Height Datums on the Mississippi and Illinois River Systems: An Inconvenient Feast
For historical reasons, numerous vertical datums are used along the Mississippi River and its tributaries in Illinois (e.g., Memphis Datum, Mean Gulf Level, and the Fourth General Adjustment of 1912). While most of these are of historical interest only, one—the Fourth General Adjustment of 1912—is still actively used by the U.S. Army Corps of Engineers for water-level measurements in the upper Mississippi. Approximate shifts between these systems and NAVD 88 were determined by identifying benchmarks with valid heights in both datums. These height differences are not small; for example, differences of nearly 8 ft are observed for the Memphis Datum, and even the Fourth4th General Adjustment of 1912 shows a maximum datum shift of nearly 0.8 ft.
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