A surface that converts ellipsoid heights to chart datum, or to another vertical datum, at any point in an area. It is built from tide stations tied in by GNSS, hydrodynamic tide models and a geoid model, and hydrographic offices publish them for many coasts. Its accuracy, often a decimetre or so offshore, feeds straight into the vertical uncertainty of an ellipsoid-referenced survey.
Separation model's only good to 15 centimetres out here, that eats half our TVU budget.
Also heard as
- separation surface
- datum separation model
- SEP
Related terms
ERS
Ellipsoidally referenced survey: depths taken from GNSS ellipsoid heights and converted to chart datum with a separation model.
Chart datum
The low-water level from which charted depths are measured, set so actual depth is seldom much less than shown.
TVU
Total vertical uncertainty: the depth part of a sounding's uncertainty, judged against a depth-dependent limit.
RTK
Real-time kinematic: GNSS positioning that uses carrier-phase corrections from a base station to reach centimetres live.