Candidates most commonly trip up by treating ECDIS as though it is a perfect, always-current system. In the oral, this manifests as vague answers about 'updating the charts' without demonstrating understanding of why the data itself may be fundamentally unreliable — regardless of how recently it was updated. The sharper failure is not knowing what RCDS mode means or when it applies.
The core distinction: SENC vs RCDS
ECDIS operates in two modes depending on the chart data loaded:
- ECDIS mode (full functionality): uses an ENC — a vector chart compiled to IHO S-57/S-101 standards, issued by an authorised hydrographic office, forming the System ENC (SENC) after processing by the ECDIS.
- RCDS mode (Raster Chart Display System): uses a raster image — essentially a scanned paper chart. The system can display position over it, but critical ECDIS functions are not available.
Limitations of RCDS mode
When operating in RCDS mode, candidates must know the following limitations apply:
- Safety contours and depth areas cannot be interrogated — no automatic grounding alarm based on chart data.
- Chart objects cannot be selected for attribute information.
- Route-checking against chart features is significantly degraded.
- The system must be used in conjunction with an appropriate folio of up-to-date paper charts. This is a regulatory requirement, not a best-practice suggestion.
RCDS is used in areas where ENCs are not yet available. Candidates who do not know this requirement for paper backup will fail on this point.
Limitations of ECDIS regardless of mode
- Survey data quality: ENCs are only as accurate as the underlying hydrographic survey. Much of the world's coastal and offshore data was surveyed to 19th- or early 20th-century standards. The IHO CATZOC (Category Zone of Confidence) attribution indicates data reliability — many candidates cannot name this.
- Datum shifts: the chart may be referenced to a local datum rather than WGS-84. GPS output is WGS-84. If the ECDIS does not apply an automatic datum shift, a positional offset exists between the vessel's plotted position and the actual charted features.
- Generalisation and scale: ENCs compiled at small scale may not contain features present on a larger-scale paper chart. Always use the largest-scale ENC available for the area.
- Overscaling: displaying an ENC at a scale larger than its compilation scale gives a false impression of accuracy. A well-configured ECDIS warns of this; candidates should know what that warning means.
- Update currency: ENCs must be kept current via the authorised updating service. An updated ENC does not correct errors in the original survey data.
- Software and type-approval: ECDIS must be type-approved to the IMO performance standard IMO Resolution MSC.232(82); IEC 61174 is the associated type-testing standard against which equipment is tested. A non-type-approved system cannot be used as a SOLAS chart-carriage alternative.