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The ETOPO5 file for the northern hemisphere is an array of
elevations or depths, spaced at 
of arc. We extracted
4 
subsets, starting 56 rows down from the North Pole, and starting at the
first column, and at multiples of 1024 columns later. Fig. f:etopo4 shows the fourth section. Then we tried gzip,
progcode, and sp_compress on them, with the results
shown in Table t:etopo. sp_compress looped on section 2,
so we used progcode for that one. This data does not compress so
much since the elevation range is much greater (about 19,000).
Sp_compress or progcode are still better than
gzip.
Table 9: Comparison of Various Compression Methods on ETOPO5 Sections