Definitions of Vertical Datum, CoordinateSystem, and CoordinateReferenceSystem VerticalCRSDictionary WaterSurfaceAtTimeInstantDatum Surface waters of the earth with water levels that vary over time. Gravity-related depths below the surface of water whose level varies due to wind, currents, tides, temperature, rain, drought, or other influences, at particular time instants. WaterSurfaceAtTimeInstantCS depth depth down WaterSurfaceAtTimeInstantCRS urn:x-nooa:ioos:def:crs:WSATI Surface waters of the earth with water levels that vary over time.