ONDECK
Academy
OOW Oral Prep
Module 04 — OOW 3000 GT Oral Preparation
ECDIS
Electronic Chart Display and Information System — types, limitations, alarms, carriage requirements, backup arrangements
What is ECDIS?
ECDIS is a navigation information system that displays electronic chart data, integrated with position fixing (GPS/GNSS), and other navigational data. A compliant ECDIS can be used in lieu of paper charts if it meets IMO performance standards and uses the correct chart type.
ENC — Electronic Navigational Chart
- Official chart — produced by or on behalf of a hydrographic office
- Issued in S-57 format (IHO standard)
- Used in SENC (System ENC) inside ECDIS after conversion
- Meets carriage requirement — can replace paper charts
- Updated via AVCS (Admiralty Vector Chart Service) or equivalent
- Has CATZOC rating — allows assessment of data reliability
RCDS — Raster Chart Display System
- Digital scan of a paper chart — no vector data
- Displayed on an ECDIS but is not a true ECDIS chart
- Cannot replace paper charts on its own
- Can only be used when ENCs are not available for the area
- Must be used with a folio of up-to-date paper charts
- All ECDIS alarms and functions are reduced in RCDS mode
Carriage Requirements SOLAS V Reg 19 / MSC.282(86)
| Vessel type / size | Requirement |
|---|---|
| New passenger ships ≥500 GT (from 2012) | ECDIS mandatory |
| New tankers ≥3000 GT (from 2012) | ECDIS mandatory |
| New cargo ships ≥10,000 GT (from 2013) | ECDIS mandatory |
| New cargo ships 3000–10,000 GT (from 2014) | ECDIS mandatory |
| Existing vessels (phased schedule to 2018) | ECDIS mandatory per GT/type |
| Vessels below carriage threshold | ECDIS not mandatory — paper charts or type-approved ECDIS acceptable |
FOR SUPERYACHTS: Many superyachts below 3000 GT are not subject to mandatory ECDIS carriage, but most operators fit ECDIS as standard. Know whether your vessel is operating on ENCs or RCDS — the backup requirement differs.
ECDIS Limitations — What the Examiner Wants to Hear
| Limitation | Risk |
|---|---|
| Chart data quality (CATZOC) | Many ENCs are based on old survey data. A Zone of Confidence (CATZOC) rating tells you how reliable the depth data is. Low-rated areas may have significant depth errors — never assume the chart is accurate |
| GPS / GNSS dependency | Position accuracy depends entirely on the GNSS input. Signal loss, jamming, or spoofing will give a false position. Always cross-check with visual bearings and radar |
| Incorrect alarm settings | Safety contour, safety depth, and XTE alarms must be set correctly for the vessel's draught and the passage. Wrong settings = no alarm when needed |
| Operator over-reliance | The most dangerous limitation. ECDIS gives a false sense of security — it shows the chart on screen but it does not think. The OOW must apply judgement |
| ENC update status | Charts must be kept up to date with weekly updates. Out-of-date ENCs show hazards that may no longer exist — or miss new ones |
| RCDS mode limitations | No vector data — many ECDIS alarms and functions do not work. Must carry paper charts alongside |
| Equipment failure | Single ECDIS failure with no backup = potentially no chart. Backup arrangement is mandatory for vessels on paperless navigation |
| Scale distortion | Zooming in too far on a small-scale chart gives false precision — the chart data does not become more accurate because you zoom in |
Critical ECDIS Alarms — Must Know Before Departure
| Alarm | What it is | Set based on |
|---|---|---|
| Safety Contour | Vessel will alarm if it approaches a depth contour you have defined as unsafe | Vessel's maximum draught + UKC allowance + safety margin |
| Safety Depth | Soundings shallower than this depth are highlighted on the chart display | Vessel's draught + UKC margin |
| Cross Track Error (XTE) | Alarm when vessel deviates beyond a set distance from the planned track | Sea room available and proximity of hazards on each side of the track |
| Anti-grounding cone | Forward-looking alarm — projects vessel's track ahead and alarms if hazard lies within the cone | Vessel speed and required warning time |
| Look-ahead | Time or distance based — warns when an upcoming waypoint or hazard is within a set time/distance | Speed and manoeuvring characteristics |
ALARM MANAGEMENT: An alarm that is always triggering will be muted or ignored. Set alarms correctly so they are meaningful. An alarm-fatigued OOW is a danger.
Backup Arrangements
MSC.232(82): Vessels navigating with ECDIS must have an appropriate backup arrangement. This is either a second independent ECDIS, or a folio of up-to-date paper charts for the area.
| Scenario | Required backup |
|---|---|
| Single ECDIS on ENCs (paperless navigation) | Second independent ECDIS — or paper charts for the area |
| ECDIS in RCDS mode | Paper charts mandatory — RCDS cannot be used alone |
| ECDIS failure at sea | Revert to backup — if no second ECDIS, plot on paper charts immediately. Call Master. |
Passage Planning on ECDIS — Key Points
- Route must be checked at appropriate scale for each phase — do not plan a coastal approach on a small-scale overview chart
- Safety contour and safety depth must be set before loading the route
- All waypoints must be checked visually on the chart — ECDIS route planning can produce straight lines through shallow water if waypoints are not carefully placed
- Route check function must be run — the system will flag any hazards along the planned track
- No-go areas must be identified and wheel-over positions confirmed
- Plan must be checked and approved by the Master before departure
- ENCs must be up to date — check update status before sailing
Setting It Up — Safety Values Are Passage-Plan Items
| Setting | What it does | The trap |
|---|---|---|
| Safety depth | Bolds every sounding shallower than the value (draught + squat + UKC margin) | Display aid only — it drives no alarm |
| Safety contour | THE machine's no-go line: drives the anti-grounding alarm, look-ahead warnings and shallow-water shading | ENCs carry only certain contours, so the system snaps your request to the next DEEPER one — ask for 4 m, get 5 or 10. Plan against the contour you actually got |
| Look-ahead | The sector ahead scanned for the safety contour and dangers | Sized for your speed: a 6-minute sector at 20 knots is 2 miles of warning; at the default it may be seconds |
| Cross-track corridor (XTD) | The width the route check scans and the off-track alarm watches | Set to the margins the plan drew — a 5-mile XTD makes the route check meaningless |
Settings are recorded in the passage plan, checked at handover, and changed deliberately — an ECDIS still wearing last week's ocean settings inshore is silently useless, and "the alarm never sounded" is not a defence when the alarm was configured not to.
The Route Check — Let the Machine Read the Whole Corridor
After the route is laid, the automated route check interrogates every leg's corridor against the safety contour and charted danger objects. The discipline is in the handling: every warning individually examined and resolved — most are seconds each, but the isolated wreck inside your XTD looks identical to the trivia until it is read. Clicking through warnings wholesale is the signature behaviour in ECDIS-assisted groundings. Two display disciplines to pair with it: review the route at compilation scale (over-zoomed-out, SCAMIN suppresses symbols and isolated dangers can simply not display), and check the update status of every cell on the route — ENCs are corrected by weekly digital updates, applied and logged, and the ECDIS can show each cell's currency, which is exactly what a Port State inspector asks it to do.
CATZOC — The Honesty Layer Under the Chart
| ZOC | Position accuracy | Working response |
|---|---|---|
| A1 / A2 | ±5 m / ±20 m, full seafloor search | Trust to close margins |
| B | ±50 m, uncharted hazards unlikely but possible | Standard coastal margins |
| C | ±500 m, depth anomalies expected | Widen UKC and passing distances; favour daylight; sounder is a primary sensor |
| D / U | Worse / unassessed | Old survey or none — navigate as if the chart is a sketch |
ECDIS draws a 19th-century leadline survey with the same crisp confidence as last year's multibeam. Interrogating the ZOC layer at the planning stage is what converts that false confidence back into honest margins — in ZOC C water, "the chart shows deep water" is not a complete sentence.
What ECDIS Inherits — and the Over-Reliance Pattern
The displayed ship position is the GNSS feed: every jamming, spoofing and datum problem arrives on the ECDIS screen wearing the chart's authority, with the AIS and autopilot agreeing because they drink from the same source. The defence is the same independence doctrine as the chartwork module: radar overlay or ranges compared against the charted land, visual bearings, the echo sounder against expected depths — and the instinct that when the radar picture and the ECDIS disagree, you slow down and believe neither until resolved. The MAIB's ECDIS grounding pattern is stable across reports: safety values wrong or defaulted, route check warnings dismissed, over-zoom hiding the danger, alarms silenced as noise, and a bridge team watching the screen instead of the water. Every element is procedural — which means every element is preventable by the OOW who treats ECDIS as a powerful instrument to be configured, not an authority to be obeyed.
Additional Question
You join a vessel and the ECDIS shows no alarms on a route you know passes close to a drying patch. First actions?
Suspect the configuration before trusting the silence: check the safety contour actually selected — if it defaulted deep or snapped somewhere unexpected, the shading and alarms are describing a different ship; check the safety depth against our real draught; check the XTD and look-ahead; then re-run the route check and read the warnings it now raises. Then the wider join-the-ship checks: cell update status, position source and its cross-check, and the previous officer's settings recorded anywhere. An ECDIS is only ever as safe as its last deliberate configuration — and joining is when you make it yours.
Oral Exam Practice Questions
Question 1
What is the difference between an ENC and RCDS?
An ENC is an official vector chart produced by a hydrographic office to IHO S-57 standard. It contains object-based data that ECDIS can interrogate — enabling alarms, safety contours, and route checking. An RCDS is a rasterised scan of a paper chart. It looks the same on screen but has no vector data — ECDIS cannot use its information for alarms or safety functions. RCDS can only be used when ENCs are not available, and must be accompanied by a folio of up-to-date paper charts.
Question 2
What are the limitations of ECDIS?
Chart data quality varies — CATZOC ratings show how reliable the depth information is, and many areas are based on old surveys. GPS dependency means position accuracy relies entirely on GNSS — jamming, spoofing, or signal loss gives a false position. Alarm settings must be correctly configured for the vessel's draught or they are meaningless. Operator over-reliance is the greatest risk — ECDIS does not navigate the vessel, the OOW does. ENCs must be kept up to date. Equipment failure without backup leaves the vessel without a chart.
Question 3
What alarms must be set on ECDIS before a passage?
Safety contour — set to vessel's draught plus required UKC margin, so the system alarms if approaching a depth shallower than this. Safety depth — soundings shallower than this are highlighted. Cross track error — set to the sea room available on each side of the track. Anti-grounding cone — forward-looking hazard alarm. All alarms must be reviewed and confirmed correct by the OOW before departure.
Question 4
Your ECDIS fails at sea. What do you do?
Call the Master immediately. Switch to the backup arrangement — second ECDIS if fitted, or paper charts. Plot the vessel's current position on the paper chart. Confirm the safe track and continue the passage using the paper chart with manual position fixing. Log the failure. Advise the engine room to maintain standby readiness. Do not attempt to navigate without a chart — if no backup is available, consider seeking a safe anchorage until the system is restored.