Raster Chart Display System

When an ECDIS system does not have an ENC for the area being navigated, and an RNC is used in its place, it is said to be operated as a Raster Chart Display System (RCDS). The following limitations significantly reduce the functionality expected from the ECDIS.

 

• Unlike within the ECDIS where the ENCs have no chart boundaries, RNCs are based on paper charts and as such have boundaries which are evident when passing from one chart to the next. This may cause confusion or distract the user at areas on or near to chart boundaries.

• RNCs will not trigger automatic alarms (e.g. anti-grounding). However, some alarms and indications can be generated with the manual addition, during passage planning, by the user (example: clearing lines, ship safety contour lines, isolated danger markers and danger areas). To recover some of the safety functionality of the ECDIS system and mitigate these limitations a significant amount of data set-up is required.

• Horizontal datum and chart projections may differ between raster charts. Mariners should understand how a chart’s horizontal datum relates to the datum of the position fixing system in use. In some instances, this may appear as a shift in position. This difference may be most noticeable at grid intersections and during route monitoring.

• Some raster charts cannot be referenced to WGS-84. If any electronic chart cannot be referenced to a WGS-84 chart datum the ECDIS equipment should give a continuous indication of this inaccuracy in order to highlight the position error.

• The display of RNCs features cannot be simplified by the removal of data and features to suit a

particular navigational circumstance or task at hand. When you zoom in or out on scale within the system the raster chart image is only magnified, it does not jump step the available information and provide clear display detail to suit the scale as with an ENC. This could affect the superimposition of radar/ARPA and overload the amount of data being displayed.

• Without selecting different scale charts the look-ahead capability may be limited. This may lead to inconvenience when determining range and bearing or the identity of distant objects.

• Raster charts are drawn in the north-up orientation and the information is written on the chart in that same orientation. When the orientation of RNC within the RCDS display is arranged in anything other than north-up the readability of chart text and symbols may be affected (e.g. when in course-up, route-up display modes).

• It is not possible to interrogate RNC features to gain additional information about charted objects.

• With RNC it is not possible to display a ship’s safety contour or safety depth and highlight it on

the display, unless these features are manually entered during route planning.

• ECDIS systems using ENCs apply prescribed colour and intensity regimes for day, dusk and night time so as to not impair the night vision of the user. Depending on the source of the RNC, different colours may be used to show similar chart information and these may again vary within the day and a night regime from what the user is used to.

• An RNC is intended to be used at the scale of the equivalent paper chart. Excessive zooming in or zooming out can seriously degrade the displayed image or overload the display with too much unreadable data. If the RNC is displayed at a larger scale than the equivalent paper chart, the ECDIS will provide an indication.

• ECDIS can provide an indication of the quality of Hydrographic data used in the ENC. When using RNCs, mariners are require to consult the source diagram or the zone of confidence diagram, if available, to gain this data.

• Lastly, when a Pilot boards a vessel they need to interface with the bridge equipment and bridge team as quickly and seamlessly as possible. The standardised nature of ECDIS greatly assists in this, However, it is important to notify the Pilot when the system is in RCDS mode so that incorrect assumptions about available functionality are avoided.

 

Due to these limitations the vessel must use up-to-date paper charts for the intended voyage if no ENC are available.

 

Alarms and Indications

 

 

While operating in RCDS mode the electronic chart system is not capable of automatically triggering an alarm about events such as possible grounding, passing safety lines or approaching danger areas. Such alarms can only be pre-set when using RNCs in RCDS mode if the mariner first identifies all the hazards concerned during his passage planning process, and then manually enters each of them as individual electronic markers on the RNC. These alarms can include:

 

 

• clearing lines vessel safety contour lines

• isolated dangers

• danger areas

 

 

In full ECDIS mode when using ENCs, however, the system can automatically trigger such alarms, as well as responding to a wider range of additional data about navigational hazards, such as signals picked up from VTS traffic information relayed from outside the vessel's radar and visual range, or lines of magnetic variation. In full ECDIS mode the system is also capable of triggering alarms based on notes entered earlier during passage planning.

 

 

The Master and all deck officers must be familiar with:

 

· The types of alarms available on their ECDIS

· The types of warning (visual, audible or a combination of both) associated with each alarm

· The procedure to switch on and set alarm parameters

 

 

The vessel/voyage specific parameters/alarms should be set during the appraisal stage of passage planning and must be approved by the Master.

 

For setting Safety depth, Safety Contour, Shallow contour, Deep contour and Safety Frame see section on Safety settings

A la r m setting adjustments:

It should be noted that the alarm parameters may require to be amended from their previous settings when beginning a new voyage. Changes to the alarm settings are to be done with the Masters consent. Only the Master is authorized to disable alarms when entering shallow waters to adjust the safety contour etc.

 

It is further recommended that Alarm parameters are adjusted by the Master throughout the voyage to ensure that they are optimised for the prevailing circumstances and conditions. Checks should be made to ensure that user defined limiting parameters, such as the safety contour, are not violated when the passage plan is adjusted after departure.

 

The five mandatory alarms (indicated by audible means or audible and visual means indicating a condition requiring attention) are:

I. crossing safety contour;

II. deviation from route;

III. positioning system failure;

IV. approach to critical point; and

V. different geodetic datum.

 

The guard zone (also known as Safety Frame) provides the user with an advance warning of dangers/cautions. The user sets the dimensions of this guard zone which must be altered according to the prevailing circumstances to prevent unnecessary alarms or to give adequate warning. The navigators need to remember that not all dangers are enclosed by a contour and guard zone remains active even if it is not selected to display on the screen. In order for the alarm system to be properly effective (when the route is being monitored) the own ship’s guard zone must be set in a seamanlike manner, i.e. with a sensible time or range warning depending on proximity to hazards and planned speed etc. It is recommended to set the guard zone “As large as possible as the circumstances allow”.

 

 

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