2.2 Operational Considerations 2.2.1 Similarity to the paper chart Because much experience is embodied in the paper chart, and to avoid confusion in the extended period while paper charts and ENCs coexist, the two presentations should be similar wherever possible. While recognising that the application of ECDIS extends beyond that of the paper chart, paper chart practice should be followed unless otherwise specified in these specifications. However, because the ECDIS display uses emitted light, compared with reflected light for the paper chart, ECDIS must switch to a negative image of the chart at night, using a dark background in place of the white background of the paper chart, in order not to impair night vision. In addition, studies and early experience indicate that the need for good visual communication between the ECDIS display and the user may require simplification and change of symbols traditionally used on paper charts. Some alternative display methods have been introduced as options in the Presentation Library. 2.2.2 Distinguishing between features and between sources It should be possible to distinguish clearly on the display between a very large number of features. These are listed as classes in section 2.1, such as different water depths, various types of buoys and beacons, chart lines compared with navigators lines, and soon. It should also be possible to distinguish between sources, such as ENC information hand-entered N to Ms, local information, and mariner's notes and manufacturer's additional information. The means available for coding these distinctions are limited. They include - coloured areas, lines and symbols, - coded lines (e.g. dashed) and areas (e.g. patterned, - symbols, - text labels (but these cause clutter, - cursor interrogation, - switching a class of information on/off either automatically or by hand, - splitting information into separate windows, - commonsense interpretation (e.g. a red buoy, a red arc indicating alight sector, and a dotted red line indicating planned route, are unlikely to be confused, even though all are coloured red. Because there are more demands for making distinctions than there are coding methods available, duplicate coding will sometimes be unavoidable, as in the above example. Distinctions should be made logically and systematically, giving priority to features that have greater operational significance. 2.2.3 Route Planning / Route Monitoring Look-ahead The IMO Performance Standards for ECDIS distinguish between the route planning and route monitoring modes of using ECDIS. It is expected that in route planning the display will be viewed, without urgency, from the normal screen viewing distance of about 70 cm, and so the display can contain considerable detail without causing confusion.
16 S, Edition 6.0 March 2010 Experience up to now of route monitoring indicates that in that mode the display will be used for immediate decision-making, sometimes under stress, and that it maybe viewed from a distance of several metres. The route monitoring display should therefore be planned to present only the immediately relevant information, in a manner that ensures it can betaken in quickly, clearly and without ambiguity. In particular, text is difficult to read and tends to cause clutter. It should be kept to a minimum on the route monitoring display. If there is a delay in preparing data for the route monitoring display (e.g. due to a request for scale change, or look-ahead to another area) the ECDIS should inform the mariner. The previous display should be maintained, and updated, until the new display is ready for draw.