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Next Generation Height Reference Frame NEXT GENERATION HEIGHT REFERENCE FRAME PART 2/3: USER REQUIREMENTS Nicholas Brown, Geoscience Australia Nicholas Bollard, RMIT University Jack McCubbine, Geoscience Australia Will Featherstone, Curtin University 0 <Insert Report Title> Document Control Version Status & revision notes Author Date 0.1 Initial draft Nicholas Brown 31/1/2018 0.2 Update to make it clear that the scope of the project is to Nicholas Brown 16/3/2018 assess user requirements for a height reference frame to be available alongside AHD 1.0 Incorporate feedback from User Requirements study Nicholas Brown 8/11/2018 1.1 Incorporate feedback from Jack McCubbine Nicholas Brown 20/11/2018 2.0 Review and alignment with Technical Options document Nicholas Brown 23/11/2018 and Executive Summary 2.1 Minor adjustments following review by Jack McCubbine Nicholas Brown 3/12/2018 2.2 Minor adjustments following review by Nic Gowans, Mick Nicholas Brown 17/12/2018 Filmer and Jack McCubbine 3.0 Minor corrections following a review by Phil Collier Nicholas Brown 14/2/2019 1 Next Generation Height Reference Frame Contents 1. PURPOSE AND SCOPE ............................................................................................................ 3 2. INTRODUCTION .................................................................................................................... 3 2.1 Changing world of geospatial .......................................................................................... 3 2.2 Introduction to the AHD ................................................................................................. 3 2.3 Problems with the AHD ................................................................................................... 4 3. STAKEHOLDER SURVEY ......................................................................................................... 5 3.1 Analysis methodology ..................................................................................................... 5 3.2 Overview of respondents ................................................................................................ 5 3.3 User satisfaction with AHD .............................................................................................. 5 3.4 Access and efficiency of AHD .......................................................................................... 7 3.5 Accuracy of AHD ............................................................................................................ 8 3.6 Why respondents use AHD ............................................................................................. 9 3.7 Using an alternative height reference frame ................................................................... 10 3.8 Monuments or model .................................................................................................... 11 3.9 Summary ..................................................................................................................... 11 4. RECOMMENDATIONS ........................................................................................................... 14 4.1 Choice of reference frame ............................................................................................. 14 4.2 Name of reference frame .............................................................................................. 14 4.3 Communication and Education ...................................................................................... 14 4.4 Tools / Products ........................................................................................................... 14 4.5 Future Work................................................................................................................. 15 APPENDIX A: STAKEHOLDER SURVEY .......................................................................................... 16 2 Next Generation Height Reference Frame 1. Purpose and Scope The purpose of this study was to assess both the User Requirements (Part 2/3) for height determination and the Technical Options (Part 3/3) which could be implemented to meet the user requirements. The only other user requirements study of physical heights in Australia was undertaken in 1988 (Kearsley, 1988) which predates the widespread use of GNSS. Given the technological advances of the past two decades and the modern methods some now use to access physical heights, we felt it necessary to reach out to the user community to assess their needs for physical heights now and into the future. The results of the user requirements study should be reviewed in conjunction with the technical options report which reviews the height system and height datum options which could be implemented in Australia based on current and future data holdings. The objective is to identify what requirements users have for height datums in Australia and what can technically be developed to provide users with what they need. The Executive Summary (Part 1/3) brings the recommendations of both reports together to describe the preferred option to satisfy the needs of users for physical heights in Australia. 2. Introduction 2.1 Changing world of geospatial Within five years users will have the capacity to position themselves at the sub-decimetre level using mobile Global Navigation Satellite Systems (GNSS) technology, providing efficient and accurate positioning for industrial, environmental and scientific applications. The Geocentric Datum of Australia 2020 (GDA2020) was introduced in 2017 in recognition of the increasing reliance on and accuracy of positioning from GNSS. The new geocentric datum enables precise positioning relative to the ellipsoid, that is, coordinates computed in terms of geocentric Cartesian coordinates (XYZ) that are then converted to latitude, longitude and ellipsoidal height in terms of the Geodetic Reference System 1980 (GRS80) reference ellipsoid. Nonetheless, ellipsoidal heights do not take into account changes in gravitational potential and therefore cannot be used to predict the direction of fluid flow. For this reason we have a physical datum, known as the Australian Height Datum (AHD) and a model to convert ellipsoidal heights to AHD heights known as AUSGeoid. 2.2 Introduction to the AHD The Australian Height Datum (Roelse et al., 1971; AHD) is Australia’s first and only national height reference system. It was adopted by the National Mapping Council in 1971 based on a (staged) least squares adjustment of 97,320 km of ‘primary’ levelling (used in the original adjustment) and approximately 80,000 km of ‘supplementary’ levelling (included in a subsequent adjustment). Levelling observations ran between junction points (see Figure 1; Filmer et al., 2010) in the Australian National Levelling Network (ANLN), and were known as level sections. These level sections were created by combining levelling observations along level runs (usually following major roads). The interconnected network of level sections and junction points was fixed to mean sea level (MSL) observed for 1966-1968 at 30 mainland tide gauges, with MSL assigned a value of zero AHD at each gauge. The staged least squares adjustment propagated heights above 1966-1968 MSL (or defined AHD zero 3 Next Generation Height Reference Frame reference), across the ANLN. The ANLN contains systematic, gross and random errors which have propagated into the AHD as local and regional biases (e.g. Morgan 1992). Figure 1: The Australian National Levelling Network (ANLN). First order levelling sections are in yellow, second order sections in light green, third order in fine grey, fourth order in dark green, one way (third order) in red and two-way (order undefined; Steed 2006, pers. comm.) in blue. The junction points are the intersection points of the levelling loops. Lambert projection (figure and description from Filmer et al., 2010). The Australian Height Datum (Tasmania) 1983 (AHD–TAS83) zero reference is fixed to MSL observed for 1972 only at tides gauges in Hobart and Burnie. It was propagated throughout Tasmania using mostly third order differential levelling over 72 sections between 57 junction points and computed via least-squares adjustment on 17 October 1983. Because AHD (mainland) and AHD (Tas) are referenced to MSL observed at different times and locations, there is a vertical offset between the two datums, which has been estimated (from various methodologies and data) to be ~10 cm (Rizos et al. 1991), between 12 cm and 26 cm (Featherstone, 2000), and ~1 cm (Filmer and Featherstone, 2012). For the purposes of this document, AHD is used to refer to both the Australian Height Datum 1971 (AHD71; Australian mainland) and Australian Height Datum (Tasmania) 1983 (AHD–TAS83). 2.3 Problems with the AHD The AHD is known to have a number of biases and distortions. The primary bias is due to the manner in which AHD was realised. In the adjustment of the levelling data, the measured mean sea level of tide gauges around Australian coastline was fixed to zero AHD. Due to the effect of the ocean’s time-mean dynamic topography (MDT), AHD is about 0.5 m above the gravimetric geoid in north-east Australia and about 0.5 m below the 4 Next Generation Height Reference Frame gravimetric geoid in south-west Australia (e.g. Featherstone, 2004, 2006; Featherstone and Filmer, 2008). Secondary effects are uncorrected gross, random and systematic levelling errors in the ANLN (e.g., Roelse et al., 1975; Morgan, 1992; Filmer and Featherstone, 2009) and the use of the (truncated) Rapp (1961) normal- orthometric
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