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:
Amendments to the IMO ECDIS Performance Standards (A.817) to
allow for an RCDS mode of operation.
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).
when operating ECDIS in RCDS mode.
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.
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 vessels 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.
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 vessels 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 (http://services.ecc.as/ihocc/public)
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.
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.
Information for small fishing vessels and commercial vessels is
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.
Circular SN/Circ.207 - 7
under revision January 2007)
BETWEEN RCDS AND ECDIS
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).
amendments permit ECDIS equipment to operate in two modes:
the ECDIS mode when ENC data is used; and
the RCDS mode when ENC data is not available.
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.
The mariners attention is therefore drawn to the following
limitations of the RCDS mode:
unlike ECDIS where there are no chart boundaries, RCDS is a
chart-based system similar to a portfolio of paper charts;
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:
safety contour lines
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;
features cannot be simplified or removed to suit a particular
navigational circumstance or task at hand. This could affect
the superimposition of radar/ARPA;
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
of the RCDS display to other than chart-up, may affect the readability
of chart text and symbols (e.g., course-up, route-up);
may not be possible to interrogate RNC features to gain additional
information about charted objects;
is not possible to display a ships safety contour or safety
depth and highlight it on the display, unless these features
are manually entered during route planning;
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;
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;
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.
Governments are requested to bring this information to the attention
of the relevant authorities and all seafarers for guidance and
action, as appropriate.