The step that converts depths measured from the moving sea surface into depths below chart datum by subtracting the observed water level at the time of each ping. Water levels come from tide gauges near the survey, adjusted for time and range differences across the area. Errors show up as steps between lines run at different stages of tide, and they often dominate the vertical uncertainty far from a gauge.
Tide correction from the harbour gauge won't hold out at the bar, use the zoned reducers.
Steps that line up with line start times point to tides; steps that line up with survey days point to draft.
Also heard as
- tide reduction
- tide reducers
- water level correctors
Related terms
Chart datum
The low-water level from which charted depths are measured, set so actual depth is seldom much less than shown.
Tide zoning
Dividing a survey area into zones, each with time and height adjustments for transferring tides from a reference gauge.
Bust
A vertical or horizontal mismatch between overlapping lines or surveys that exposes an error somewhere.
ERS
Ellipsoidally referenced survey: depths taken from GNSS ellipsoid heights and converted to chart datum with a separation model.