Data Acquisition, Processing and Geo¬Statistical Evaluation

The advantage of Geosat data from the Gendetic Mission (1985–86) is their denser ground-track pattern resultant from the fact that the satellite was allowed to drift instead of being forced to repeat wider spaced orbits exactly (as in the Exact Repeat Mission 1987–89). Data processing, projection, the atlas-mapping problem and geostatistical evaluation are treated in Reference Herzfeld, and Matassa,Herzfeld and Matassa (in press a). We acquired data from the Ice Sheet Altimetry Group at NASA Goddard Spare Flight Genler (H.J. Zwally, J. DiMarzio and co-workers). Data processing at NASA GSFC included reiracking, reduction using Goddard Earth Model (GEM) T2 orbits, correction for atmospheric effects, water vapour and solid Earth tides, and slope corrections (for methods see Reference Herzfeld, and Matassa,Herzfeld and Matassa, in press a). Only points with retracked and slope-corrected data are used in mapping Geographic coordinates of data locations are convened to Universal Transverse Mercator (UTM) coordinates to facilitate rapid interpolation and to reduce distortion (Reference Snyder,Snyder, 1987; Reference Herzfeld, and Matassa.Herzfeld and Matassa, in press b).

Grid values are calculated using ordinary kriging with search algorithms developed for geophysical track-line data. Ordinary kriging is a least-squares-based optimum interpolation technique from geostatistics, the theory of regionalized variables, and proceeds in two steps. spatial analysis (variography) and estimation. The relatively high error in radar altimeter data over ice surfaces requires special precautions in the variography (see Reference Herzfeld, and Matassa,Harzfeld and Matassa, in press a). A grid spacing of 3 km is used; elevations are above the World Geodetic System 1984 (WGS84) ellipsoid.

Mertz and Ninnis Glacier Maps: Presentation and Interpretation

The resultant topographic map of the Mertz and Ninnis Glacier area (142.0−148.5° E,66.5−68.5 S) is given in Figure 1. The map is based on a digital terrain model (DTM) with 3 km grid spacing. Surface elevations are in meters above WGS84. Isohypses are drawn for 0,10,20,...,140 m (in steps of 10m) and for 200, 400,...,1400 m (in steps of 200 m).

To enhance the details in the glaciers and ice tongues, enlarged, individual topographic maps of Mertz Glacier (143.5−146.0° E, 67.0−68.0° S) and Ninnis Glacier (147.0−-148.5° E, 68.0–68.0° S) are given in Figures 2 and 3. In both maps, isohypses are drawn for 0,10, 20,....,140 m (in steps of 10 m). for 140, 160,...,400 m (in steps of 20 m) and for 400, 600, 800 m (in steps of 200 m). A 20 m spacing is used between 140 and 400m to map the transition between the level of the ice cap and the level of the ice tongues.

Mertz Glacier is at UTM 360 000– 440 000 E, 7 525 000–7 440 000 S. Its drainage is a broad valley that is distinguishable at the southern map boundary. The sides of the western valley are transected by erosional features. Al its head. the glacier is fed by a narrow valley, approximately 4 km wide. The distance from the 80 m contour line at the head to the ice front is 85 km. In the eastern arm of Mertz Glacier there is a 20m overdeepening. centered at (415000,–751 000). This is located below the steepest part of the valley walls. It is not an interpolation error, since the contours are really smooth.

Mertz Glacier is about 27 km wide. The glacier tongue appears to extend about 45 km seaward of the coastline (see discussion below). The grounding line of Mertz Glacier is probably located in the vicinity of the 40–50 m contour. The 60 m contour still exhibits the indentation of the valley that continues subglacially from the valley leading to the head of the glacier, while the 50 m contour does not. The 40 and 50 m contours show the signs of the eastern side valley. At 30 m, the tongue is definitely floating.

The following surface slopes were calculated along local gradients of the Mertz Glacier map: 0.049° (= 8.57E-4) between the 30 and 0 m contours for the floating tongue; 0.078° in the direction of the head valley and 0.143° farther east (=1.36E-3 and 2.5E-3, respectively) between the 40 and 70m contours on the grounded ice; 0.286°(5E-3) between the 80 and 140 m contours in the head valley. The western side of the Mertz Glacier valley has a slope of 0.573° (= 1E-2) between 80 and 140m, and a slope of 0.8275°(= 1.4E-2) between 140 and 400 m; above that, the slope increases with increasing elevation. The steepest area in the flanks of Mertz Glacier is located southeast of the overdeepening; it is inclined 2.0045°(= 3.5E-2) on average between 50 and 400 m. with a steeper section (2.0617°= 2.6E-2) between 50 and 140 m, and a shallower section (1.2412° = 2.16E-2) between 140 and 400 m. In this region, the slope lessens above 400 m elevation, Ninnis Glacier is at UTM 500 000– 540 000 E, 7550 000–7580000 S. The glacier tongue appears to extend about 10– 15 km seaward from the coastline (the coastline being identified by the steep gradient of contours). Ninnis Glacier lies below a sleep cliff, at 90 m above WGS84 and lower. The entire extension from the break in slope at the fool of the cliff to the 0 m contour line is 20 km. (Notice that the 0 m contour does not coincide with sea level, because the reference level is the ellipsoid) Ninnis Glacier is about 35 km wide. It does not really have a “tongue” anymore (as opposed to 1913), as noted by Reference Wendler,, Ahlnäs and Lingle,Wendler and others (1996).

Discussion on Accuracy and reliability

Mapping of atlas type and careful analysis of the spatial structure and the distribution of noise levels in RA data allows us to extend the limits of use of altimeter data for mapping. Bamber (1994) produced a map of Antarctica (from ERS-1 altimeter data) with 20 km grids, reliable only in areas with a slope of less than 0.65° according to the author. The drainages of Mertz Glacier and the Mertz and Ninnis Glacier area are considerably sleeper than that. Accuracy depends on topographic relief, as calculated in Reference Herzfeld,, Lingle, and Lee,Herzfeld and others (1993, Reference Herzfeld., Lingle, and Lee,1994). with sub-meter accuracy on ice Streams, However, the accuracy of a kriged map in areas of high topographic relief refers to the pointwise error rather than to the error of the “mean” surface elevation mapped. The “pointwise error” is the probable difference between a point on the map (estimated surface elevation) and a radar-altimetry-derived surface elevation; it depends on the averaging process of kriging. Shape and elevation of the surface at the Mertz Glacier drainage and on the glacier itself, however, are more accurate than inferred from the noise calculation, which is best conceived from inspection of the maps. The contour lines are smooth on the resolution of the 3 km grid. Smooth contour lines indicate a continuous or differentianle surface function with low error; little islands and edging contour lines indicate a rougher surface function with higher errors. That means the maps are reliable also in areas of steep terrain. An area with high relief is located on the western side of Mertz Glacier. Notice that the grid sparing of the subarea enlargements is the same 3 km as for the large map, and that a wealth of information only becomes available in the enlargement. Mertz Glacier tongue appears to extend about 40 km seaward of the coastline, Ninnis Glacier tongue about 20 km.

Neither satellite altimetry nor kriging is a tool designed to track the location of an ice cliff. Because of the effect described in Reference Thomas,, Martin, and Zwally,Thomas and others (1983) and Reference Partington,, Cudlip,, Melntyre, and King-Hele.Partington and others (1987) (snagging of altimeter), the location of the ice edge is systematically wrong, differently so in descending and ascending orbits. Reference Thomas,, Martin, and Zwally,Thomas and others (1983) attempted to trace the ice edge from altimeter data, designing a technique not used here. In the Ice Data Record the ice edge is in the middle of both errors. Kriging employs a moving window averaging procedure, which results in smoothing of steep edges. Consequently, the exact location of the ice edge cannot be inferred from altimetry, but the cliffs may be identifiable as a sequence of dense contours. Comparing our map with the images in Reference Wendler,, Ahlnäs and Lingle,Wendler and others (1996), however, we note that the results of kriging altimeter data are surprisingly good. On their map in Figure 2. Mertz Glacier tongue extends about 80 km off the (averaged) coastline, Ninnis Glacier tongue 20–30 km. (Also. 1993–85 = 8 years, 0.9 km year⁻¹ for 1962–93; 8 × 0.9 km = 7.2 km or 8 × 1.2 km = 9.6 km advance since 1985–86, according to Reference Wendler,, Ahlnäs and Lingle,Wendler and others (1996), assuming constant rale of advance.)

This indicates that, while SAR images are Superior to altimetry-based maps for location of the ice edge, changes in advance/retreat can still be monitored from satellite-altimeter derived digital-terrain models (DTM), which are available for a longer time-span (since 1978; Seasat mission). SAR imagery shows surface features in the floating ice tongue. The slope and elevation of the glacier surface are lost in die grey-shaded image. Additionla information on ice flux can be derived from the surface elevation in the altimetry-based DMSs, calculated for the drainage basins.

Another advantage of RA data over SAR data is that records have been available since the 1978 Sensat mission. A maximal amount of information may be expected from a quantitative combination of SAR and RA data.

12 May 1997