Coordinate, Height and Tide Datums - Tasmania

Geocentric Datum of ​Aus​​​tralia 2020 (GDA2020)

The Geocentric Datum of Australia 2020 is the new official geodetic datum recommended for recording the horizontal location of all spatial information in Australia. 

It is defined in the National Measurement (Recognized-Value Standard of Measurement of Position) Determination 2017​ by the coordinates on 109 geodetic reference stations known as the Australian Fiducial Network (AFN). These coordinates are referenced to the International Terrestrial Reference Frame 2014 at epoch 2020.0. 

Geocentric Datum of Australia 2020 geographic coordinates (latitude & longitude) are known as GDA2020, and the equivalent UTM grid coordinates are known as Map Grid of Australia 2020 or MGA2020.

Elevation values returned by Global Navigation Satellite System (GNSS), including Global Positioning System (GPS), are not referenced to AHD83. These instruments nominally return elevation values relative to a reference ellipsoid, or simplified mathematical representation of the earth, that the particular GNSS system has been configured to adopt. The difference between AHD83 and ellipsodial heights is dependent upon location and the particualr ellipsoid and it varies by around 10 metres across Tasmania.  The relationship between reference ellipsoid​s, Mean Sea level and the AHD is discussed at a conceptual level on the ICSM Fundamentals of Mapping Datum pages, whilst the Geoscience Australia website​ has tools to calculate the difference between the AHD and GDA94 / GDA2020 ellispoidal heights.​

Australian Height Datu​​m (Tasmania) AHD79

The Australian Height Datum 1979 is also based on mean sea level for 1972 at the Hobart and Burnie Tide Gauges. It was propagated throughout Tasmania via the same differential levelling used to establish AHD83 and a levelling adjustment computed on 3 April 1979.

However, in 1983 an error of approximately 0.2 m was discovered in the level section which crossed the Mersey River at Devonport. It was decided to re-compute the adjustment using the correct height difference for this section. This was carried out on 17 October 1983 to create the Australian Height Datum (Tasmania) as described above.

Height values based on the 3 April 1979 adjustment are labelled AHD79 in the Survey Control Marks Database (SurCOM). These heights have been superseded by AHD83 values for most of the level networks in Tasmania. AHD79 values are sometimes still encountered in the height values determined by trigonometric levelling carried out between 1979 and 1983.

State ​Datum

State Datum is based on a determination of mean sea level made prior to 1905 at the old Marine Board of Hobart tide gauge at Castray Esplanade, Hobart. A bench mark cut on the stone foundation of tide gauge building was assigned the number 1371 by proclamation in the Tasmanian Government Gazette on 21 July 1948. According to the State Register of Permanent Survey Marks established under the Survey Coordination Act 1944, this bench mark has a height of 12.43 feet above mean sea level determined over a period of 30 years previous to 1905.

State datum heights were propagated throughout Tasmania via a variety of levelling techniques, including precise levelling by the Hydro Electric Commission and trigonometric levelling by the Lands and Surveys Department.  The difference between State Datum heights and AHD83 heights varies across Tasmania since State​ Datum was constrained by one tide gauge and the AHD83 was constrained by two, whilst the levelling measurements comprised in each network propagation were not the same.  This location dependency can only be ascertained by calculating the actual difference at Bench Marks that have both a State Datum height and a AHD83 height.

A comparison of AHD83 and State Datum heights at the old Castray Esplanade tide gauge bench mark, now known as SPM 1371, is as follows:

To convert State Datum heights to AHD83 heights in the vicinity of the old Castray Esplanade tide gauge, subtract 0.165.

King Island L​​​ocal Datum

The datum used for topographic mapping carried out in 1974 is mean sea level (MSL) as determined by the Royal Australian Navy (RAN).

In 1974 the RAN advised a bench mark at Grassy harbour was 3.524m above mean sea level. This bench mark was an arrow in the concrete wharf capping on the south side of the eastern end of the old jetty - it has since been destroyed. Heights based on this determination of mean sea level were propagated throughout the island by trigonometric levelling.

Subsequent determinations of mean sea level carried out in 2007 at Currie and Grassy Harbours by 3D Marine Mapping indicate the RAN determination of mean sea level (MSL) was incorrect. Survey control mark 170/2 located at Currie Harbour has a height of 2.25m above MSL as determined by the RAN. According to the 2007 determination, as recomputed by the National Tidal Centre in 2009, this mark is 2.006 above MSL.   

This value was adopted as the definitive height of MSL for King Island in 2014, and the heights of all survey control marks measured by GPS on King Island were re-computed, and elevations changed and labelled accordingly in the Su​rvey Control Marks Database​.​  Survey control marks on King Island with heights not determined​ by GPS, and denoted as determined by Trigonometric heighting, have not necessarily been amended. 

​Flinders Island Local​​​ Datum

The datum used for topographic mapping of the Furneaux Group carried out in 1972 is mean sea level as advised by the Division of National Mapping.

Survey control mark station summaries published by the Division of National Mapping at that time showed the heights of Brougham Sugarloaf (ST343) and Vinegar Hill (ST354) as 452.46m and 107.095 respectively above MSL as determined at the Lady Barron Tide Gauge. These values were adopted and held fixed in a trigonometric height adjustment used to propagate heights throughout the Furneaux Group.

Tide Prediction Datums

Following is a brief discussion about the relationship between the Australian Height Datum (Tasmania) and the tide prediction datum at some Tasmanian ports.

As from the beginning of 2009 the Australian Hydrographic Office has adopted Lowest Astronomic Tide (LAT) as the prediction datum for all Standard and Quasi Standard Ports.

WARNING

The following information has been compiled from information supplied by the Australian Hydrographic Office, the National Tidal Centre and the Tasmanian Ports Authority. Subsequent changes to the data supplied by these authorities may not be reflected in the information shown below. Before using this data for critical applications, users should consult these authorities for up to date information.

Explanation of Terms (Extracted from the Australian National Tide Tables 2011):

HAT (Highest Astronomical Tide), andLAT (Lowest Astronomical Tide): The highest and lowest levels respectively which can be predicted to occur under average meteorological condition and under any combination of astronomical conditions.
These conditions will not be reached every year, nor are HAT and LAT the most extreme levels that can be reached. Storm surges may cause considerably higher and lower levels to occur - for example HAT for Sydney is 2.0m, but the highest recorded tide is 2.4m.

MSL (Mean Sea Level): MSL is the average level of the sea surface over a period of time (preferably 19 years), or the average level which would exist in the absence of tides.