Page 127 - Electronic Navigation Cyber Book

computer network processes the positioning information and controls the
integrated bridge system’s display and control functions.

• Chart Data Base: At the heart of any integrated bridge system lies an
electronic chart. An electronic chart system meeting International
Maritime Organization (IMO) specifications for complying with chart
carrying requirements is
an Electronic Chart Display and Information System (ECDIS). All
other electronic charts are known as Electronic Chart Systems (ECS).
Following sections discuss the differences between these two types of
electronic charts.

An integrated bridge system may receive electronic chart data from the
system manufacturer or from the appropriate government agency. The
mariner can also digitize an existing paper chart if the system
manufacturer provides a digitizer. Electronic charts can differentiate
between and display different types of data far better than conventional
charts. Paper charts are usually limited to four colors, and they display
all their data continuously. An electronic chart can display several colors,
and it can display only the data the user needs. If the electronic chart is
part of an ECDIS, however, it must always display the minimum data
required by IMO/IHO. The database for a typical civilian electronic chart
contains layers consisting of hydrography, aids to navigation,
obstructions, port facilities, shoreline, regulatory boundaries and certain
topographic features. Other layers such as communication networks,
power grids, detailed bathymetry, and radar reflectivity can also be made
available. This allows the user to customize his chart according to his
particular needs, something a paper chart cannot do.

• System Display: This unit displays the ship’s position on an electronic
chart and provides information on sensor status and ship’s control
systems. It displays heading data and ship’s speed. It provides a station
where the operator can input warning parameters such as minimum
depth under the keel or maximum cross track error. It plots the ship’s
position and its position in relation to a predetermined track.
There are two possible modes of display, relative and true. In the relative
mode the ship remains fixed in
the center of the screen and the chart moves past it. This requires a lot
of computer power, as all the screen data must be updated and re-drawn
at each fix. In true mode, the chart remains fixed and the ship moves
across it. The operator always has the choice of the north-up display. On
some equipment, the operator can select the course-up display as well.
Each time the ship approaches the edge of the display, the screen will
re-draw with the ship centered or at the opposite edge.
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