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A Unified Datum for the Antarctic Geodetic Infrastructure

Background Paper


Background

A datum is essential for all activities that need to locate a position on the earth’s surface. Positions are normally defined by latitude and longitude, or by map grid coordinates that are directly related to the latitude and longitude. However, latitude and longitude alone do not give a unique position, as the same position on the earth’s surface can have different latitude & longitude, depending on which datum is used.

A datum is the reference framework for positions and was traditionally defined by one or more "origin" survey stations with well-defined positions, together with a suitable mathematical model of the earth’s size and shape (an ellipsoid). Traditionally, some form of astronomical observation determined the origin positions and if there was only a single origin station, orientation of the datum was also supplied by astronomically determined azimuth. These origin coordinates were then propagated throughout the region by conventional surveying techniques, supplying the fundamental coordinates for spatial data applications. For convenience, these datums were often locally implemented, meaning that a large region could have a number of datums and the coordinates of a point differ by many hundreds of metres when determined in terms of the different datums (e.g. see www.ga.gov.au/nmd/geodesy/datums/). These local datums may change not just with locality, but also with time, as updated positions are adopted (e.g. the Australian Geodetic Datum 1966 and the Australian Geodetic Datum 1984).

Antarctica is a classic example of diverse local datums. A number of nations have established geodetic datums for their various spatial data applications, but the datums vary considerably. In many cases the different datums are remote from each other, but in others they can overlap, giving rise to conflicting and confusing positions for the same points. This is of particular concern when time-based studies are being used (e.g. glaciology).

Global Datums

World Geodetic System 1984

The World Geodetic System 1984 (WGS84) is the reference frame used with GPS. It was developed by the United States Department of Defense and is widely used for navigation, mapping and charting. Stations around the world monitor the WGS84 system (figure 1). Initially, the WGS84 system had an accuracy 1-2 metres (NIMA, 2000, pp2-3), but since then it has been refined on several occasions to bring it into close alignment with the International Terrestrial Reference Frame (ITRF) (ibid, pp2-5).

Prior to 1 May 2000, WGS84 point positions obtained by a single GPS receiver may have had a positional error of up to 100 metres (at 95% confidence) due to the deliberate degradation of the signal (Selective Availability) by the US Department of Defense. However, on 1 May 2000, Selective Availability was removed from GPS, so that WGS84 point positions obtained by a single GPS receiver now have a positional error of about 10-20 metres. Better relative accuracy can be obtained by using differential techniques, but the resulting positions still depend on the position and datum of the base station. It is not possible to routinely obtain WGS84 positions with accuracy better than about 10 metres.

GPS tracking stations

Figure 1: GPS tracking stations

International Terrestrial Reference Frame

With the advent of accurate space positioning systems, such as the Global Positioning System (GPS), rapid and accurate determination of positions on the earth’s surface in a single datum has become possible. Since the late 1980s a combination of space-based positioning techniques has been used to regularly produce a globally distributed set of very accurate positions known as the International Terrestrial Reference Frame (ITRF). These are in effect the "origin stations" for a global datum.

ITRF is a global, accurate (a few cm), readily accessible reference frame recommended by the International Association of Geodesy). It is provided by the International Earth Rotation Service (IERS) and is based on the combination of sets of coordinates and velocities of points on the surface of the Earth, derived from observations of space-geodetic techniques (VLBI, LLR, SLR, GPS (since 1991) and DORIS (since 1994)) (lareg.ensg.ign.fr/ITRF/ITRF97/). More than 550 stations located at 355 well-distributed sites around the world are used in this analysis (see figure 2) and local ties between collocated stations are included (Boucher et al, 1999). The ITRF is periodically refined to take advantage of additional observations and techniques, but the changes between versions are typically of the order of a centimetre. The current version is ITRF97 and the next will be ITRF2000.

The ITRF is being used by many countries to establish or update their local datum to an earth-centred (geocentric) datum that is compatible with GPS positioning and international functions such as marine and air transport, environment, science and defence (e.g. The Geocentric Datum of Australia 1994, the New Zealand Geocentric Datum 2000).

Sites contributing to the ITRF

Figure 2: Sites contributing to the ITRF

(lareg.ensg.ign.fr/ITRF/network.html)

Regional Datums

A common geocentric datum has already been adopted for some areas where a many countries share a continental region (e.g. North American Datum 1983, European Datum 1989).

In the Australian Antarctic Territory (AAT), ITRF positions have been adopted for the permanent GPS tracking stations at Casey, Davis and Mawson and used with the GRS80 ellipsoid to define the Australian Antarctic Geodetic Datum 1998. This provides a single, accurate, globally compatible datum for the AAT. Records of activities in terms of previous datums can be related to this datum for comparison and exchange of information. As further observations are added and the ITRF is further refined in the future, this datum can be updated to any particular version of ITRF, with only a small adjustment necessary to account for the change in the reference frame (a few centimetres).

With suitable geodetic GPS observations and post-processing, ITRF positions may be obtained anywhere with an accuracy of a few centimetres. Although the post-processing required to obtain such positions is meticulous and highly specialised, on-line services are now available on the Internet, which allow suitable GPS data to be uploaded and processed and the ITRF positions returned, generally in ten or fifteen minutes.

It is therefore comparatively simple to establish an accurate ITRF based datum by observing with geodetic GPS receivers on key sites in the geodetic survey network; computing the ITRF positions; and then propagating these positions through the network by readjustment.

 

Antarctica

In Antarctica the WG-GGI originally resolved to use the International ellipsoid with locally determined origin stations as the reference frame. Many local origins were established from astronomic observations and it was assumed that the geoid and ellipsoid coincided at these points.

As a consequence of space geodesy and the wide use of GPS for modern surveys, pragmatically, a geocentric datum has replaced the local datums. Some old networks have been recomputed using ITRF at key points, but many old surveys have not yet been converted.

ITRF97 includes a number of Antarctic GPS base stations. In 1999, Reinhard Dietrich submitted positions of all observed GPS from the SCAR surveys to IERS for inclusion in ITRF2000.

To avoid a plethora of individual national datums being used across Antarctica (although they may be based on ITRF) and to avoid the use of a dynamic system of datums, it is proposed to standardise on the use of ITRF2000 (at an epoch of 2000.0) as an Antarctic datum for the geodetic infrastructure and as the basis for spatial data. Each permanent base station defined in ITRF2000 will also have computed velocities so that observations taken at different epochs in the future can be translated back into the Antarctic 2000 datum.

Summary

A common geocentric datum for Antarctica would facilitate the use of spatial data for all applications.

ITRF uses a large number of well-distributed global positions and is based on the best available global data. It can be used for all applications, from mapping and navigation to high accuracy geodesy and geodynamics. Where necessary, ITRF positions can be re-labelled as WGS84, without loss of accuracy.

Recommendation

Recognizing:

Noting that the International Terrestrial Reference Frame (ITRF) is an accessible, global datum that can be realised accurately,

Recommends that:

  1. The SCAR WG-GGI adopt the International Terrestrial Reference Frame 2000 (ITRF2000) at an epoch of 2000.0, together with the GRS80 ellipsoid, as the geodetic datum for all Antarctic activities.
  2. This Antarctic ITRF2000 datum be known as the SCAR Geodetic Datum 2000 (SCARGD2000)

 

References

National Imagery and Mapping Agency, 2000, "Department of Defense World Geodetic System 1984 – Its Definition and Relationships with Local Geodetic Systems", NIMA TR8350.2, Third Edition, 3 January 2000.

Boucher C., Z. Altamimi, P. Sillard, 1999, "The International Terrestrial Reference Frame (ITRF97)", IERS technical Note 27, central Bureau of IERS, Paris, France.