Reduced Tide

A reduced sounding is an observed depth corrected to chart datum by subtracting the height of tide at the place and time of sounding, the core step in reducing soundings during a hydrographic survey. The tide height comes from a local gauge or a transferred predicted curve, and removing it expresses every depth relative to one datum, usually Lowest Astronomical Tide or Mean Lower Low Water, so charted depths are comparable. The seed gloss tying reduced tide to atmospheric correction is wrong: reduction references soundings to datum, while meteorological residual is a separate correction.