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Navigating Chile's Southern Ice Cap
A SPATIAL CHALLENGE
Months earlier, during our extensive preparations, my initial inquiries to the Chilean authorities about maps of the region provided us with a set of poor photocopies from a 1:500,000 series of topographic maps. Clearly, these would not suffice for glacier travel or planning an approach route. We wanted to enter onto the 400-kilometer-long ice cap from the northern access point at Jorge Montt glacier, one of the SPI's 48 distinct glacial outflows that combine to cover an area of 13,000 square kilometers. These glaciers, though little known, are among the most dynamic in the world, with annual precipitation exceeding 700 centimeters and ablation rates as high as 6 centimeters per day.
Unfortunately, we could not find any maps of the towns in the area of Jorge Montt glacier's terminus, and every tourist guidebook stopped about 500 kilometers north of the SPI. After my initial despair, I realized that remotely sensed, geographic optical satellite data could be a perfect fit for our needs. However, the ephemeris data from U.S. government-owned satellite imagery would not be sufficient to rectify the imagery, and the U.S. government-supplied vector data sets were too coarse. I had a spectacular, crisp image, yet no way to georeference it. At this point, I decided that one of our expedition's goals would be to collect GPS control points to rectify the image. The resulting map could then be used as a base map for future climbers and explorers such as ourselves.
We continued our research, exhausting the international commercial satellite imagery archives and turning up only a handful of cloud-free scenes, as nearly perpetual cloud cover keeps the SPI obscured from satellites. We therefore purchased an optical satellite imagery scene from January 1986 because of its low cost and good image quality. We realized, however, that a dynamic glacial region such as this would have experienced significant geomorphic change in 13 years, so we still looked for other imagery.
Luck of the Draw. A stroke of good luck came our way when I won a synthetic aperture radar (SAR) image in a sponsored drawing. Given radar's capability to penetrate through clouds, I knew that this technology held great promise. Geographic coordinates of the SPI were provided to the SAR-based commercial satellite system operator and in January 1998 we acquired imagery for the northern part of the ice cap at a nominal resolution of 11 meters.
We were counting on the radar data, used with GPS, to guide us through the maze of eroded ice fins and bottomless crevasses we had to circumnavigate to reach the altiplano, or high plateau, of the SPI. Rectifying the radar and optical data to a common coordinate system, though, in an area that had no available maps better than 1:250,000 scale was going to be a problem. We were at the mercy of a small-scale map to guide us through large-scale features and could not verify our maps in any way before departure.
We then reached another breakthrough. We were loaned a full suite of software tools for processing our radar satellite data and discovered that one of the software's tools could use an existing digital elevation model (DEM) combined with ephemeris data from the radar satellite to georeference the imagery without any ground control.
Although we had only a coarse DEM at 1-kilometer resolution from a global DEM topographical dataset, and no assurances of what the final accuracy would be from the software manufacturer, it was our best chance at getting a basemap suitable for GPS navigation. The software's root-mean-square (RMS) error report, which is an estimate of the positional accuracy of the image, put the error at around one pixel, which would have to do. Next we matched the optical satellite image to the radar satellite basemap to generate a series of false color basemaps showing vegetation and snow conditions.
We planned to use the radar data to find the smoothest snow with the fewest crevasses and supplement that information with the snow conditions visible in the 1986 optical satellite data, so we could avoid areas of hard blue ice visible in the imagery. We prepared a set of paper basemaps in triplicate with a Universal Transverse Mercator (UTM), Zone 18 South, World Geodetic System of 1984 (WGS84) ellipsoid graticule and placed them securely in clear waterproof map pouches.