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Because of movement of the earths crust, the "datum" or elevation reference system used to define water levels previous within the Great Lakes-St. Lawrence River system must be adjusted every 25 to 35 years. The current datum was known as the International Great Lakes Datum, 1955 (IGLD 1955). These few questions briefly explains the development and impacts of the revision to this datum, known as the International Great Lakes Datum, 1985 (IGLD 1985). The date, 1985, is the central year of the period 1982- 1988 during which water information was collected for preparing the datum revision.
Click to return to the IGLD Datum Home Page.
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