Cite as: 521 U. S. 1 (1997)
Opinion of the Court
The question, then, is whether the 1949-1950 survey party's conclusion that Dinkum Sands is three feet above mean high water, taken together with visual observations of Dinkum Sands above water, undermines the Master's factual finding that Dinkum Sands is "frequently below mean high water." Report 309. It does not.
Alaska emphasizes that Dinkum Sands has been observed "many times . . . above water" and only "occasionally . . . submerged." Alaska Exceptions Brief 44. But visual observations of Dinkum Sands are not dispositive; the question is not whether Dinkum Sands is above or below high tide on any given day, but where the feature lies in relation to mean high water. To address precisely this problem, the parties jointly commissioned a $2.5 million study to calculate mean high water in the feature's vicinity and to determine the feature's elevation in relation to that datum. First, using a year of tidal readings, the National Ocean Survey computed a mean high-water datum at Dinkum Sands and calculated an error band to account for the fact that the level would ordinarily be based on 19 years of readings. Second, an engineering firm measured Dinkum Sands' highest points in March, June, and August 1981.
Comparing the feature's highest elevation measurements to the mean high-water level, the Master found that Dinkum Sands was not above mean high water at any time it was surveyed. The two highest points of the survey were within the error band for the mean high-water level, but the Master found this fact to be of little weight because the measurements were likely taken from piles of gravel disturbed by the March measurements, rather than from Dinkum Sands' true highest points. Alaska continued to measure Dinkum Sands in relation to mean high water in 1982 and 1983. The feature was found to be above mean high water on a visit in July 1982. By September, the feature had fallen in elevation, possibly by more than a foot, see Report 281-282, placing it below the mean high-water datum.
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