Final report for Co-operative Research Centre for Spatial Information CRC-SI Project 114: Reconciling height datums in Australia: the bathymetric component

Vertical height datums are necessary as a reference for height measurements over the Earth. However the choice of reference is dependant on the purpose for the height data. For terrestrial users (especially on the coast), heights above a Mean Sea Level (MSL) are desired, whilst marine uses that depend on under keel clearance, prefer a low water tidal datum. The result of these preferences has produced two different types of height datum in Australia. Topography heights are referenced to the Australian Height Datum (AHD), Roelse (1975), and bathymetry heights are referenced to Chart Datum (CD) or Lowest Astronomical Tide (LAT), calculated through a tidal observation. Both are not directly compatible with satellite based data in ellipsoidal heights, which span the land/sea interface. Users of these data sets are increasingly aware of difficulties in joining these adjacent datasets, primarily a result of datum disparities. The focus of the research in this report is to reconcile the various bathymetry datums to allow joining of bathymetry datasets. A methodology has been presented to calculate uncertainty for short span observation MSL's relative to a long term reference epoch. This will increase the number of available MSL observations around the coast, which will aid in the creation of an ellipsoidal MSL surface with uncertainties. Despite their inclusion in the original project scope, methodologies for computing uncertainties of the Lowest Astronomical Tide (LAT) and Highest Astronomical Tide (HAT) tidal planes have not been developed. Several alternative options for high water and low water tidal planes with associated uncertainties to define the intertidal zone about MSL in Australia will be presented for consideration.