms used to define various tidal definitions that make up the tidal interface.  The definitions supplied with the terms have been taken from the Australia Hydrographic Office Tidal Glossary.  It should be noted that these are tidal datums and not limits as such.  The tidal interface descriptions are the intersection of the datum with the land, or the line where the sea meets that land at these various tide heights.

Key TermsPurposeDefinition

1. HAT

  • Landward limit of the tidal interface.
  • Chart datum for high tide (clearances)
  • Limit of landward extent of tidal water under normal atmospheric circumstances.

Highest Astronomical Tide: 
The highest level of water which can be predicted to occur under any combination of astronomical conditions.2. MHWS (and MHHW)

  • Tidal datum for cadastral (boundary) purposes for some jurisdictions (eg New Zealand, Queensland)

Mean High Water Springs (MHWS): 
The average of all high water observations at the time of spring tide over a period time (preferably 19 years).  Applicable in semi-diurnal waters only.
Mean Higher High Water (MHHW): 
The mean of the higher of the two daily high waters over a period of time (preferably 19 years).  Applicable in mixed and diurnal waters.3. MHW

  • Common law datum for cadastral (land boundary) purposes.  Used in Australia unless amended by legislation (as in Queensland for example)
  • Frequently used as the coastal limit on topographic mapping

Mean High Water (MHW): 
The average of all high waters observed over a sufficiently long period.4. MSL

  • Average limit of the tides.

Mean Sea Level (MSL): 
The arithmetic mean of hourly heights of the sea at the tidal station observed over a period of time (preferably 19 years).5. MLWS (and MLLW)

  • Tidal datum for administrative and jurisdictional extentx

Mean Low Water Springs (MLWS): 
The average of all low water observations at the time of spring tide over a period of time (preferably 19 years).  Applicable in semi-diurnal waters only.
Mean Lower Low Water (MLLW): 
The mean of the lower of the two daily low waters over a period of time (preferably 19 years).  Applicable in mixed and diurnal waters.6. MLW

  • Has been used as the limit of Australian States as the definition of ‘low water’.

Mean Low Water: 
A tidal level.  The average of all low waters observed over a sufficiently long period.7. LAT

  • Chart low water datum.
  • Baseline for the purposes of defining Australia's maritime boundaries in compliance with the UN Convention on the Law of the Sea.

Lowest Astronomical Tide (LAT): 
The lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions.

It should be noted that variation of these definitions might apply in current legislation. It should also be noted that some jurisdictions have adopted pragmatic procedures to approximate these values in the field (eg. Surveying guidelines, vegetation limits, etc).

The Compendium demonstrates that many, but certainly not all, terms currently used can be mapped against these terms.

Rationalising Terminology

Theoretically, it might be possible to amend all legislation and terminology to align with the key terms above. This may reduce some uncertainty in defining the limits of relevant legislation and make it easier to visualise and realise these limits. However, even if this were desirable, it is an unrealistic short-term goal for the following reasons:

  • Legislation amendment, even to proclamations and regulations, should not be taken lightly.
  • Many legal documents are inter-related. For example, the term ‘low water’ may be used simply because it was used in the original ‘Letters Patent’ which defined the limit of a State. In cases such as this, adopting a more clearly defined term may unintentionally lead to greater uncertainty.
  • Some legislation contains definitions that differ from those suggested by this paper. To change the definition used within existing legislation may not be possible without serious implications.
  • In some cases a ‘less technical’ term may be preferred for public acceptance and understanding.