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Australian Height Datum: Saving AHD in New South Wales the Australian Height Datum (AHD) Celebrates Its 50Th Anniversary This Year

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Australian Height Datum: Saving AHD in New South Wales the Australian Height Datum (AHD) Celebrates Its 50Th Anniversary This Year [ SURVEYING Australian Height Datum: Saving AHD in New South Wales 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 Levelling 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, Australia 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 mining, 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-Victoria 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 coast 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
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