Datum Discrepancy at the Quay Wall
You are tasked with a pre-dredge survey of a berth pocket measuring 220 m by 45 m. The design drawings specify depths to Chart Datum (CD), and the local Lowest Astronomical Tide is 0.0 m CD. Your survey vessel uses an RTK GNSS system referenced to a published geoid model, and you have a real-time tide gauge at the berth reading 2.35 m above CD. During processing, you notice that the RTK-derived ellipsoidal heights minus the geoid separation at the quay wall control point disagree with the published charted land level by 0.18 m. The tide gauge calibration certificate expired two weeks ago, and the dredging contractor is pressuring you to release preliminary volumes for a tender. You have a total of 9 hours of field time remaining before the vessel demobilises.
1. What is your immediate priority before releasing any depths or volumes, and how do you reconcile the 0.18 m discrepancy?
Read the worked answer
Immediate priority: Do not release depths or volumes to CD until the vertical datum is verified. A 0.18 m error is significant relative to typical dredge tolerances (often ±0.25 m) and could alter pay volumes materially.
Reconciliation steps:
- Re-observe the quay wall control point with a static or long-occupation GNSS session and compare against the published benchmark using levelling, not just the geoid model. The 0.18 m could be a geoid model limitation, a benchmark error, or an RTK antenna height blunder.
- Check the RTK antenna offset (measured vs entered) and the vessel's dynamic draft/settlement corrections.
- Treat the expired tide gauge certificate as a data-quality flag: either recalibrate (if time allows) or cross-check the gauge against a nearby secondary gauge or a verified tide prediction over a full tidal cycle.
- Document the resolution in the survey report and state the vertical uncertainty achieved.
Only after the datum is confirmed should preliminary volumes be issued, and they must be labelled preliminary with stated uncertainty.
2. The contractor demands volumes within 4 hours. How do you respond professionally, and what do you deliver?
Read the worked answer
Response: Decline to certify final volumes but offer a defensible interim product. Explain that releasing unverified CD depths risks both under-dredging (safety/navigational hazard) and over-dredging (cost/dispute), and that the Authorized Surveying Officer's duty is to the accuracy of the deliverable, not the schedule.
Deliverable within 4 hours:
- A clearly labelled preliminary volume estimate with an explicit vertical uncertainty statement (e.g., ±0.20 m) and the datum assumptions used.
- A caveat that the tide gauge certificate is expired and the geoid/benchmark discrepancy is under investigation.
- A written plan and timeline for resolving the discrepancy within the remaining field time.
This protects the contractor's planning while preserving the integrity of the final certified survey. Record the request and your response in the project log.