Is It Time for a New National Height Datum?

Uses of AHD71

There are many different forms of uses of the Australian Height Datum. Since 1971, AHD71 has formed the framework for heights in Australia (Filmer & Featherstone, 2009). Traditional users of heights include surveyors and engineers, and these use heights in a number of different ways - including in design and construction of buildings, roads, pipelines, etc. Heights are also very important in GIS data, and are used in emergency planning in order to plan and monitor evacuation routes, and also for flood or inundation mapping and modelling (NOAA, 2013). 

Scientific Uses

The AHD is used – implicitly or explicitly – in many Earth-science-related applications, such as for resource and environmental management, river geomorphology and hydrological modelling, height-change analysis associated with seismicity and the computation of gravimetric terrain corrections (Filmer & Featherstone, 2009).

Scientists need accurate heights to monitor and model long to medium term changes in heights in the study of crustal motion, subsidence, isostatic readjustment and seasonal changes such as frost heave, as well as an issue of high current importance, sea level rise (NOAA, 2013). ). Scientific uses require a very high accuracy of height determination.

GNSS Uses

Positions obtained by a Global Navigation Satellite System (GNSS) such as GPS, GLONASS or the planned Galileo include height information referred to a reference ellipsoid, and these heights are ellipsoidal heights (Janssen et al., 2010). These ellipsoidal heights (h) can be converted to orthometric heights (H) by applying the geoid undulation (N), also known as geoid-ellipsoid separation, geoid height or N value through the use of the equation    H = h – N.

The use of GPS for such height determination was first discussed by Engelis et al. (1984; 1985). GNSS measurements are widely used in surveying, however the ellipsoidal height is the least accurately GNSS-determined coordinate, mainly because of atmospheric refraction coupled with the geometry of the resection, and thus will probably never reach the same accuracy (Featherstone, 2008). GNSS heights are currently of a reasonable accuracy, however an increase in accuracy is required for GNSS to replace traditional levelling in many modern applications.

GNSS heighting has increased in popularity over time, and in recent times there has been an increased use of GNSS for absolute rather than relative height determination  (Featherstone, et al., 2010), where the quasigeoid height (aka height anomaly) is subtracted from the GNSS-derived ellipsoidal height to yield an AHD height at a single point. Relative heights are determined through the use of GNSS baselines, and as these were simply a measure of ellipsoidal height differences to determine AHD height differences, the determination of these heights did not require accurate geoid-ellipsoid separation values. As a result of this, the deficiencies in AUSGeoid98 went largely unnoticed (Featherstone, et al., 2010).

Other Uses

Agriculture is an example of a relatively new user of heights, as heights are used to efficiently control farming equipment and to apply fertilizer as per planning for optimal agricultural efficiency. This is only a relatively new application as it was not until the advancement of accurate GPS heighting that this technology became possible. This is an example of how development of heights can bring about many unforeseen advancements in technology, in areas in which accurate heights had little importance. This technology is also one which could be vastly improved on with further developments into accurate height determination, as any improvements in height determination would ultimately improve the efficiency of such equipment.

There are many current applications which use heights, which would be further improved if vertical measurements were improved. Some of the more accurate applications of heights outlined by the National Oceanic and Atmospheric Administration (NOAA) of the United States of America (2013) include:

Further to this, it is hoped that improvement in height datums would lead to significant efficiency gains in industries such as mining, agriculture and construction that will no longer need to develop their own localised geometric models.

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