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Mitsubishi 4WD Owners Club of Qld (Inc) |
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GPS - Part 8 |
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GPS-101 Part 8 (Australian Datum) This month I’ll be discussing more about some other acronyms related to mapping in Australia, including: GDA, AGD, WGS, AHD, ANG and why you can be 200meters out if you don’t have the correct Australian datum. Most of this info comes from the Geoscience Australia web site: www.ga.gov.au. The Geocentric Datum of Australia 1994 (GDA94) is the current Australian coordinate system. It replaced the Australian Geodetic Datum (AGD). GDA is part of a global coordinate reference frame and is directly compatible with the Global Positioning System. It is a UTM (Universal Transverse Mercator) projection and the coordinate system is called the Map Grid of Australia 1994 (MGA94). Older Australian maps may be based on older data (plural of datum), such as AGD66 and AGD84. The difference between these two coordinate sets varies from about 2m in south east Australia to about 5m in the north west, but the variation is not uniform. AGD and GDA94 positions differ by about 200 meters. Prior to 1966, there were some twenty different datums used in Australia, using four different figures (shape representations) of the earth. The ANG (Australian National Grid) was based on one of these, but I’m not going to head down that ‘rabbit hole’. Suffice it to say, this is why it is important that you use the same datum on your GPS and the map you are using. If you have a map with an AGD66 datum and you’re taking readings from your GPS unit based on GDA94, then you could be 200m out. If you’re using your GPS in conjunction with a moving map on a PDA or Notebook PC, then you also need to be aware that some GPS units output real-time data in a different datum to what is shown on the display. You’ll need to check your specific GPS unit for details and make sure that you configure your PDA or Notebook software to the datum that is output by your GPS unit via the serial, USB, or bluetooth port. Some software will already know this, if you tell it which GPS units you are using. Another acronym that you’ll come across regularly in regard to GPS datum is WGS84 (World Geodetic System 1984). WGS84 provides an earth-centred reference system established by the US Defence Mapping Agency and is the basis for the NAVSTAR GPS satellite navigation system. Apart from datum, the WGS also takes into consideration the non-spherical shape of the Earths surface. For most practical purposes, WGS84 and GDA94 are the same. We’ve discussed how the GPS system has to deal with accuracy of orbits and atmospheric conditions and apply corrections due to relativistic effects. We’ve also discussed how we have to deal with the distortion of projections of the Earth’s surface to make convenient maps. There’s another complicating factor that has to be dealt with in regard to GPS and the representation of the Earth’s surface. The problem is that the Earth’s surface isn’t stationary. GDA94 is based on the International Terrestrial Reference Frame 1992 (ITRF92) held fixed at 1 January 1994 – hence the name GDA94. However since 1994, the Australian tectonic plate has been shifting approximately northeast at a rate of about 7cm per year. Because the whole of Australia is moving uniformly, this movement is not an issue when obtaining positions relative to other GDA94 positions. However, if a position is obtained directly from a global positioning system (such as GPS) the difference may be apparent. Fortunately, as far as our navigating goes for 4WD trips, this doesn’t represent a problem. But, if you were a surveyor using a highly accurate differential GPS then the uncertainty of hundreds of millimetres is very relevant. So far I’ve only mentioned datum in regard to two-dimensional navigation. In 4WDing we don’t usually need to know our altitude, other than for curiosity purposes, or statistics for trip reports. It really doesn’t play a role in our navigation. So, I’m only going to touch on it quickly. Back in my days as a Civil Engineering draftsman (many moons ago) we used to deal with the AHD (Australian Height Datum). Mean sea level for 1966 - 1968 was assigned the value of zero on the Australian Height Datum in 1971. GPS height is based on the ellipsoid model of the Earth’s surface. The difference between AHD and the GDA94 ellipsoidal heights is typically within ±0.5m. Again, unless you’re a surveyor, this is not a big deal, because the standard GPS altitude accuracy is about ±10m. Happy Navigating. Greg Conlon |
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