Three viable options for a new Australian vertical datum

While the Intergovernmental Committee on Surveying and Mapping (ICSM) has stated that the Australian Height Datum (AHD) will remain Australia's official vertical datum for the short to medium term, the AHD contains deficiencies that make it unsuitable in the longer term. We present and discuss three different options for defining a new Australian vertical datum (AVD), with a view to encouraging discussion into the development of a medium- to long-term replacement for the AHD. These options are a (1) levelling-only, (2) combined, and (3) geoid-only vertical datum. All have advantages and disadvantages, but are dependent on availability of and improvements to the different data sets required. A levelling-only vertical datum is the traditional method, although we recommend the use of a sea surface topography (SSTop) model to allow the vertical datum to be constrained at multiple tide-gauges as an improvement over the AHD. This concept is extended in a combined vertical datum, where heights derived from GNSS ellipsoidal heights and a gravimetric quasi/geoid model (GNSS-geoid) at discrete points are also used to constrain the vertical datum over the continent, in addition to mean sea level and SSTop constraints at tide-gauges. However, both options are ultimately restricted by the requirement to upgrade the Australian National Levelling Network (ANLN). It is also desirable that the ANLN be kept in reasonable shape for the validation or testing of height products. On the other hand, a geoid-only vertical datum, where GNSS-geoid is used to continuously define the vertical datum, has advantages primarily because it avoids the requirement to level long distances to upgrade the levelling network. However, it is not routinely possible to realise a geoid of the desired 1-2 cm accuracy necessary to develop a geoid-only vertical datum, especially to a local precision that can match levelling, such that a geoid-only vertical datum is considered a long-term proposition. In the meantime, a combined vertical datum is a more suitable option for any new AVD in the next decade or so, although a geoid-based vertical datum which retains only the higher-quality parts of the ANLN in Australia's densely settled areas, but connected by a geoid model rather than continent-wide levelling, may also have merit.