Annex 14 - Electronic Charts

IMO Safety of Navigation Circular SN/Circ.207 (which follows these guidance notes) explains the differences that exist between various official modes of operation of electronic chart systems. (This circular is to be revised in 2007.) The following notes give advice and explain current requirements for the use of electronic charts on UK vessels:

Key Points

  • Amendments to the IMO ECDIS Performance Standards (A.817) to allow for an RCDS mode of operation.
  • Requirement to use an appropriate portfolio of up-to-date paper charts when operating ECDIS in the RCDS mode and, until further notice, when operating ECDIS with Electronic Navigational Charts (ENCs).
  • Limitations when operating ECDIS in RCDS mode.

1.) In 1998 IMO adopted amendments to the performance standards for Electronic Chart Display and Information System (ECDIS) to permit ECDIS to operate in the Raster Chart Display System (RCDS) mode, with Raster Navigational Charts (RNCs), when Electronic Navigational Charts (ENCs) are not available. In addition, IMO has approved a Safety of Navigation (SN) Circular which describes differences between RCDS and ECDIS. The text of the Circular SN/Circ.207 follows these notes.

2.) RNCs are raster charts that conform to International Hydrographic Organization (IHO) specifications and are produced by digitally scanning a paper chart image. The image may be either the finished chart itself or the stable colour bases used in the multi-colour printing process. The resulting digital file may then be displayed in an electronic navigation system where the vessel’s position, generally derived from electronic position fixing systems, can be shown. Since the displayed data are merely a digital photocopy of the original paper chart, the image has no intelligence and, other than visually, cannot be interrogated.

3.) ENCs are vector charts that also conform to IHO specifications. They are compiled from a database of individual items (‘objects’) of digitised chart data which can be displayed as a seamless chart. When used in an electronic navigation system, the data can then be reassembled to display either the entire chart image or a user-selected combination of data. ENCs are intelligent in that systems using them can be programmed to give warning of impending danger in relation to the vessel’s position and movement. It should be noted that whilst ENCs have been developed for many areas, it may be several years before comprehensive worldwide coverage is available. Up to date information on ENC coverage can be found in the IHO Global ENC Catalogue (

4.) The IMO Circular highlights the fact that ENCs possess more capabilities than RNCs. As a result, ECDIS operating with ENCs is a more powerful navigational tool than ECDIS operating in the RCDS mode. For example, when operating with ENCs, alarms are triggered automatically by the data but when operating with RNCs the same alarms only occur if the mariner first identifies and electronically marks the features concerned during passage planning.

5.) Further details on compiling an appropriate folio of paper charts when operating ECDIS in the RCDS mode are contained in MGN 285. The primary consideration must be that navigational safety is not compromised in the event of failure of the electronic chart system and that the vessel is able to navigate to a safe haven.

6.) Information for small fishing vessels and commercial vessels is contained in:
MGN 319 - Acceptance of Electronic Chart Plotting Systems for Fishing Vessels under 24m and small vessels in commercial use (Code boats) up to 24m load line length.


IMO Circular SN/Circ.207 - 7 January 1999
(Currently under revision January 2007)


1.) The Maritime Safety Committee, at its seventieth session (7 to 11 December 1998), adopted amendments to the performance standards for Electronic Chart Display and Information Systems (ECDIS) to include the use of Raster Chart Display Systems (RCDS).

2.) These amendments permit ECDIS equipment to operate in two modes:

2.1) the ECDIS mode when ENC data is used; and

2.2) the RCDS mode when ENC data is not available.

However, the RCDS mode does not have the full functionality of ECDIS, and can only be used together with an appropriate portfolio of up-to-date paper charts.

3.) The mariners’ attention is therefore drawn to the following limitations of the RCDS mode:

3.1) unlike ECDIS where there are no chart boundaries, RCDS is a chart-based system similar to a portfolio of paper charts;

3.2) Raster navigational chart (RNC) data, itself, will not trigger automatic alarms (e.g. anti-grounding). However, some alarms can be generated by the RCDS from user-inserted information. These can include:

  • clearing lines
  • ship safety contour lines
  • isolated dangers
  • danger areas

3.3) horizontal datums and chart projections may differ between RNCs. Mariners should understand how the chart horizontal datum relates to the datum of the position fixing system. In some instances, this may appear as a shift in position. This difference may be most noticeable at grid intersections and during route monitoring;

3.4) chart features cannot be simplified or removed to suit a particular navigational circumstance or task at hand. This could affect the superimposition of radar/ARPA;

3.5) without selecting different scale charts, the look-ahead capability may be somewhat limited. This may lead to some inconvenience when determining range and bearing or the identity of distant objects;

3.6) orientation of the RCDS display to other than chart-up, may affect the readability of chart text and symbols (e.g., course-up, route-up);

3.7) it may not be possible to interrogate RNC features to gain additional information about charted objects;

3.8) 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;

3.9) depending on the source of the RNC, different colours may be used to show similar chart information. There may also be differences in colours used during day and night-time;

3.10) an RNC should be displayed at the scale of the paper chart. Excessive zooming in or zooming out can seriously degrade RCDS capability, for example, by degrading the legibility of the chart image; and

3.11) mariners should be aware that in confined waters, the accuracy of chart data (i.e., paper charts, ENC or RNC data) may be less than that of the position-fixing system in use. This may be the case when using differential GNSS. ECDIS provides an indication in the ENC which allows a determination of the quality of the data.

4.) Member Governments are requested to bring this information to the attention of the relevant authorities and all seafarers for guidance and action, as appropriate.



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