Ellipsoidally referenced surveying uses high-accuracy GNSS heights of the vessel to measure the seabed relative to the ellipsoid, then converts to chart datum through a separation model. Because the vessel's height is measured directly, errors from tides, squat, draft change and long-period heave largely drop out. It depends on RTK or PPK heights and a sound separation model, and it removes the need for zoned tides far from gauges.
We're running ERS on this job, so no tide gauges, just the separation surface and PPK.
ERS moves the vertical risk from the tide gauges to the separation model, so a bad model gives a perfectly consistent but wrong survey.
Also heard as
- ellipsoidally referenced survey
- ellipsoid-referenced survey
- GNSS tides
Related terms
Separation model
A grid giving the height difference between the GNSS ellipsoid and chart datum across an area.
RTK
Real-time kinematic: GNSS positioning that uses carrier-phase corrections from a base station to reach centimetres live.
PPK
Post-processed kinematic: GNSS positions computed after the survey from logged rover and base data, at centimetre level.
Chart datum
The low-water level from which charted depths are measured, set so actual depth is seldom much less than shown.
Tide correction
The adjustment that removes the height of the tide from measured depths so they refer to chart datum.