Coordinate, Height and Tide Datums - Tasmania

​​​​This page contains information about the Geocentric Datum of Australia 1994, and coordinate and height datums currently and previously used in Tasmania. Also included is a brief discussion about tide prediction datums. Links to information regarding national proposals to modernise and ultimately replace the current coordinate and height datums are also provided.
A comprehensive general overview of maps, mapping and datums is available at ICSM fundamentals of mapping pages.

Coordinate Datums

Geocentric Datum of Australia 1994 (GDA94)
The Geocentric Datum of Australia 1994 is the current recommended datum for recording the horizontal location of all spatial information in Tasmania.

The coordinates of the geodetic survey stations used as the origin points for GDA94 were published in the Commonwealth of Australia Government Gazette on 6 September 1995. These coordinates are referenced to the International Terrestrial Reference Frame 1992 at epoch 1994.0

GDA94​ coordinates were propagated throughout Australia via a series of national geodetic campaigns and a national geodetic adjustment that produced a network of approximately 80 geodetic stations with a nominal spacing of 500km. These stations are known as the Australian National Network (ANN). There are five ANN stations in Tasmania, their station IDs are: ST770, ST556, SPM9089, SPM9261 and AU016. GDA geographic coordinates (latitude & longitude) are known as GDA94, and the equivalent UTM grid coordinates are known as MGA94.

Further information about GDA94 and MGA94 is available from ICSM Geocentric Datum of Australia, Geoscience Australia ​and the ICSM GDA Technical Manual.

Modernisation of GDA94
Locations reported by Global Navigation Satellite System (GNSS), including the Global Positioning System (GPS), are not directly referenced to GDA94. These instruments nominally return location values relative to the reference frame that the particular GNSS system adopts - either the World Geodetic System 1984​ (WGS84) in the case of GPS or the International Terrestrial Reference Frame (ITRF) in the case of other GNSS . ​ 

At the official adoption of GDA94 on 1 January 2000 the difference in coordinates of any feature measured in the "plate-fixed" GDA94 and "earth-fixed" ITRF/WGS84 was approximately 40cm. In 2015 the difference is about 1.5 metres and by 2020 the difference will be approximately 1.8 metres. Given the current (2015) measurement accuracy for consumer grade GNSS receivers typically cannot be relied upon to better than +/-5 metres, for many practical purposes the difference between GDA94 and these reference frames can currently be ignored. 

However, it is envisaged that within several years consumer grade GNSS will be capable of measuring location relative to ITRF / WGS84 at sub-metre accuracy.  For this reason, ICSM has proposed a Datum Modernisation Roadmap to bring the national datum into closer alignment with GNSS reference frames.  Further information is available from the​ ICSM Datum modernisation pages​

Australian Geodetic Datum (AGD)
The Australian Geodetic Datum preceded the GDA in Australia. It was proclaimed in the Australian Commonwealth Gazette on 6 October 1966. This proclamation included the parameters of the adopted ellipsoid (The Australian National Spheroid), and the position of the origin point (the Johnston Geodetic Station)

AGD coordinates were propagated throughout Australia by the 1966 National Adjustment. The geographic coordinates (latitude & longitude) are known as ADG66, and the equivalent UTM grid coordinates are known as AMG66.

Further information about the AGD is available from the Geoscience Australia website and ICSM GDA Technical Manual.

Australian Geodetic Datum 1984
In 1982 a national adjustment was carried our using all data previously included in the 1966 National Adjustment, together with more recent observations. This new adjustment used the same gazetted ellipsoid and origin point as the AGD. It used improved software and included a geoid model.

The Coordinates of this adjustment were accepted by the National Mapping Council in 1984. The geographic coordinates (latitude & longitude) are known as ADG84, and the equivalent UTM grid coordinates are known as AMG84.

AGD84 coordinates were adopted by the surveying and mapping authorities in Queensland, Northern Territory and Western Australia. They were not adopted in Tasmania because the new adjustment did not reveal any significant discrepancies in the relative accuracy of the coordinates determined by the 1966 National Adjustment.

Australian National Grid and 'Clarke' Coordinates
From the early 1940s until 1966, a coordinate system based on the Clarke 1858 spheroid was used as the datum for most mapping activities in Tasmania. Coordinates were quoted in yards and were derived from a local Transverse Mercator (TM) projection of latitudes and longitudes determined in relation to the Tasmanian origin station.​

The rectangular grid coordinates were alternatively referenced as Australian National Grid (ANG), 'Clarke' or Transverse Mercator (TM) coordinates. The TM projection in Tasmania adopted a coordinate origin of S34 degrees latitude and E146 degrees longitude, with the false origin 1 800 000 further south.

Refer to the ICSM GDA Technical Manual​ for further details and references on 'Clarke 1858'/ANG and the Geoscience Australia website for advice on transforming between the 'Clarke 1858'/ANG​ and AGD/AMG.

The Tasmanian origin station was the Lochmaben Astronomical Station:

     Latitude: S 41deg 38min 23.389sec
     Longitude E 147deg 17min 49.725sec.
    Semi Major Axis: 20926348 British Feet (6378293.645 metres)
    Flattening: 1/294.26

Height Datums

Detailed below is information about the Australian Height Datum (Tasmania), datums previously used in Tasmania and datums currently in use on Tasmanian Islands.

The Australian Height Datum (Tasmania) AHD83
The Australian Height Datum (Tasmania) is based on mean sea level for 1972 at the Hobart and Burnie Tide Gauges. It was propagated throughout Tasmania via third order differential levelling and an adjustment computed on 17 October 1983. Mean sea level at both Hobart and Burnie was assigned the value of zero in this adjustment.

Height values based on this, and subsequent adjustments, are labelled AHD83 in the DPIPWE Survey Control Marks Database​.

WARNING
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 the reference ellipsoid, or simplified mathematical representation of the earth, that the particular GNSS system adopts. The difference between AHD83 and ellipsodial heights is dependent upon location and it varies by around 10 metres across Tasmania.  The relationship between reference ellipsoid​s, Mean Sea level and the AHD is discussed on the ICSM Fundamentals of Mapping Datum pages.  

Australian Height Datum (Tasmania) ADH79
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 DPIP​​WE Survey Control Marks Database. 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:


Datum
SPM 1371 (height)
AHD83
3.624
State Datum
3.789
Difference
0.165


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

King Island Local 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 DPIPWE Survey 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), and LAT (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.
AHD Heights of LAT, HAT and MSL
PortRef BM & Height
HAT
MSL
LAT
HobartSPM 194 1.859
0.86
0.05
-0.83
Spring BaySPM 8521 7.236
0.69
-0.02
-0.76
Low HeadSPM 9214 1.863
1.62
-0.03
-2.02
StanleySPM 6235 3.430
1.60
-0.07
-2.06
BurnieSPM 8857 3.553
1.64
-0.01
-1.96

Contact

Program Manager Geodetic System
Scott Strong
134 Macquarie Street
HOBART TAS 7000
Phone: 03 6165 4175
Email: scott.strong@dpipwe.tas.gov.au

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