 |  |  | |||
A datum, in surveying and geodesy, is a mathematical surface on which a mapping and coordinate system is based. The definition includes:
The Geocentric Datum of Australia (usually referred to as GDA94, or just GDA) is a coordinate system for Australia. That is, it's a system of latitudes and longitudes, or east and north coordinates, which we can use to keep track of locations.
GDA is GEOCENTRIC - it is a system of coordinates centred at the centre of the earth's mass.
GDA94 is compatible with modern positioning techniques such as the Global Positioning System (GPS). It supersedes the existing Australian Geodetic Datum 1984 (AGD84) and older coordinate systems. GDA94 is based on a global framework, the IERS Terrestrial Reference Frame (ITRF), but is fixed to a number of reference points in Australia.
The main advantage of GDA94 is that the coordinates are immediately compatible with global coordinates obtained from GPS and with other coordinate systems adopted in many parts of the world. GDA94 will also provide a single uniform coordinate system within Australia. This will reduce our current problems caused by different states using different datums. It will allow more efficient and more confident exchange of spatial information between digital systems.
All Australian governments - state, territory and commonwealth - have agreed to adopt the GDA94 datum during 2000/2001. This includes changing Acts and Regulations which determine the datum used for many dealings between government and the community, such as mining tenements, a great range of maps, reporting of various information. It is this interaction which means that most people who use maps or coordinates will need to change their methods.
There is no single date. However, most Western Australian government bodies changed during 2000. DME, DOLA and other WA government bodies will change between 4/12/2000 and 18/12/2000, and these are critical dates for operators in WA. The Commonwealth has not yet passed the necessary parallel legislation. Dealings with the Commonwealths where datum is specified in regulations, mainly deeper water offshore petroleum tenements, are expected to remain AGD-based until the middle of 2002.
No. There is no absolute compulsion on anyone (and therefore no compensation). For most purposes, you can work with any coordinates (AGD or local). However, some work which is controlled by regulation, such as Exploration Licenses and surveys or mapping under government contract, will be obliged to be completed based on GDA94. As others change to GDA, it will be increasingly inefficient to convert inputs and outputs.
MGA is a metric rectangular grid system (i.e. east and north), comparable to the AMG grid in use since the 1980s. It is a Cartesian coordinate system based on the Universal Transverse Mercator projection and the Geocentric Datum of Australia 1994. The unit of measure is the metre. Because both AMG (old) and MGA (new) are Universal Transverse Mercator projections, the parameters for both grids are identical (zone numbers, zone width, central meridians, central scale factor, false easting & northing), but they are projected from the different ellipsoids used by the different datums.
The Australian Geodetic Datum (AGD) was established before satellite techniques were available and was based on a model of the Earth, which was suited to the Australian region as well as could be calculated at the time. GDA94 is based on an international mathematical model which "best fits" the shape of the whole earth, with its centre coinciding with the earth's centre of mass. Coordinates on the earth's surface will change approximately 200 metres in a northeasterly direction with the new datum (GDA94). The exact change and orientation of the change will vary from area to area.
WGS84 is the datum used by the GPS system, and also for many small scale maps (e.g. maritime charts).
When GDA94 was defined, it was a requirement that coordinates of points in these 2 systems should be within a metre of each other. This was achieved, and is generally still the case in 2000. The spheroids used for WGS84 and GDA84 are also almost identical, and both systems are geocentric.
Thus for most mapping, exploration and GIS uses, WGS84 and GDA94 coordinates will be the same. If you use a GPS receiver (alone, or with a differential service) and the GDA datum is not a built-in option, you can select WGS84.
For precise surveys, however, the difference between WGS84 and GDA94 may be significant, and changes slowly over time. There are two reasons:
The difference between GDA94 and WGS84 is approximately 45cms in 2000.
Caution: Parameters for transformation of WGS84 to AGD84 data were derived and publicised in the 1980s by the Australian government – the so-called "Higgins" parameters. They were derived to the best standards available at the time, however they are not exactly the same as those now derived from modern satellite measurements. These parameters have sometimes been used to convert AGD84 positions to "WGS84". If the modern parameters now available are used to convert the AGD84 locations to GDA94, the results will be different to the "WGS84" figures previously derived. If these small differences (up to 1 metre) are important to your work, recompute the old conversions or contact Geoproject Solutions for assistance.
The detail on maps will appear to shift, relative to the map grid. The magnitude of this shift depends on the map scale (divide 200 metres by the map scale to assess the effect). At 1:1,000,000 the shift is barely significant at 0.2 mm. It is 0.8 mm on a 1:250,000 scale map and 200 mm at 1:1000 scale, as might appear on a site map of a facility - the facility could disappear off the sheet if the map was redrawn with the neatlines set to the same numeric values, but on the new datum.
Legal boundaries on land are generally defined by physical features or marks on the ground. These boundaries will not change, though the coordinates of them may.
Offshore boundaries may have been defined by coordinates in terms of the AGD (or another system). These boundaries too will not change, although the coordinates will be different when converted to GDA. Some boundaries have been defined by coordinates without reference to a coordinate system. In these cases the physical location has always been uncertain and this predicament will continue until that doubt is resolved in each individual case.
The GDA94 will not affect the heights which we normally use for survey, positioning, construction and legal purposes. The Australian Height Datum (AHD) is based on mean sea level and will remain so.
Yes, zero degrees longitude will still pass through Greenwich.
No, but zero degrees latitude relative to the GDA94 datum is a better estimate (than zero degrees AGD), of the actual position on the earth's surface of a line equidistant from the poles.
Yes, however the resulting maps may look better if new GDA neatlines are used. If the old neatlines (map edges) are used, they would be have to be re-labelled with the new GDA94 values, which would be non integer numbers (e.g. 30°30'05.6218" in lieu of 30º30'). Strictly, these values will be different at the top and bottom of a line, since the two grids are at a small angle to each other.
If the neatlines were given the same GDA94 values as they had in AGD (e.g. 30°30'), then the detail covered by the map will appear to shift (by about 200 metres).
If the map neatlines were defined along integer values of AMG (in metres), they will have non-integer values on MGA.
The Australian plate is moving in a north easterly direction at a rate of about 7 cm per year, but there is little known distortion within the plate. That means the latitude and longitude of points in Australia, measured in a world-wide system, are changing slowly but constantly.
The definition of the GDA94 datum includes fixed values for certain geodetic reference points in Australia. The values of these will remain fixed. GDA94 position values, therefore, are NOT affected by continental drift. The RELATIONSHIP of positions to each other, such as between 2 seismic shots, or of an object relative to the Trigonometric beacon used to locate that object, will NOT change.
However, ABSOLUTE positions in Australia, measured in a world-wide system, will change over the long term. WGS84 is such a world-wide system. This is one of the reasons why the exact relationship between GDA94 and WGS84 coordinates for points is not constant.
In the future, GPS systems which measure absolute positions to centimetre precision, may become commonly available. This will necessitate new rules for dealing with the gradual change of absolute positions.
The combined wisdom of state and federal governments was applied to the search for a new name in the early 1990s. The result was the decision that AGD should be replaced by GDA, and AMG by MGA. Geoproject Solutions offers a free coordinate to the person who submits the best mnemonic for remembering these confusingly different acronyms.
| | | |||