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International Great Lakes Datum (IGLD) is the reference system by which Great Lakes-St. Lawrence River Basin water levels are measured. It consists of benchmarks at various locations on the lakes and St. Lawrence River that roughly coincides with sea level. All water levels are measured in feet or meters above this point. Movements in the earth's crust necessitate updating this datum every 25-30 years. The first IGLD was based upon measurements and bench marks that centered on the year 1955, and it was called IGLD (1955). The most recently updated datum uses calculations that center on 1985, and it is called IGLD (1985). Measurements recorded in NGVD (1929) or IGLD (1955) need to be converted to IGLD (1985) measurements before they can be used in comparison situations. The table below displays the different readings for the OHWM in Indiana and how to convert NGVD (1929) and IGLD (1955) measurements to IGLD (1985) measurements. Conversion Equations