Operational Guide · Registers & Logbooks

# Datum Shift and Chart Reference in ECDIS

A 2026 analysis of 5,000 bridge team assessments found recurring gaps in understanding of datum: a silent error, because the GPS position still appears plausible on the ECDIS screen.

datum shift ECDIS WGS84 chart reference system

## Operational Explanation

Many nautical charts worldwide were originally produced on local geodetic datums different from WGS84 (the reference system used by GPS); the shift to WGS84 for ENCs derived from these charts can be unknown or unreliable, generating small but significant positional differences between adjacent ENC cells converted with slightly different offsets.

The risk is not theoretical: a 2026 analysis of roughly 5,000 bridge team assessments found that misunderstanding of WGS84 discrepancies, sounding datums and the implications of vertical datum is a recurring error, with a direct impact on under keel clearance calculation, air draft assessment and overall positional accuracy: factors that concretely increase the risk of grounding.

## Regulatory Reference

There is no single IMO requirement mandating a specific datum-verification procedure; warnings about ENC cells not accurately referenced to WGS84 are included in the cell's own notes, and industry guidelines (e.g. the ECDIS Procedures Guide) recommend cross-checking position with alternative navigation techniques near hazards.

## Scope of Application

Every ship navigating with ECDIS as the primary means of navigation, with particular attention to areas where charts are derived from historical surveys on local datums not reliably converted to WGS84.

## Procedure / How to Complete It

1. Verify the ENC cell notes relating to datum and its reliability of conversion to WGS84 before placing full reliance on the displayed position.
2. Confirm the GPS position shown by the ECDIS using alternative navigation techniques (radar, visual, soundings) when approaching the coast or known hazards, especially on cells with uncertain datum.
3. Understand the difference between horizontal datum (position) and vertical datum (depth/height), since both affect critical calculations such as under keel clearance and air draft.
4. Train the bridge team specifically on interpreting datum warnings, not treating them as technical notes of secondary importance.
5. Integrate datum verification into the appraisal phase of passage planning for routes crossing areas with cells of known uncertain datum.

## Practical Example

Example: during the appraisal phase of passage planning for a coastal route in an area with ENC cells derived from historical surveys, the bridge team checks the datum notes, plans cross-checks of position with radar near the coast, and does not place exclusive reliance on the GPS position displayed by the ECDIS.

## Real Cases

The 2026 analysis conducted on roughly 5,000 bridge team assessments found that misunderstanding of safety contours, datum, ENC updates, alarms, display levels and data-quality indicators creates a high risk of grounding, unsafe voyage planning, alarm fatigue and degraded situational awareness: while acknowledging that ECDIS has overall reduced groundings compared with paper-chart navigation, significant room for improvement remains in crews' practical understanding of the system.

## Common Mistakes Mistake Library

Mistake | Consequence | How to avoid it
--- | --- | ---
GPS position displayed by the ECDIS considered reliable without checking the datum notes of the ENC cell in use | Positional error not detected in areas with unknown or unreliable conversion to WGS84 | Always check the cell's datum notes before placing full reliance on the displayed position
Horizontal and vertical datum treated as a single concept, without distinguishing their respective implications | Errors in calculating under keel clearance or air draft due to confusion between the two types of datum | Train the bridge team on the distinction between horizontal datum (position) and vertical datum (depth/height)
Datum verification not integrated into the appraisal phase of passage planning for routes in areas of known risk | Bridge team not specifically prepared before navigating in an area with uncertain datum | Always integrate datum verification into the appraisal phase for routes crossing areas known for this risk

## PSC Observations

Understanding of datum and its implications is not typically the subject of direct, specific PSC verification, but may emerge indirectly in the general assessment of the bridge team's competence in ECDIS use during an inspection.

## Operational Tips

- Never treat the ECDIS GPS position as automatically reliable: always check the cell's datum notes, especially near the coast.
- Train the bridge team to clearly distinguish horizontal and vertical datum: confusing the two generates different but equally dangerous errors.
- Integrate datum verification as an explicit step of the passage planning appraisal phase, not as an implicit note.

## Checklist

- ENC cell datum notes checked before relying on the displayed position
- Cross-checking of position with alternative techniques applied near coast/hazards on cells with uncertain datum
- Distinction between horizontal and vertical datum understood by the bridge team
- Datum verification integrated into the passage planning appraisal phase
- Specific training on interpreting datum warnings included in the ECDIS programme

## FAQ

Why are some ENC cells not accurately referenced to WGS84?
Because they derive from historical cartographic surveys conducted on local geodetic datums, for which the shift to WGS84 can be unknown or unreliable, generating positional differences even between adjacent cells.
What is the difference between horizontal datum and vertical datum?
Horizontal datum concerns the reference for position (latitude/longitude); vertical datum concerns the reference for depths and heights (e.g. sounding datum). Both affect critical calculations such as under keel clearance and air draft, but with distinct implications.
What did the 2026 analysis find regarding bridge teams and datum?
Across roughly 5,000 assessments, misunderstanding of datum discrepancies emerged as a recurring error, contributing to a high risk of grounding, unsafe voyage planning and degraded situational awareness.
🎬 Additional photos, videos and interactive diagrams for this topic will be available in a future version of the platform.

## Related Topics

ECDIS (Electronic Chart Display and Information System)Electronic Navigation and GNSS Resilience

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