Safety contours
Safety depths and contours for port approaches
I have had some discussions with my colleagues regarding the best practice for handling safety contour and safety depth.
For example, a vessel has a draft of 12 m. With squat effect and UKC added, the safety depth could be for example 14 m. If the ENC cell has depth contours for 10 m and 20 m, the ECDIS will chose the 20 m curve for the safety contour, and any depth below this will be a no go area.
Obviously a ship with 14 m draft will need to cross the 20 m contour to enter a number of ports. Is there an agreed best practice how to handle this in the ECDIS? Should the safety depth be changed to the 10 m contour? In that case no go areas can be entered manually, as it has always been done in the paper charts. Or should the vessel proceed in to the so called no go area, and maybe put a note on this in the passage plan?
What are the thoughts on having safety depth that is less than the actual draft? Can the hydrographic offices put more contours in on cells used for port entry?
RESPONSE
From Andy Norris MNI, author of ECDIS and Positioning
Similarly to paper charts, ECDIS has two methods of indicating depth- contours and spot soundings.
Contours are ascertained by the surveying hydrographic office from the detailed soundings of the original survey used to compile the ENC. The contours are put onto the ENC in two separate ways. Firstly, as a curve that represents the depth contour and secondly as an area with a depth that lies within a defined range of values- effectively the values between the adjacent depth contours. Not surprisingly, the depth contour is a line object, the depth area is an area object and soundings are put onto the ENC as point objects.
On ECDIS the user needs to select a safety contour. If a safety contour is entered that is not available on the particular ENC in use then the system will use the next deepest available contour. Areas that are deeper than the safety contour will have a grey or white background in the normal daytime viewing mode, depending whether in two or four colour depth area indication settings.
If within a specified time set by the user, the ship is about to cross the safety contour an alarm will sound. If equipment follows the IMO requirements on this, the alarm should only need a single acknowledgement and should not alarm again, except (perhaps) when about to recross the safety contour. However, it is possible that this is implemented differently on different machines and feedback to the ECDIS Forum concerning this would be appreciated.
The user also needs to set a safety depth. This affects the indication of spot soundings when selected to be displayed on ECDIS. They will appear as bold numerals when the spot sounding is less than the safety depth, highlighting the potentially unsafe areas. However, the safety depth feature does not trigger an alarm but it may provide an indication.
There is an IMO requirement that "An indication shall be given to the mariner if, continuing on its present course and speed, over a specified look-ahead time or distance set by the mariner, own ship will pass closer than a user-specified distance from a danger (e.g. obstruction, wreck, rock) that is shallower than the mariner's safety contour or an aid to navigation". It is to be hoped that no design interprets this as applying to soundings that are less than the selected safety contour. A sensible approach here would be either to give no indication or to base the indication on the selected safety depth. In fact, it is unlikely that any ECDIS does provide distance-specific indications on soundings that are less than the safety depth but feedback to the ECDIS Forum if such a feature is available on specific equipment would be appreciated.
As mentioned in the enquiry, depth contours on some ENCs can be relatively far apart, and sometimes do not permit their simple use when navigating close to shallow waters, such as on port entry. In such areas soundings enhanced by appropriately set safety depths must be used. However, providing the ECDIS in use does not trigger a recurrent alarm that needs continuous acknowledgement, it should not be necessary to change the safety contour when navigating in such waters. Instead, the ECDIS will continue to provide a visual reminder on the chart that the situation requires care, simply because own ship will be displayed in an area that is coloured blue, indicating that the waters are shallower than the safety contour. It is, of course, good practice to make a note of this impending situation on the passage plan.
In such waters close and regular inspection of the chart is always necessary, whether using paper or ECDIS. However, if the ship is kept away from depth soundings displayed in bold numerals then there should be sufficient water, providing an appropriate safety depth has been entered.
Many port areas have more closely spaced depth contours than in the quoted example. For instance, Harwich has them at 2, 5, 10, 15 and 20 metres. It is always worth lobbying the hydrographic office responsible for the compilation of the ENC data if it is considered that insufficient contours are available. However, as described above, safe navigation is still quite possible where there are few contours- providing there are sufficient soundings, that they are turned on and an appropriate safety depth has been set. Also, on many systems user-specified lines and areas can be generated and used to highlight a rough approximation to the required safety contour and can also be set to alarm if the vessel approaches the boundary- but the safe navigator never relies on alarms!