What is low water datum?
Low water datum is a height above sea level used to measure water depth. All ports on Lake Erie use a LWD of 568.6 feet above sea level. This provides a constant reference point when using depths shown on a chart. Each lake has a different LWD. Water levels can change hourly due to winds, atmospheric pressure or rainfall and usually vary from +3.0 to -3.0 LWD.