Knowledge of the surface topography of the Antarctic is very important because it is used as input to estimate the surface temperature, precipitation and katabatic wind intensity and direction (Reference Marsiat and BamberMarsiat and Bamber, 1997). Elevation data can be used to determine the locations of ice divides, drainage basins and ice-flow directions (Reference DrewryDrewry, 1983). In addition, together with ice-thickness data, driving stress, deformational velocity and subsurface and basal conditions may be inferred (Reference Bamber and BindschadlerBamber and Bindschadler, 1997).

Ice flows very slowly in the flat, inland polar summit regions, and thus the dynamics are difficult to measure. The surface and bed topography at Dome Concordia, Antarctica, (Dome C; Reference Tabacco, Passerini, Corbelli and GormanTabacco and others, 1998; Reference Capra, Cefalo, Gandolfi, Manzoni, Tabacco and VittuariCapra and others, 2000; Rémy and Tabacco, 2000), Dome Fuji, Antarctica, (Dome-F Deep Coring Group, 1998) and the summit of Greenland (Reference Hodge, Wright, Bradley, Jacobel, Skou and VaughnHodge and others, 1990; Reference Hvidberg, Keller, Gundestrup, Tscherning and ForsbergHvidberg and others, 1997) have been determined in recent years. So far, however, little has been published on the surface topography at Dome Argus (Dome A), Antarctica.

Dome A, the highest ice feature in Antarctica at just over 4000 m elevation, is located near the centre of East Antarctica. Details of its morphology were first determined by the Scott Polar Research Institute (SPRI)–US National Science Foundation (NSF)–Technical University of Denmark (TUD) airborne radio-echosounding program, 1967–79. As a contribution to the International Trans-Antarctic Scientific Expedition (ITASE), Chinese National Antarctic Research Expedition (CHINARE) glaciological studies were conducted along a traverse line from Zhongshan station to Dome A during the austral summers from 1996/97 (13th CHINARE) to 2004/05 (21st CHINARE) (Reference QinQin and others, 2000, Reference Qin2004; Reference Xiao, Ren, Qin, Li, Sun and AllisonXiao and others, 2001, Reference Xiao, Allison, Ren, Qin, Zhang and Li2004; Reference ZhangZhang and others, 2002; Reference Ren, Sun and QinRen and others, 2004).

During the 21st CHINARE, more than 20 global positioning system (GPS) sites were established at approximately 50 km intervals along the 1228 km traverse route, to monitor the surface ice velocity, which will be reported elsewhere. Here, we present the surface topography of the summit of Dome A measured by real-time kinematic (RTK) GPS, a tool that is well suited to surface topography measurements in the interior of the Antarctic as it allows high-precision measurements to be made relatively quickly and with minimum logistical support.

Based on an estimate of the summit position from the topography around Dome A, obtained from the RADARSAT-1 Antarctic Mapping Project (RAMP) digital elevation model (Reference Liu, Jezek and LiLiu and others, 1999), a survey was carried out during 9–11 January 2005 using a total station to obtain a second estimate of the summit position. A camp was set up at this new position which was established as a reference station, initially occupied by a GPS receiver for 36 hours. The location of the reference station was calculated using Leica Geo Office (LGO) V1.0 point positioning software. The World Geodetic System 1984 (WGS84) ellipsoid coordinates were 80°22’01.62888” S, 77°22’22.90269” E, 4092.457 m.

Two Leica SR530 dual-frequency GPS receivers were used for the kinematic survey. A rover receiver was installed on an over-snow vehicle. The survey was carried out on a ‘star’ grid, centered on the camp, with a radius of 4–5 km. The sampling space was 200 m. The speed of the vehicle ranged from 4 to 8kmh^–1. After 5 days of fieldwork, more than 1000 points had been surveyed.

GPS data were processed using GAMIT/GLOBK software (Reference KingKing, 2002). During the data processing:

1. 1. International GPS Service (IGS) precise ephemerides were used;

2. 2. several IGS stations around Antarctica (e.g. CAS1, DAV1, MAW1, MCM4, PALM, SYOG and VESL) were tightly constrained (within 1 cm) at their ITRF2000 values while the site at Dome A was loosely constrained (within 100 m);

3. 3. an elevation cut-off angle of 15° was set;

4. 4. antenna phase centre variation corrections were applied;

5. 5. the ionospheric-free linear combination of the L1 and L2 frequencies was used;

6. 6. corrections were applied for both the solid-Earth and frequency-dependent tides;

7. 7. the dry component of the zenith tropospheric delay was estimated by the Saastamoinen model; and

8. 8. the wet component was estimated during the inversion, with zenith-delay adjustments every 2 hours.

The coordinates of the reference station were 80°22’ 01.64502” S, 77°22’22.90070” E, 4092.528 m. The difference between the GAMIT result and the LGO result was <1.0m horizontally, and <0.1 m vertically. All topographic point coordinates were corrected, based on the GAMIT result.

GPS coordinates were used to build the topography of the Dome A surface. Points with GPS errors >0.10 m in the vertical were eliminated. Taking into account the tracked- vehicle penetration into the ice between one passage and the next (estimated at ~0.10 m), our topography accuracy is estimated to be ~0.20m. Eventually, 480 points were used to build the topographic map of the Dome A summit area.

Surfer V7.0 software was used to generate a contour map of surface elevation (Fig. 1). The ‘kriging’ gridding technique was chosen. Contour levels are plotted at every 0.40 m, with highly smoothed contours based on the tested accuracy of the GPS data. The dome surface and the topographic summit are defined. The lowest surface height within the grid is 4090.2 m, giving an elevation range of 2.7 m over the entire 60 km² area. The surface slope is <0.08% in the summit area, indicating a very flat and smooth surface.

Fig. 1. Dome A surface topographic map based on GPS data from the 21st CHINARE. Heights are relative to the WGS84 ellipsoid. Contour interval is 0.40 m. The black triangle indicates the position of the summit of Dome A.

Because of logistical constraints, an area of only ~60 km² was measured during 2005. A larger area is expected to be mapped during the expeditions of the International Polar Year (IPY, 2007–08). In addition to the kinematic GPS survey, ground-based ice-penetrating radar measurements were made. As ice flow is related to ice thickness and bedrock slope, precise surface morphology is important to determine the ice divide and ice-flow parameters. Precise surface topography, bedrock morphology and internal layering geometry are all important for the future selection of the best site for deep ice-core drilling at Dome A. The GPS data and precise topography at Dome A will also provide ground truth for satellite missions such as ICESat (Ice, Cloud and land Elevation Satellite), GRACE (Gravity Recovery and Climate Experiment) and CryoSat.