Why Limitations Matter at Command Level
An examiner expects you to demonstrate that you understand why an ECDIS or RCDS can mislead you, not merely that limitations exist. Your safety decisions — passage planning approval, alter-course authorities, anchor watch parameters — rest on that understanding.
ECDIS: The Underlying Data Problem
ECDIS displays an Electronic Navigational Chart (ENC), which is a vector database. The system itself is only as reliable as the survey data behind it. Much of the world's coastal and offshore chart coverage is based on surveys that pre-date modern hydrography — some areas have not been re-surveyed to modern standards since the 19th century. The ENC carries metadata (source data diagrams, survey date, scale of original survey) accessible from within the system; a Master must interrogate these before passage, not delegate that check. Shallow, under-surveyed areas may show a clean bottom with no hazards precisely because nothing was recorded, not because nothing is there.
Sounding datum varies by chart origin. Unless you have confirmed the datum relationship to your vessel's draught reference, a charted depth is not directly comparable to your echosounder.
Display and Operator Limitations
Over-zooming is a persistent hazard. ENCs are compiled at a specific scale; zooming beyond that scale gives a false impression of precision — the chart data does not improve with the zoom level, but the displayed position of hazards appears more precise than it truly is. ECDIS should be used at the compilation scale for the area or no larger.
Route-checking alarms are only as good as the safety contour and safety depth values entered by the operator. If those values are set incorrectly — or never reviewed after a change of draught — the system will not warn of genuine danger. This is a master's responsibility to verify before departure.
ECDIS sensor inputs (GPS, AIS, radar overlay) can fail or degrade silently. A GPS offset or HDOP degradation may not trigger an obvious alarm. Position must be cross-checked by independent means — visual bearings, radar ranges, an independent GPS receiver — at appropriate intervals throughout the passage.
Type-Approval and Performance Standards
For an ECDIS to be accepted as satisfying a statutory chart carriage requirement, it must be type-approved to IMO Resolution MSC.232(82) (Performance Standards for ECDIS), with equipment testing carried out to IEC 61174. An examiner may probe whether you understand that not every electronic chart display constitutes a type-approved ECDIS — many vessels carry chart plotters or software-based chart systems that do not meet this standard and cannot be counted as ECDIS for regulatory purposes.
Paper Chart Requirements Under the Red Ensign Group Yacht Code (REG YC)
For vessels operating under REG YC, the question of whether paper charts may be dispensed with entirely depends on the ECDIS fit:
- Dual-ECDIS fit (a primary unit and a fully independent backup ECDIS on a separate power supply — including UPS — and independent position inputs): a paperless passage is permissible, as the backup satisfies the secondary means requirement.
- Single-ECDIS fit: the vessel must carry a complete, up-to-date, and corrected paper chart folio for the intended voyage. The paper folio is the legal secondary means; the single ECDIS alone does not satisfy the requirement for an independent backup.
This distinction is important at the command level: fitting one ECDIS does not authorise a paperless vessel. A Master must know the configuration of his own navigational fit before deciding whether paper charts may lawfully be left ashore.
RCDS: A Different, More Limited System
A Raster Chart Display System (RCDS) uses a scanned image of a paper chart. It lacks the interrogatable database of an ENC: you cannot query a depth sounding, reveal source data age, or receive automatic safety contour warnings. Anti-grounding alarms and route checking are either absent or severely limited.
IMO requires that when RCDS mode is used in place of ECDIS, a full folio of up-to-date paper charts must be carried and used alongside it. RCDS alone does not satisfy the chart carriage requirement for vessels required to carry ECDIS.
Correcting an RCDS is limited to adding manual overlays or notation — the underlying raster image cannot be updated in the way an ENC cell is updated by an automatic service.
The Master's Oversight Obligation
At command level, the obligation is to establish — and enforce through standing orders — how ECDIS is to be used, what cross-checks are mandatory, what scale limits apply, and how the system is to be verified at handover. An unconfigured or poorly supervised ECDIS is a navigationally degraded vessel regardless of what the certificate of fitness states.