Is It Time for a New National Height Datum?

Feasibility Study

Advantages of a New National Height Datum

The full range of height uses cannot be fully realised until accurate, reliable and up-to-date heights are available in real time to a variety of users (NOAA, 2013). It is quite possible that a new range of applications of height could result from improved height determination methods and technology, which could bring a wide range of unforeseen advantages.

The expected advantage in Australia is that a height datum could be produced which would address the issues present in the current height datum, mitigating or even eliminating them. Whilst AHD is widely accepted in practice, it is not sufficient for many scientific applications (Janssen, 2009). Although serving Australia well since its inception in 1971, the Australian Height Datum no longer meets all the requirements of a modern vertical datum (Filmer, 2010). A new national height datum would seek to be sufficient for all current uses of heights, whilst also ensuring that any foreseeable uses of heights will be accommodated.

As a push grows for a new Global Vertical Datum (GVD), it is important to realise that the AHD in its current form is not compatible with any Global Vertical Datum. The north-south slope and regional distortions mean that any vertical offset to the GVD will be a function of its horizontal position, rather than a constant offset. Therefore the development of a new Australian Height Datum is necessary if Australia is to benefit from any future GVD (Filmer, 2010, p. 256).

Disadvantages of a New National Height Datum

Although there are many positive aspects which would be brought about by a new improved national height datum, there are equally many negative aspects which would arise from a new national height datum, and these must be considered in detail before any change is implemented to the current national height datum. The idea of changing vertical datums and the possible upheaval that can accompany the change will naturally cause concern among users (Featherstone & Filmer, 2012, p. 20).

There are many potential methods of upgrading the Australian Height Datum. However each option comes with a different set of difficulties, challenges and costs. The cost of re-levelling an entire continent would be massive, and to ensure that the quality of the resultant datum was sufficient, care would have to be taken in the determination procedures, and so slower, more costly procedures are likely to be used.

A change is not only a scientific debate, as political opinions will also influence the decision. Scientific progress and political changes produce changes in technical procedures (Augath & Idhe, 2002). Any change would have to be government-funded, and due to the very large costs involved, it would be difficult to obtain such a grant, especially if there is any doubt whatsoever as to whether or not it is entirely necessary.

Although change is inevitable in some format, it will bring with it a large period of inconvenience as the difference in height systems is being determined and accommodated. A New National Height Datum effectively means changing the heights of all benchmarks and tide gauges around Australia, which is far from an easy or instantaneous event. All benchmarks and tide gauges are currently with respect to the AHD71, and so a new national height datum would have differing values. All National and State topographic maps, and those maps upon which they are based (geological, land use, forestry, etc.) relate to the AHD, and the cost of transforming these – even in digital form – to a new height datum would be enormous. Before a change could be implemented, an effective plan would have to be made to ensure that the transition is as smooth as possible, and all surveys done in this transitional period are compliant with future needs for heights.

Currently, the major impediment in developing a new Australian Height Datum of any type is the low-quality and incomplete data available from previous height datums and height surveys (Featherstone & Filmer, 2012, p. 28). This is likely to cause a problem for a potential upgrade, as it means that in order to ensure that a new national height datum is of sufficiently high quality, it would require large scale re-definition of the datum, and in this activity the previous data is not helpful in many sections.

Overall Feasibility

This section of the report outlines the various advantages and disadvantages which are likely to result from a new national height datum. Whilst the true success of the upgrade cannot be measured until after such an upgrade has been implemented, the preliminary research shows that, while it is a feasible option for Australia in the long term future, at the present time it is not quite feasible.

The disadvantages listed above are mostly short-term issues, and these are likely to be an issue for any suggested height datum upgrade in the future, whenever that may be. However as survey procedures improve, effecting an upgrade will become a much cheaper and simpler option, and as more and more height uses are found and developed, and the need for higher quality height data becomes even more apparent, the advantages may outweigh the disadvantages, and the upgrade would be deemed feasible given the increasingly smaller costs and larger benefits which would result from it. It is well understood that national vertical datums have a finite life-span, and should be upgraded or replaced as required (Featherstone & Filmer, 2012, p. 21). With this in mind, it is clear that change is inevitable in terms of a new national height datum, and so addressing the costs and logistical issues expected from a new national height datum in Australia is something which will have to be done whenever this change occurs. As the key purpose of an upgrade is to bring about a vast improvement in height quality in Australia, the focus should not be on minimizing costs and ease of upgrade, and instead the emphasis should be on the quality of the new height datum. The challenges and difficulties are still an important aspect, despite not being the primary priority.

The listed advantages, in the author’s view, will eventually outweigh the disadvantages listed above for a new national height datum. However, such an upgrade is not entirely urgent. Of most importance currently is an improvement in definition of the height datums, and the most feasible option for the short-term future is to improve on those issues present in the current national height datum.

Thomas Pollard z3323750 | School of Civil and Environmental Engineering | University of New South Wales