NOAA chart soundings are referenced to a chart datum rather than today’s instantaneous water surface. Great Lakes charting uses low-water reference concepts maintained through the International Great Lakes Datum system.
A rough planning comparison may combine charted depth with an appropriate water-level departure from datum, but only when units and datums match. Survey age and bottom change still matter.
Keep an independent depth sounder where appropriate, use current official charts, and maintain under-keel clearance for waves, squat and uncertainty.