Display of Isolated Dangers and Soundings

03 Apr 2012 Resource

A recent software update on our ECDIS has revealed a change in the way isolated dangers are indicated.

A recent software update on our ECDIS has revealed a change in the way isolated dangers are indicated.

On the previous version, isolated dangers of depth less than the user defined safety depth would only show in depth areas greater than the safety contour.
In the new version, the isolated dangers of depth less than the user defined safety depth are indicated in all areas regardless of safety contour.

Is this in line with new guidelines on the display of ENC data on an ECDIS?

Additionally, the option to display soundings only in areas less than the safety contour has been removed and replaced by an option to show either all soundings or none. Is this, again, in line with new guidelines or is this a manufacturer specific issue?
It was a useful option to have only "deep soundings" displayed, therefore further highlighting the areas in which more cautious navigation was required.

RESPONSE (from UKHO)

There have not been any amendments to IMO or IHO standards or any new guidelines issued that would require a manufacturer to change the display functionality for isolated dangers and soundings. Manufacturers have some freedoms in how they design their systems to meet the requirements of the IMO ECDIS Performance Standard and there are significant differences between systems in the user interfaces including terminology used and options available for various features and functions. As can be seen here, there can even be significant differences in functionality between versions of the same manufacturer’s software. This is something mariners need to be aware of; hopefully manufacturers provide documentation to highlight any such changes in new software releases.

The functionality provided in both old and new versions of this ECDIS would appear to be compliant with the requirements of the IMO Performance Standard in relation to the display of isolated dangers; these are:

1) ECDIS must always display the isolated danger symbol for hazards ‘within the safe waters defined by the safety contour’ (ie in what would otherwise be clear water)

2) ECDIS should be capable of displaying isolated dangers inside the safety contour (to cover the circumstances where a vessel has to navigate through these shoaler waters - see detail below).

In common with many other ECDIS the earlier version of this particular system only displayed 2) when the mariner opted switch them on as part of the ‘All’ display or an extended ‘Standard’ display mode. In the new version it appears that the manufacturer has chosen to combine 1) and 2) in the default standard display. This does have the advantage of simplifying the system and ensuring that the mariner is always able to see all the hazards - although this may well be at the expense of some additional screen clutter.

NB If navigating through areas inside the displayed safety contour it is essential to display all soundings as well as isolated dangers to get a complete picture of the possible hazards.

The query on the display of soundings is a similar situation; the decision of the manufacturer to show all soundings either on or off (at least in the default mode) looks to be an attempt to simplify the system for the user.

More detail on Isolated Dangers:

The term ‘isolated danger’ refers to a distinctive magenta symbol in the shape of a ‘screw head’ that is used to display dangers (wrecks, obstructions etc - but not soundings) that are a hazard to a vessel.

By applying the rules set out in the IHO S-52 Conditional Symbology Procedure, ECDIS determines whether or not a feature is a hazard to a vessel and whether an isolated danger symbol should be displayed. Essentially this is done by comparing the depth clearance value of the feature that is encoded in the ENC with the user selected safety contour setting.

The IMO ECDIS Performance Standard (PS) requires that isolated danger symbols are always visible on the display where they occur ‘within the safe waters defined by the safety contour’ (ie in what would otherwise be clear water). If the ECDIS determines that the feature is not a hazard then, in default ‘Standard’ display, it will not be shown at all. If ‘All’ display (or an extended standard display) is selected then the feature will be shown using the ‘normal’ chart symbol for that feature.

To complicate matters the PS goes on to state that in the ‘All’ display mode the mariner should, on demand, be able to show ‘details of all isolated dangers’. This is a reference to those features that could be hazards that fall inside (ie are shoaler than) the selected safety contour. This requirement was included to cover the times when it was necessary for the ship to navigate through waters shallower than the displayed safety contour - for example where a contour appropriate for a vessel’s draft is not present on the ENC. In this circumstance the ECDIS will display all wrecks and obstructions between the zero depth contour and the selected safety depth contour with the isolated danger symbol.

Whilst the concept of an ‘isolated danger’ is unambiguous in waters otherwise safe for navigation it is less so when applied to waters inside the selected safety depth contour where isolated shoal soundings depths might be more significant to a vessel than the isolated dangers themselves. For this reason if navigating a vessel through waters shoaler than the safety depth contour it is essential to ensure that soundings are displayed as well as isolated dangers.

Another issue related to the isolated danger symbol that has already been reported is where the depth of water over a potential hazard is not known. In this case the ECDIS will always assume the worst case (zero depth clearance) and display an isolated danger; this can lead to the display becoming cluttered in some instances.

A few screenshots are given below to illustrate the various combinations available in some ECDIS and the problem with features having no clearance value.