ECDIS: Safety Settings, Alarms and Back-up

What ECDIS Is and Who Must Carry It

Read first: the SOLAS V/19 carriage list, chart datums and chart correction.

In short: ECDIS (Electronic Chart Display and Information System) is a type-approved chart system that, with an adequate back-up, can replace paper charts. An ECS cannot.

• Primary function: to contribute to safe navigation (IMO MSC.232(82) 1.1). It does this by:
  • sounding alarms and alerting the watchkeeper;
  • making chart updating simple and reliable (ENCs and, where carried electronically, publications);
  • and so reducing the OOW’s navigational workload compared with paper (IMO MSC.232(82) 1.4–1.5).
• ECDIS vs ECS:
  • ECDIS is approved: it can be your primary means of navigation and meets the chart carriage rule of SOLAS V/19 and SOLAS V/27.
  • An ECS (Electronic Chart System) is not approved and can never be the primary means of navigation. It can even run on a mobile device. With an ECS, up-to-date paper charts stay primary (MGN 379 (M+F) 5.1).
• To count as “the chart”, an ECDIS must (IMO MSC.1/Circ.1503/Rev.1 §6):
  1. be type-approved;
  2. use up-to-date official ENCs;
  3. run software kept to the latest IHO standards;
  4. have adequate, independent back-up in place.
• Who must carry it: carriage became mandatory through SOLAS V/19.2.10, phased in from 1 July 2012 to 1 July 2018 (high-speed craft from 1 July 2008). New passenger ships of 500 GT and over and new cargo ships of 3,000 GT and over on international voyages were first. Existing ships followed by size and type.
• On the bridge: a yacht with a plotter on the helm console has an ECS. Unless the plotter is a type-approved ECDIS with official ENCs and a back-up, the paper chart (or approved ECDIS) is still the legal chart.

Sensors, Voyage Recording and Training

In short: an ECDIS needs position, heading and speed to work, it keeps a 12-hour “black box” record, and every OOW on an ECDIS ship needs generic training plus type-specific familiarisation.

• Inputs required by IMO (IMO MSC.232(82) 15.2): the position-fixing system (GNSS), the gyro compass (heading) and the speed and distance measuring device (log). Without a gyro, a transmitting heading device.
• Depth: many installations also connect the echo sounder, and revision lists often include it under “requirements”. The IMO standard does not require it — other connected items are optional.
• Alarm on lost input: ECDIS must alarm if the position, heading or speed input is lost (IMO MSC.232(82) 11.4.8).
• Voyage recording (IMO MSC.232(82) 11.5):
  • previous 12 hours at 1-minute intervals: time, position, heading and speed, plus the ENC source, edition, date, cell and update history;
  • the whole voyage track with time marks at 4-hour intervals or less;
  • the record cannot be changed.
• Training (IMO MSC.1/Circ.1503/Rev.1 §28):
  • generic ECDIS training to STCW A-II/1 / STCW A-II/2 (2010 Manila amendments, in force 1 July 2013);
  • type-specific familiarisation with the ship’s own ECDIS, sensors and back-up (STCW I/14, ISM Code 6.3);
  • officers who serve only on ships without ECDIS get a limitation on their endorsement.

Chart Standards: IHO, S-52, S-57 and S-101

In short: the IHO writes the chart data standards and coordinates them with IMO. S-52 is what you see, S-57 is how the data are coded, and S-101 is replacing S-57.

• IHO (International Hydrographic Organization) coordinates world hydrographic charting policy and procedures with IMO.
• S-52 — display: colours and symbols, so every ECDIS shows the chart the same way. Memory aid: S-52 is for your eyes.
• S-57 — data transfer: the same idea seen from the code side — the digital format used to exchange ENC data.
• S-101 (S-100 framework) is replacing S-57:
  • new ECDIS may follow the S-100 standard (IMO MSC.530(106)/Rev.1) voluntarily from 1 January 2026;
  • all ECDIS newly installed from 1 January 2029 (including retrofits) must be S-100 compatible;
  • existing ECDIS can keep using S-57. There is no withdrawal date for S-57 yet: both run in parallel (“dual fuel”).
• Cell names: an official ENC code starts with the producing hydrographic office, e.g. GB203804 (UK) or JP248NFO.007 (Japan).
• Updates (IMO MSC.232(82) 4.3–4.8): ENC contents cannot be altered; updates are stored separately and the ECDIS keeps a record of updates applied. Official updates are cumulative, so only the latest weekly update is needed. An expired licence may leave the cell on screen, but it stops updating and is not valid for navigation — check licences in the passage plan (MGN 379 (M+F) 5.5).
• Software: an ECDIS not kept to the latest IHO Presentation Library may show “?” symbols, miss features or fail to alarm (IMO MSC.1/Circ.1503/Rev.1 §13–15).

ENC, SENC and Raster Charts

In short: a raster chart (RNC) is a picture of a paper chart; a vector chart (ENC) is a database the ECDIS can question. Only ENC data give anti-grounding alarms — this is the most common ECDIS question.

Raster charts (RNC)

• A scanned copy or digital photograph of a paper chart: a dense grid of dots (pixels), each tied to a latitude and longitude.
• The system can lay position, bearing lines, AIS and ARPA targets over the image.
• Cannot give grounding alarms — the picture has no depth data the ECDIS can read.

• Approval: an official RNC is used only in RCDS mode (Raster Chart Display System), where no suitable ENC exists, and only with an appropriate folio of up-to-date paper charts (IMO MSC.1/Circ.1503/Rev.1 §26–27; MGN 379 (M+F) 5.1). The UK risk assessment for RCDS use is in MGN 285 (M+F).

Vector charts (ENC) and the SENC

• ENC (Electronic Navigational Chart): the official database of chart objects, coded to IHO standards and issued by or for a hydrographic office (IMO MSC.232(82) 3.2). Vector charts can also be private (unofficial) — only official ENCs meet carriage rules.
• SENC (System ENC): the ECDIS’s own internal database, converted from the ENC plus its updates. The display and the alarms are drawn from it (IMO MSC.232(82) 3.3). It is not “the display” itself.

Display Categories, Scale and SCAMIN

In short: ENC data come in three display categories. Navigate on Standard as a minimum, and watch the scale — overscale and SCAMIN can hide or distort what you need.

• Display Base — always on screen, cannot be removed: coastline, own safety contour, isolated dangers inside safe water, scale and north arrow, units, display mode. You see the contour lines but no buoyage. Use it for initial planning only — IMO says it is not enough for safe navigation (IMO MSC.232(82) 3.5, App. 2).
• Standard Display — the minimum for navigation (planning and monitoring): Display Base plus drying line, buoys, beacons, other aids to navigation, fixed structures, fairway and channel limits, conspicuous features, prohibited and restricted areas, chart scale boundaries, cautionary notes, routeing systems. One button must bring it back (IMO MSC.232(82) 3.4, 5.3).
• All Other Information — everything: spot soundings, cables and pipelines, details of dangers and aids, magnetic variation, place names, and more.
• If you remove Standard Display categories, ECDIS shows a permanent “customised display” indication.
• Scale indications (IMO MSC.232(82) 6.1):
  • Information overscale — you have zoomed in beyond the scale the ENC was made for. Detail looks precise but is not.
  • Larger scale ENC available — a better chart for your position exists; load it.
• SCAMIN (scale minimum): the smallest display scale at which an object is still drawn. Zoom out past it and the object disappears to declutter the screen — a buoy or wreck can vanish. Press the 1:1 button to return to the ENC’s own (compilation) scale for the best reading.

Safety Contour and Safety Depth

In short: the safety contour is the line between safe and unsafe water and drives the anti-grounding alarm; the safety depth decides which spot soundings stand out. Calculate both from your draught, squat and UKC — and set them exactly.

What each one does

• Safety contour: a depth contour chosen by the mariner. ECDIS draws it as an extra-wide line between “safe water” and shallow water and alarms if, within a time you set, own ship will cross it (IMO MSC.232(82) 5.8, 11.4.3).
  • Nothing set → defaults to 30 m.
  • Value not in the ENC (ENCs carry only certain contours, e.g. 5, 10, 20 m) → ECDIS uses the next deeper contour and shows an indication.
  • When you change cells, the contour can jump to a deeper one — check it after every chart change (MGN 379 (M+F) 5.9.1).
• Safety depth: a value set by the mariner. Spot soundings equal to or shallower than it are shown bold black; deeper ones are grey (IMO MSC.232(82) 5.9). It gives no alarm — it only highlights soundings.
• Only on ENC: none of this works on a raster chart.

How to set them

• Follow the UKC policy in the SMS — calculate safety depth and contour as part of the passage plan.
• Factors to consider:
  • static draught (deepest, at the time of passage);
  • squat at planned speed;
  • minimum UKC required by the SMS;
  • CATZOC of the area — extra margin where surveys are poor;
  • allowances for roll, pitch, heel and swell;
  • height of tide, only if the SMS allows it (see tides) — many companies ignore tide for the contour.
• Worked example: draught 7.5 m + squat 0.6 m + UKC 1.5 m → safety depth 9.6 m. The ENC has contours at 5, 10 and 20 m, so a 9.6 m safety contour is shown as the 10 m contour. Where a cell has no 10 m line, it jumps to 20 m.
• Set the values exactly — the danger area is only right if the numbers are (MGN 379 (M+F) 5.9.1). Check settings at every watch handover (MGN 379 (M+F) 5.9.8).

CATZOC: How Good Is the Survey?

In short: CATZOC (Category Zone of Confidence) tells you how far to trust the depths in an ENC area.

• Shown on the ENC by stars inside a triangle or ellipse (IHO S-67):

• Quality falls from A1 → A2 → B → C → D. U = Unassessed, not “uncharted”.
• CATZOC covers bathymetry only (depths, rocks, reefs) — not lights, buoys or the coastline.
• In B, C, D or U areas, especially shallow ones, add extra safety margin to the safety contour and stay well off (MGN 379 (M+F) 5.9.2).
• An ENC can look modern and still rest on an old survey — CATZOC is how you find out.

The Five Alarms and the Indications

In short: an alarm is audible (and visual) and demands action; an indication is visual information only. Learn the five alarms by heart.

The five alarms (often asked)

1. Crossing the safety contour — within the time set by the mariner (IMO MSC.232(82) 11.4.3).
2. Deviation from route — cross-track limit (XTE) exceeded (11.4.5).
3. Positioning system failure — GPS lost (11.4.8).
4. Approach to a critical point — at a set time or distance before it, e.g. a wheel-over (11.4.9).
5. Different geodetic datum — position system and chart not on the same datum (11.4.10).

• These five are exactly the plain “alarm” lines of IMO MSC.232(82) Appendix 5.
• Some revision lists add “non-ENC area” as a sixth alarm. In the IMO table, “No ENC available” is an indication, not an alarm.
• Alarm or indication (the mariner chooses): area with special conditions (e.g. restricted area, TSS, anchorage, PSSA, user-defined areas to avoid) and malfunction of ECDIS.
• Loss of heading or speed input also gives an alarm (11.4.8).

Indications (“warnings” — information, not alarms)

• Information overscale and larger scale ENC available.
• Route planning across the safety contour and route planning across a specified area.
• System test failure.
• Also: default safety contour in use, different reference system, no ENC available, customised display, crossing a danger in route monitoring mode.

Alarm fatigue

• MGN 379 (M+F) 5.7: an alarm is a high-priority alert, audible and visual, active until acknowledged.
• Too many alarms → alarm fatigue: watchkeepers silence them without looking and miss the one that matters (5.7.1). Set limits to suit the traffic, nearness of danger and conditions.

Passage Planning and Route Monitoring in ECDIS

In short: depending on the ship’s certification, ECDIS may be the primary passage-planning tool. Plan with the right settings, check the route automatically and by eye, then monitor it with alarms on.

Planning the route in ECDIS

• Safety depth and safety contour calculated and set to the SMS UKC requirements.
• A maximum cross-track distance (XTD) for each leg, to SMS rules and right for the area.
• ENCs available for the whole passage, at a suitably large scale.
• Reusing an old plan? Recheck the whole route. Remove old routes from the display.
• Select the chart symbols and detail needed (Standard Display as a minimum).
• Set the ETA correctly (by hand or with the planning tools). A wrong ETA spoils tidal data and other time-dependent information.
• Apply current and tidal data if integrated with ECDIS and up to date.
• Add critical points and wheel-over positions — see parallel indexing and wheel-over.

Checking the route

• Run the route check (scan) — then inspect the whole route visually in “All” display, at the largest scale ENC (IMO MSC.1/Circ.1503/Rev.1 App. 1). Small islets and some dangers may not trigger the automatic check.
• The scan only covers the XTD corridor: hazards outside it are not checked or alarmed. Leave the corridor and you are in unvalidated water — reassess the plan (MGN 379 (M+F) 5.9.3–5.9.4).

Monitoring the route

• Keep these active and checked: cross-track error (XTE) limit, display scale (not overscale), look-ahead (anti-grounding) zone, and all alarms.
• Increase anti-grounding settings when needed and ask if unsure (MGN 379 (M+F) 5.9.5).
• Use the correct night colour scheme in darkness — and not in daylight, where it hides information (5.9.7).
• Radar overlays and maps on the ECDIS are covered in radar maps and ECDIS overlay.

Integrity Monitoring and Over-reliance

In short: ECDIS is only as good as its chart data and sensors. Integrity monitoring means the system and the OOW keep checking both.

• Integrity monitoring: continuously assessing the quality and reliability of the chart data and position shown. ECDIS helps by:
  • detecting chart errors;
  • monitoring position accuracy;
  • handling sensor failures;
  • keeping the display reliable;
  • monitoring safety parameters;
  • generating alarms.
• The OOW’s part: ECDIS only alarms for the position it is fed. A wrong GNSS position looks perfectly safe on screen (MGN 379 (M+F) 4.5). Check it by independent means — position fixing by radar ranges, visual bearings, depths.
• Radar overlay is the best cross-check of GPS integrity: if radar echoes sit on the charted coast, the position is good (MGN 379 (M+F) 5.8, 5.10).
• Second position source: where possible, a second independent source, ideally of a different type; ECDIS should flag discrepancies (IMO MSC.232(82) 11.4.7). See GNSS errors.
• Datum: set the GNSS receiver to the datum the ECDIS expects. A datum shift can be applied twice — once by the receiver, once by the ECDIS (MGN 379 (M+F) 5.11).

Back-up Arrangements and ECDIS Failure

In short: an ECDIS only replaces paper charts with an adequate, independent back-up that can take over at once and carry the ship safely to the end of the voyage.

• The rule: if the chart function is done electronically, a back-up arrangement is required (SOLAS V/19.2.1.5; IMO MSC.232(82) 1.2, section 14).
• Two purposes (IMO MSC.232(82) App. 6):
  1. a safe take-over of ECDIS functions, so a failure never becomes a critical situation;
  2. safe navigation for the rest of the voyage.
• Accepted back-ups (MGN 379 (M+F) 5.12):
  • an appropriate folio of up-to-date paper charts; or
  • a second ECDIS on a power supply independent of the first.
• Typical twin-ECDIS fit: 2 ECDIS units, 2 GPS inputs, 2 independent power supplies plus a back-up (emergency) supply.
• What the back-up must do (IMO MSC.232(82) App. 6):
  • show the chart information needed for safe navigation, up to date for the whole voyage;
  • take over the route plan and allow changes to it;
  • plot own position (automatically or by hand), take courses, distances and bearings, show the route and time labels;
  • if electronic: at least Standard Display content, a chart area of at least 250 × 250 mm (or 250 mm diameter), a power supply separate from the ECDIS, a position-fixing input, and a record of the ship’s track.
• Power: ECDIS must run on the emergency source of power, and an interruption of up to 45 seconds must not need manual re-initialisation (IMO MSC.232(82) 16).
• Test the back-up as regularly as the main system (MGN 379 (M+F) 5.12) — in confined waters it must be ready for immediate use.
• On the bridge — ECDIS fails: call the Master; switch to the back-up ECDIS or paper chart; put the last known position on it; fix by radar and visual means; in pilotage waters consider reducing speed until the back-up is fully running.