[ Australian Height Datum: Saving AHD in The Australian Height Datum (AHD) celebrates its 50th anniversary this year. This paper outlines the datum maintenance and modernisation efforts undertaken in New South Wales (NSW) to not only preserve but improve access to AHD

Volker Janssen The Australian Height Datum (AHD) is Present challenges Senior Technical $XVWUDOLD¶V¿UVWDQGRQO\OHJDOYHUWLFDO Surveyor in the Geodetic datum, celebrating its 50th anniversary this One important issue affecting the Operations team at year. The adjustment of the Australian availability of AHD is mark destruction. DCS Spatial Services, National Network (ANLN) in Despite the best efforts in the Preservation NSW Department of 0D\SURYLGHGIRUWKH¿UVWWLPH of Survey Infrastructure (POSI, see DCS Customer Service, in a nationwide network of physical AHD Spatial Services, 2020), entire sections Bathurst, heights (Roelse et al., 1975). For most of original ANLN spirit-levelled AHD spatial professionals, AHD has been have been destroyed. Mark destruction ubiquitous for the entire duration of is far higher in eastern NSW, with some Simon McElroy their careers, being the vertical datum level runs completely lost in city regions Senior Surveyor, of choice because it was the only one. or along highways. In rural and remote leading the Geodetic In a previous paper published in May areas, marks often still exist but can Operations team at 2021 issue of Coordinates magazine, we EHGLI¿FXOWWR¿QGGXHWRWKHUHPRYDO DCS Spatial Services, celebrated the achievements and longevity of all physical connections listed on NSW Department of of AHD, outlined its shortcomings and locality sketch plans (e.g. road mile Customer Service, in looked ahead to a new era of vertical posts, telegraph lines and relocated Bathurst, Australia datum determination, culminating fences or gates) and road realignments, in the Australian Vertical Working which alter chainages or deviate far Surface (AVWS, see ICSM, 2020a) as from the original road corridors. an alternative for those who need it. Fortunately, many ANLN marks DCS Spatial Services, a business unit of SUHYLRXVO\LGHQWL¿HGDVGHVWUR\HGRUQRW the NSW Department of Customer Service found in SCIMS are being successfully (DCS), is responsible for the maintenance recovered in good condition, maintained of the NSW survey control network. and upgraded using Geoscience Australia’s Relevant information is made available free online GPS processing service, via the Survey Control Information AUSPOS (see Janssen and McElroy, Management System (SCIMS), the state’s 2020; GA, 2021). On some level runs, database containing about 250,000 survey '&66SDWLDO6HUYLFHV¿HOGFUHZVUHSRUW marks on public record (Janssen et al., a recovery rate of 20% or better for lost 2019). This paper describes the datum marks. Finding marks that had reference maintenance and modernisation efforts blazes cut on trees is even more successful. undertaken by DCS Spatial Services through its ongoing Saving AHD projects. Another challenge is mark movement. As AHD is showing its age and slowly How well has the mark been able to deteriorating, these projects aim to not hold its initial AHD height over 50 only preserve but improve access to AHD. years? Thankfully, Australia enjoys

6 | CCoordinatesoordinates June 2021 Port Botany, Newcastle East, Port Kembla One important issue affecting the availability of AHD and Eden (Janssen et al., 2013). This helps answer the question “Is the sea is mark destruction. Despite the best efforts in the level rising or the land falling, or both?” Preservation of Survey Infrastructure (POSI, see DCS Sampling ANLN level Spatial Services, 2020), entire sections of original runs with AUSPOS

ANLN spirit-levelled AHD have been destroyed. Mark AHD is not homogenous. While multiple ANLN level runs may meet destruction is far higher in eastern NSW, with some level at a junction point, each has its own characteristics. Level runs may include runs completely lost in city regions or along highways (positive or negative) systematic errors, one-foot blunders, and may have been rather stable tectonics, where vertical GHWHUPLQDWLRQDQGWUDQVIHUPRUHHI¿FLHQW subject to mark movement over the movements are generally infrequent than with the traditional techniques last 50 years. Transferring or linking and not substantial. However, there are employed in the 1970s and 1980s. AHD heights from different ANLN exceptions, most notably in subsidence runs can lead to serious complications. areas caused by , major While the role of DCS Spatial Services Sampling can model these issues. construction activities or reactive soils. is to maintain NSW’s survey control network, in the last 10 years it has taken In a dedicated large-scale effort, AHD While AHD has been lost in regions of on a more active role in both POSI and its marks were sampled across NSW, as reactive black soil and any new value effort towards saving AHD. To this end, quickly as possible, for improvement would soon be invalid following the next several projects have been undertaken. of the national AUSGeoid model. wet or drought season, problems also This was conducted in a series occur in less obvious regions. A recent of Saving AHD AUSPOS survey investigation into a height anomaly Tide gauge monitoring campaigns, starting in 2015. of about 0.14 m at the NSW- border revealed that issues arise with AHD was constrained by 30 tide gauges. ,QWKH¿UVWSDVVHYHU\OHYHOUXQZDV constraining ANLN junction points when Over the years, DCS Spatial Services has investigated and sampled, generally new levelling observations are taken continued to monitor the stability of tide GXULQJDVLQJOH¿HOGGD\ZLWK¿YHWR between them. Allowing for apparent gauges via precise optical levelling, then six GNSS receivers deployed over the mark instability at one junction point and digital levelling and recently precise EDM length of the level run. Each mark was using the new levelling data resulted in height traversing. Generally conducted maintained (i.e. cleared, painted, and the discrepancy to be reduced by more HYHU\WZRWR¿YH\HDUVWKHVHVXUYH\V protected using generally three painted star than 50% (Watkins et al., 2017). Such monitor the stability of the tide gauge pickets), photographed, observed by an mark movement supports the notion that, compared to a near array of stable survey overnight AUSPOS session, and SCIMS DIWHUVHYHUDO\HDUV¿UVWRUGHUOHYHOOLQJ marks. Port Kembla tide gauge has been was updated with current metadata. surveys may deteriorate to much the same regularly monitored for over 20 years, order of accuracy as third-order levelling, while Fort Denison tide gauge has been As part of NSW’s contribution to the which becomes apparent when runs are resurveyed across 600 m of water from development of AUSGeoid2020, DCS re-levelled (Lambert and Leppert, 1976). the island in Sydney Harbour to Mrs Spatial Services collected more than Macquarie’s Point (and then back to 2,500 extended GNSS datasets (at least 6 The next challenge for AHD is the survey plug installed in 1882 on the hours but generally 12-24 hours duration) technology itself. The era of Global external wall of the former Department on levelled benchmarks for AUSPOS Navigation Satellite System (GNSS) of Lands building). More recently, Eden processing. These GNSS datasets technology led to the development of tide gauge has been similarly connected. informed the geometric component of geoid or quasigeoid models to convert AUSGeoid2020, helping to provide a GNSS-derived ellipsoidal heights to Five GNSS Continuously Operating much better connection to AHD (Janssen physical heights, including the current Reference Stations (CORS) were either and Watson, 2018). For many of these old AUSGeoid2020 for Australia (e.g. Brown VSHFL¿FDOO\EXLOWRUDGRSWHGWRDXJPHQW benchmarks, GNSS also improved their et al., 2018; Janssen and Watson, 2018; long-term monitoring of tide gauges horizontal position, which was initially Featherstone et al., 2019). Particularly located along the NSW in order to obtained by scaling off a map, often over longer distances, this has made height support sea-level studies: Fort Denison, resulting in positioning errors of several

CCoordinatesoordinates June 2021 | 7 hundred metres. This not only improved user access, but also allowed preservation efforts to be undertaken (you can only protect a mark if you know where it is).

Observing FBMs and GBMs with AUSPOS

Meanwhile, a dedicated campaign is XQGHUZD\WR¿QGPDLQWDLQDQGFROOHFW AUSPOS datasets on Fundamental Bench Marks (FBMs) and Geodetic Bench Marks (GBMs), which were installed in the 1970s. These high- VWDELOLW\PDUNVZHUHVSHFL¿FDOO\GHVLJQHG to maintain height, whether it be AHD, ellipsoidal or even AVWS height.

Figure 1: GNSS datasets of at least 6 hours duration on levelled marks observed Recovering lost levelling by DCS Spatial Services, including those contributing to AUSGeoid2020

DCS Spatial Services has invested SODQQHGMRLQWSURMHFWZLWKWKH$&72I¿FH UHSRUWHGWKHVH¿QGVVRWKH\FDQEHVKDUHG VLJQL¿FDQWUHVRXUFHVRYHUWKHODVWIHZ of the Surveyor-General to recover these IRUHYHU\RQH¶VEHQH¿W7KLVSURMHFWPD\ years in trying to recover lost levelling, RBMs was cancelled in 2020 due to the be expanded to other parts of the state. HJ¿UVWRUGHUOHYHOOLQJIURPWKH6QRZ\ COVID-19 pandemic. It has now been Mountains Hydro-Electric Authority SHQFLOOHGLQIRUWKH¿QDQFLDO\HDU Overall, the ongoing Saving AHD (SMA). This included approximately campaigns have to date yielded 900 1,000 marks over 1,000 km from Cooma additional extended GNSS datasets on to Tumut and on to the Victorian border, High fidelity (HiFi) saving AHD levelled benchmarks since the computation surveyed in the days of the Snowy of AUSGeoid2020 (Figure 1). While Mountains Hydro-Electric Scheme In the Central West, DCS Spatial Services the Intergovernmental Committee (construction from 1949 to 1974). To is nearing completion of its HiFi Saving on Surveying and Mapping (ICSM) date, nearly 15% of these marks have AHD project. In this area, every rural currently does not plan to update been recovered, maintained and observed. ANLN mark has been searched for, AUSGeoid2020 into the future, these This is a great result, considering that then maintained and upgraded. To date, datasets will be very valuable for the SCIMS basically held no levelled approximately 1,200 km of levelling continuing improvement of AVWS. heights in Kosciuszko National Park has been audited and surveyed, while and these marks are now 70 years old. another 400 km is planned. The number of recovered ANLN marks has exceeded Digitising historical Similarly, we continue to recover, expectations, with evidence that some levelling records on an ad-hoc basis, 1950s National have been used by other surveyors. While Mapping Division levelling. This data is it is fortunate to recover these marks, it In NSW, AHD is simply a set of numbers predominately located in remote regions, is disappointing that surveyors have not printed on some 3,600 cardboard sheets, with level runs reaching out to a graticule of 1° map corners that are often located in the middle of an outback paddock. Similarly, we continue to recover, on an ad-hoc basis,

DCS Spatial Services was also planning 1950s National Mapping Division levelling. This data to recover Rural Bench Marks (RBMs), installed by the Australian Capital is predominately located in remote regions, with level Territory (ACT) at the same time AHD was being observed. While about 450 runs reaching out to a graticule of 1° map corners that of the 1,200 RBMs are located in NSW, only a handful are included in SCIMS. A are often located in the middle of an outback paddock

8 | CCoordinatesoordinates June 2021 Services archive, the digitised historical There can be only one legal vertical datum, and currently levelling data is being used to generate a single, state-wide levelling adjustment there is no planned push to replace AHD. DCS Spatial for NSW. Currently, it comprises about 132,000 measurements and Services has yet to implement AVWS but continues 98,000 stations (Figure 2). While still underway, the enormity of this task and to investigate and contribute towards it. Our ongoing LWVEHQH¿WVWRWKHSURIHVVLRQVKRXOG not be underestimated. Victoria has Saving AHD projects ensure that users have access to already completed a state-wide levelling adjustment, and other jurisdictions are their vertical datum of choice. These efforts may also now also starting similar projects.

help AHD celebrate more milestones in the future It should be emphasised that there are no plans to readjust (or re-realise) AHD. Under the original rules, jurisdictions cannot modify the height of junction points without federal approval. The height of intermediate marks can be updated if a blunder or mark movement is detected and proven, which is part of the day-to-day maintenance.

Ongoing datum modernisation in NSW

DCS Spatial Services is responsible for the maintenance of the NSW survey control network. Datum modernisation and further improvement of survey infrastructure is required to accommodate the increasing accuracy and improved spatial and temporal resolution available from Figure 2: Location of benchmarks included in the NSW levelling adjustment modern positioning technologies to an ever-broadening user base. which are now safely stored in State digitised. First, they were scanned and Archives. These levelling cards summarise safely archived in a digital environment. This has led to the establishment of each level run and are abstracts of the Optical Character Recognition (OCR) CORSnet-NSW, Australia’s largest RULJLQDO¿HOGQRWHVGHWDLOLQJWKHPHDVXUHG IDLOHGWRFRQYHUWWKHVH¿OHVWRVPDUWGLJLWDO state-owned and operated GNSS CORS and adjusted height differences between ¿OHVVRHDFKFDUGZDVWKHQPDQXDOO\ network (e.g. Janssen et al., 2016; DCS benchmarks and junction points. At some FRQYHUWHGWR([FHO¿OHV RQHSHUOHYHOUXQ  Spatial Services, 2021). CORSnet- stage, these AHD values were manually Quality assurance of this laborious manual NSW is not only the backbone of the typed into SCIMS (naturally including data entry process is currently underway. Geocentric Datum of Australia 2020 unknown typos associated with all Once complete, the values in SCIMS can (GDA2020, see ICSM, 2020b) across manual data entries), and an electronic themselves be checked to remove any the state but also provides fundamental master version does not exist. Despite typos that have lain dormant for decades. positioning infrastructure that is the efforts by academics and federal authoritative, accurate, reliable and easy- agencies over the years, the original AHD to-use for a wide range of applications. values cannot be reproduced. The value Building the NSW Furthermore, with all sites contributing on the card is AHD, for better or worse. levelling adjustment to AUSPOS, it comprises a fundamental, high-density and long-term component These historical levelling cards are Together with the results of data-mining of AUSPOS infrastructure within progressively being preserved and H[LVWLQJOHYHOOLQJ¿OHVLQWKH'&66SDWLDO NSW (Janssen and McElroy, 2020).

CCoordinatesoordinates June 2021 | 9 Consequently, the use of AUSPOS push to replace AHD. DCS Spatial ICSM (2020b) Geocentric Datum campaigns has developed into a capable Services has yet to implement AVWS but of Australia 2020 technical manual, and reliable alternative to conducting continues to investigate and contribute version 1.5, https://www.icsm.gov. traditional static GNSS baseline surveys, towards it. Our ongoing Saving AHD au/gda2020-and-gda94-technical- VLPSOLI\LQJ¿HOGZRUNORJLVWLFVDQG projects ensure that users have access manuals (accessed Apr 2021). reducing processing times. This has to their vertical datum of choice. These substantially accelerated the process of efforts may also help AHD celebrate Janssen V., Commins R., Watson P. and including additional survey marks into the more milestones in the future. McElroy S. (2013) Using GNSS CORS to GDA2020 state adjustment to improve augment long-term tide gauge observations user access to GDA2020 coordinates and in NSW, Proceedings of Surveying and uncertainties on public record through References Spatial Sciences Conference (SSSC2013), SCIMS (e.g. Gowans et al., 2015; Janssen , Australia, 15-19 April, 12pp. and McElroy, 2020). AUSPOS has become Brown N.J., McCubbine J.C., Featherstone the primary survey technique used by W.E., Gowans N., Woods A. and Baran Janssen V., Gowans N., Hine S. and DCS Spatial Services to preserve AHD. I. (2018) AUSGeoid2020 combined McElroy S. (2019) Removing the gravimetric-geometric model: Location- uncertainty surrounding Positional DCS Spatial Services is currently building VSHFL¿FXQFHUWDLQWLHVDQGEDVHOLQHOHQJWK Uncertainty and Local Uncertainty an updated passive survey control network dependent error decorrelation, Journal of in SCIMS, Azimuth, 58(8), 28-32. (in the Eastern and Central Divisions) with Geodesy, 92(12), 1439-1456 & 1467. a minimum of one fundamental survey Janssen V., Haasdyk J. and McElroy mark observed by 6+ hour AUSPOS every DCS Spatial Services (2020) Surveyor- S. (2016) CORSnet-NSW: A success 10 km. Its vision is to ensure that any General’s Direction No. 11: Preservation story, Proceedings of Association future user is no further than 5 km (and of Survey Infrastructure, https://www. of Public Authority Surveyors often much less) from such a fundamental spatial.nsw.gov.au/surveying/surveyor_ Conference (APAS2016), Leura, mark providing direct connection to generals_directions (accessed Apr 2021). Australia, 4-6 April, 10-28. datum. Similarly, levelled AHD marks are observed by 6+ hour AUSPOS every DCS Spatial Services (2021) Janssen V. and McElroy S. (2020) 10 km, often at a far greater density. This CORSnet-NSW, http://www.corsnet. AUSPOS and CORSnet-NSW: A match will allow users to achieve DCS Spatial com.au/ (accessed Apr 2021). made in heaven, Coordinates, 16(7), 29-35. Services’ vision of a Positional Uncertainty of 20 mm in the horizontal and 50 mm in Featherstone W.E., McCubbine J.C., Janssen V. and Watson T. (2018) the vertical (ellipsoidal height) component Claessens S.J., Belton D. and Brown N.J. Performance evaluation of AUSGeoid2020 anywhere in the state and easily apply (2019) Using AUSGeoid2020 and its error in NSW, Proceedings of IGNSS transformation tools to move between grids in surveying computations, Journal Symposium 2018 (IGNSS2018), Sydney, current, future and various historical of Spatial Science, 64(3), 363-380. Australia, 7-9 February, 15pp. datums and local working surfaces. GA (2021) AUSPOS – Online GPS Lambert B.P. and Leppert K. (1976) A processing service, http://www.ga.gov.au/ study of the Australian national levelling Conclusion VFLHQWL¿FWRSLFVSRVLWLRQLQJQDYLJDWLRQ survey, Australian Surveyor, 28(2), 89-95. geodesy/auspos (accessed Apr 2021). As AHD celebrates its 50th anniversary Roelse A., Granger H.W. and Graham this year, this paper has outlined the Gowans N., McElroy S. and Janssen V. J.W. (1975) The adjustment of the datum maintenance and modernisation (2015) Survey infrastructure preservation Australian levelling survey 1970- efforts undertaken in NSW to not only and upgrade: Trigonometrical stations 1971, Technical Report 12 (2nd edition), preserve but improve user access to AHD. in NSW, Proceedings of Association Division of National Mapping, Through its Saving AHD projects, DCS of Public Authority Surveyors Canberra, Australia, 109pp. Spatial Services helps ensure that users Conference (APAS2015), Coffs Harbour, have continued and easy access to AHD, Australia, 16-18 March, 67-84. Watkins D., Janssen V., Woods A. while also providing a solid foundation and Grinter T. (2017) Investigating for the continuing improvement of AVWS ICSM (2020a) Australian Vertical Working an anomaly in the Australian Height as an alternative for those requiring Surface (AVWS) technical implementation Datum at the NSW-Victoria border in higher-quality physical heights. plan, version 1.3, https://www.icsm.gov.au/ Barham/Koondrook, Proceedings of publications/australian-vertical-working- Association of Public Authority Surveyors There can be only one legal vertical surface-technical-implementation- Conference (APAS2017), Shoal Bay, datum, and currently there is no planned plan-v13 (accessed Apr 2021). Australia, 20-22 March, 48-67. [

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