Read the official report into almost any modern grounding and the same sentence appears in different words: the ECDIS was not set up for the circumstances, and the alarms that should have spoken were silent or switched off. The machine was not broken. It was doing exactly what it had been told — and nobody had told it what “safe” meant for this ship, on this passage, today. ECDIS does not know your draught, your squat, or your air draught until you enter them. Until you do, it is guessing, and its default guess is dangerous.
This is the most important configuration any watchkeeper touches, and it is heavily examined for exactly that reason. The settings break down into three families named in this article’s title — the contour group, the depth setting, and the height alarm — and each does a different job. Getting them right turns the Electronic Chart Display and Information System into the best anti-grounding tool on the bridge. Getting them wrong, or leaving them at the factory default, quietly removes the safety net while leaving the screen looking reassuringly normal.
The Safety Contour: Your Line Between Water and Ground
The safety contour is the single most important setting in ECDIS. It is the line the system treats as the boundary between navigable water and water your ship cannot enter, and it does three things at once: it splits the chart into safe and unsafe shading, it decides which underwater dangers are flagged, and it drives the primary anti-grounding alarm. When your look-ahead vector or the ship itself is about to cross it in monitoring mode, the system raises an audible alarm — and under the IMO performance standards, the safety-contour crossing is one of the few alarms that is mandatory and audible by default rather than optional.
There is a crucial quirk in how the value behaves. ECDIS can only draw contours that the hydrographic office actually encoded in the ENC, so if you enter a value that does not exist on the chart in use, the system does not invent it — it selects the next deeper available contour and draws that as a thick, bold isoline. Enter eight metres on a chart that carries only five-metre and ten-metre contours and your effective safety contour becomes ten metres. This is safer, not less safe, but it means the line protecting you may sit deeper than the figure you typed, and a navigator who does not realise this misjudges how much room they really have. The contour also governs the isolated danger symbol: a wreck, rock, or obstruction shallower than your safety contour but lying in water deeper than it is drawn with the distinctive isolated-danger mark, so it stands out in otherwise “safe” water.
The default is where ships come to grief. Most ECDIS power up with the safety contour set to 30 metres. For a vessel drawing eight metres in the Dover Strait, a 30-metre setting paints almost the entire strait as unsafe blue water — and because the shallow patches that could actually ground the ship are already inside that blanket of “unsafe” shading, they stop standing out as discrete dangers. The protection collapses precisely where it is needed. A correctly chosen safety contour is calculated from draught plus the under-keel clearance you require, and reviewed for each phase of the voyage rather than set once and forgotten.
Shallow and Deep Contours: Reading the Gradient
The safety contour alone gives a two-colour picture — blue for unsafe, white or grey for safe. Switching to the four-shade display and setting a shallow contour and a deep contour subdivides that picture into something far more useful, turning a flat blue area into a readable depth gradient.
In the four-colour scheme the bands run from danger to open water: darkest blue for everything shallower than the shallow contour, medium blue between the shallow contour and the safety contour, light blue or grey between the safety contour and the deep contour, and white beyond the deep contour. The shallow contour, in effect, marks the no-go ground — many bridges enable a cross-hatch pattern over everything shallower than the safety contour to mirror the no-go hatching navigators once pencilled onto paper charts. The deep contour is the one most often left unset, which is a missed opportunity. Set it to roughly twice your draught and the grey band it creates shows you the water where squat begins to matter — the zone where a prudent navigator plans a speed reduction. Neither the shallow nor the deep contour raises an alarm; their value is entirely in what they let you see at a glance.
The Safety Depth: Bold Soundings, No Alarm
The safety depth is the setting most often misunderstood, and the misunderstanding is worth correcting precisely because examiners and inspectors probe it. The safety depth controls how individual spot soundings are drawn: any sounding equal to or shallower than the value you set is shown in bold black, while deeper soundings fade to grey. That is its entire function. It makes the dangerous numbers jump off the chart.
What the safety depth does not do is raise an alarm. It has no effect on the anti-grounding function whatsoever — it is a visual aid, nothing more. An officer who believes the safety depth is protecting them against a shoal sounding is relying on a warning the system will never give. For that reason it is usually set to the same value as, or close to, the safety contour, and it should never be set shallower than the safety contour, or some soundings inside your unsafe water would escape the bold treatment. The calculation itself is the heart of under-keel clearance management: draught, plus dynamic squat, plus your required under-keel margin, plus a correction for chart data quality (the CATZOC value), plus an allowance for swell where it applies, minus the height of tide. Get that figure right at the passage-planning stage and both the safety depth and the safety contour fall out of it naturally.
The Height Setting: The Danger Above the Waterline
Not every obstruction is on the seabed. For ships passing beneath bridges, overhead power cables, and gantries, the danger is overhead, and this is where the height alarm earns its place in the trio. You enter the ship’s air draught — the height of the highest fixed point above the waterline — plus a safe clearance margin, and the system can warn you of charted vertical clearances lower than that figure.
The margins matter and they are not symmetrical. General overhead obstacles should normally be cleared by at least a metre, but power lines demand a larger allowance, because the hazard is not only physical contact — electrical discharge can arc across a gap between the conductor and the ship. It is worth being honest about the limits of this feature: it is less standardised across manufacturers than the depth settings, and it depends entirely on the vertical-clearance attributes being correctly encoded in the ENC and on the ship’s air draught — which changes with cargo, ballast, and tide — being entered accurately. The height alarm is a valuable backstop, not a substitute for working the air-draught calculation yourself.
Where the Settings Betray You
The recurring failures cluster around a few predictable mistakes, and knowing them is half the defence. The factory 30-metre safety contour left untouched is the classic, hiding dangers in a sea of blue. The belief that the safety depth triggers an alarm is the second, and it lulls officers into trusting a setting that only changes a colour. The third is more subtle: a wreck or obstruction sitting deeper than the safety contour but shallower than your keel can remain completely silent unless the look-ahead detection function for isolated dangers is switched on — and on many systems that is a separate, manufacturer-specific setting that does not come enabled. Add the habit of muting audible alarms to stop nuisance noise, and a fully capable system can be reduced to an expensive chart display. These patterns, and the discipline that prevents them, are gathered in Common ECDIS Errors and How to Avoid Them.
Set Them for the Passage, Not Once for the Ship
The final principle ties the others together: safety settings are not a one-time configuration but a part of passage planning that changes as the voyage does. The right safety contour for an open-ocean leg is wrong for a shallow estuary approach; the right safety depth at departure draught is wrong after bunkering. Good practice is to calculate the values for each phase — departure, open sea, and arrival — as part of an under-keel clearance plan, check them against your company’s safety management system procedure, and review them at each transition. The settings are only ever applied through the route check and the look-ahead that watch over your track, so they work hand in hand with how you build and monitor the passage, covered in ECDIS Route Planning Step by Step and ECDIS Route Monitoring and Alarms.
Frequently Asked Questions
What is the difference between safety contour and safety depth in ECDIS? The safety contour is a depth-area boundary that divides safe from unsafe water, drives the shading, and triggers the anti-grounding alarm. The safety depth affects only how individual spot soundings are displayed — soundings shallower than the value appear in bold — and does not trigger any alarm.
Does the safety depth trigger an alarm? No. The safety depth is purely a visual aid that emphasises spot soundings shallower than the set value. Only the safety contour (and certain configured functions such as the isolated-danger look-ahead) drives the anti-grounding alarm.
Why is the default 30-metre safety contour dangerous? For most ships a 30-metre contour marks far too much water as unsafe, which paradoxically hides individual dangers within a blanket of blue shading and removes the contrast that makes shoals stand out. The safety contour should be calculated from the ship’s draught and required under-keel clearance.
How do you calculate the safety depth? A common approach is draught plus dynamic squat, plus the required under-keel clearance, plus a correction for chart data quality (CATZOC) and swell where relevant, minus the height of tide. It should be calculated during passage planning and reviewed by voyage phase.
What is the height setting in ECDIS for? The height alarm warns of overhead obstructions — bridges, cables, gantries — with charted vertical clearance lower than the ship’s air draught plus a safety margin. General obstacles need at least a metre of clearance; power lines require more because of the risk of electrical discharge.