Operational guide · Safety & equipment

Datum Shift and Chart Reference in ECDIS

A charted feature may be uncertain even when the GNSS fix is precise: horizontal position, sounding datum, height datum and quality/CATZOC are distinct issues to assess without improvised manual offsets.

datum shiftECDISWGS84chart reference system

Operational Explanation

ENCs and GNSS normally operate in WGS 84, but historical source surveys and transformations may make charted features and depths uncertain. The ship may appear precisely positioned against an uncertain object. Horizontal-position uncertainty, vertical sounding datum/tidal correction and height datum/air draught are not interchangeable.

Regulatory Reference

SOLAS V/19 and V/27, IMO ECDIS Guidance for Good Practice, IHO S-67 and ENC indications on CATZOC/quality, compilation/source information and cautions.

Scope of Application

ECDIS navigation, especially near hazards, coasts or historical surveys. Assess chart quality and fix quality separately.

Procedure / How to Complete It

Check ENC status, updates, cautions, CATZOC/quality and source information.
Assess horizontal position, charted depth/sounding datum and height datum separately.
Increase margins and fixing frequency where quality is limited.
Cross-check with radar/visual fixes, soundings and independent positioning near dangers.
Do not apply manual datum offsets to ENC/GNSS except under specific official instruction and controlled procedure.

Practical Example

The ship symbol aligns with the channel, but radar overlay and soundings disagree: the team considers charted-feature uncertainty and does not automatically conclude that the GPS fix is wrong.

What Typically Goes Wrong

Describing every datum uncertainty as GPS/ECDIS error, checking datum notes alone, or applying an unauthorised manual offset.

Common Mistakes Mistake Library

Mistake	Consequence	How to Avoid It
Describing every datum uncertainty as GPS/ECDIS error, checking datum notes alone, or applying an unauthorised manual offset.	Incorrect operational decision or regulatory status	Use the primary reference and ship-specific procedure

What the PSCO Checks

Updated ENCs, quality indicators, safety settings, independent checks, UKC datum/tide and caution management.

Operational Tips

Ask whether the fix, the feature or both are uncertain.
Use CATZOC as an indicator, not an absolute guarantee.
Keep UKC and air-draught datums separate.

Preparation checklist

Official updated ENC
CATZOC/source/cautions read
Horizontal and vertical datums distinct
Radar/visual/sounding checks
No improvised offset
Margins adapted to quality

FAQ

Does uncertain datum mean the GPS fix is wrong?

No. The feature position or depth derived from source surveys may be uncertain.

Does CATZOC solve everything?

No. It is a quality indicator used with source information, cautions and cross-checks.

May a manual offset be applied?

Only under specific official instruction and controlled procedure, not by trial and error.

Datum Shift and Chart Reference in ECDIS

Operational Explanation

Regulatory Reference

Scope of Application

Procedure / How to Complete It

Practical Example

What Typically Goes Wrong

Common Mistakes Mistake Library

What the PSCO Checks

Operational Tips

Preparation checklist

FAQ

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