We know the height of the Great Lakes (an average of around 260 feet) because of the International Great Lakes Datum. What’s that? It is a method of measuring heights that vary over time, known as a Dynamic Height System. All water surfaces are described using dynamic heights. The last time a major survey of the surface depth of the Lakes was undertaken was in the mid-1980’s. This base height is called IGLD 1985. These kind of dynamic height calculations are the normally-used reference for waterways throughout the world.
These measurement projects are carried out, on the Great Lakes and many other bodies of water in the United States, by the NOAA, the National Oceanic and Atmospheric Administration. Such data is compiled specifically by the NGS (National Geodetic Survey) of the NOAA. This agency is responsible for the National Spatial Reference System, the resource used to coordinate the task of gathering, relaying, charting and mapping the data on all geological and man-made aspects of the landscape that are chosen to be mapped.
One of the tasks of the NOAA is the careful, systematic and consistent recording of currents, tides and water levels. More than 50 gauges used to measure water level have been placed and are monitored in all parts of the Great Lakes region. One such gauge may soon be installed by the NOAA near Sebewaing Park. The determination of official height data for the IGLD rests with the NOAA in conjunction with their Canadian counterparts and partners.
The International Joint Commission sees to the administration of the International Treaty pertaining to the managing of the waters of the Great Lakes.All information officially published concerning these water levels is closely coordinated between Canada and the U.S. to guarantee consistency.
Part of the adjustments necessary in the numbers are being caused by the current tilt being undergone by the entire Great Lakes area. This angling is caused by glacial isostatic adjustment, the name given to the rebounding of the land, still occurring after many millennia. As a whole, the basin of the Great Lakes, a massive area, to be sure, is slanting at the rate of approximately 2 feet per 100 years.