The commonly used space-based datums are founded on the geocentric (earth-centred) 3-D Cartesian systems defined by a global ensemble of U.S. military tracking stations (specifically those of the Defense Mapping Agency -- DMA, now known as the National Imagery & Mapping Agency -- NIMA) that have been established over the decades since the start of the Space Age. As far as GPS is concerned this datum is:
The standard "GPS datum" is the so-called World Geodetic System 84 (WGS84), nominally defined as having (DMA, 1991):
This global geometric datum requires no reference ellipsoid or geoid model to be specified.
Ultimately, how well the GPS survey results relate to WGS84 is dependent on the quality of the Broadcast Ephemerides in the Navigation Message, and the quality of the Datum Station coordinates within the network. The accuracy of the GPS orbit information (related to WGS84 system) is at the few metre to few tens of metre level.
Because GPS surveying techniques only provide relative position, and hence are insensitive to absolute coordinates (that is, those relative to the WGS84 / geocentre origin), the coordinates of the datum station in effect defines the origin of the GPS survey in what is in reality a "near-WGS84" system. That is:
Hence, how well the Datum Station in the GPS network is related to WGS84 will influence how well the GPS results are referred to the WGS84 system. The implication therefore is that: (a) there may be a suitable set of published transformation parameters that will precisely relate the GPS results to a local datum, or (b) the transformation parameters relating the GPS survey to a local datum may have to be determined from the survey data itself.
In general, conventional geodetic datums are "locally defined". The label "local" may refer to an arbitrary datum of convenience, having relevance only in a very localised area, or it may be the more familiar "national" geodetic datum. There are in general two distinct components:
The datum is in general non-geocentric, the origin being the centre of the reference ellipsoid associated with the datum. However, the datum origin plays no part in defining the vertical datum. In relation to the horizontal datum:
In the Australian context, the datums are:
The Australian Geodetic Datum (AGD) for horizontal control comprising several thousand geodetic control stations distributed across Australia (Figure below). In particular the AGD is realised by the AGD66 coordinate set in the states of New South Wales, Victoria, Tasmania and the Northern Territory; and the AGD84 coordinate set in the remaining states of Australia (ALLMAN & VEENSTRA, 1984). The origin station is the Johnston Trigonometrical Station, with the assigned coordinates (NMC, 1986):
= 25°56'54.5515"S = 133°12'30.0771"E h = 571.2m (ellipsoidal height)
and the associated reference ellipsoid is the Australian National Spheroid, defined by the parameters:
semi-major axis 6378160m inverse of ellipsoidal flattening 298.25
Figure 1. Layout of the Australian Geodetic Survey.
The Australian Height Datum (AHD) for vertical control is realised by several thousand level benchmarks across Australia (Figure 2 below). However, the zero height datum surface is not coincident with any unique equipotential surface of the earth's gravity field, but is the surface of mean sea level as defined by 30 tide gauge sites around the coast of Australia (ROELSE et al, 1971).
The Australian datum has been redefined (section 12.1.5). The new datum will be known as the Geocentric Datum of Australia (GDA) (MANNING & HARVEY, 1994). As the name implies, this datum will be "geocentric" and hence very close to the WGS84 datum. However, what is even more remarkable is that the basic framework of the GDA is provided by a super-precise GPS survey linking together selected, evenly-distributed geodetic control stations (at an approximate spacing of 500km) with permanent GPS tracking stations around the world. Such a datum redefinition has already occurred in the U.S. with respect to the North American Datum 1983 (NAD83).
Figure 2. Lines of the Australian Levelling Survey.
Despite the different bases of the coordinate datums (whether GPS or conventional), all geodetic datums may be related to one another, or with any of the global space datums (including WGS84), by a number of transformation models. Associated with the transformation process are the conversion procedures relating ellipsoidal, topocentric and Cartesian coordinates (and any variance-covariance information), the projection coordinate systems and the issue of height systems.
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© Chris Rizos, SNAP-UNSW, 1999