Page 95 - Electronic Navigation Cyber Book

SOLAS (Safety of Life at Sea) requirement that charts should be
‘adequate and up-to-date for the intended voyage’.

Non-official, which describes those electronic charts which are issued by
commercial organizations which may use data owned by a hydrographic
authority but are not endorsed by that authority.

An electronic chart may be constructed using either of two types of data,
raster or vector.

Digital Chart Data Formats
One question in the development of ECDIS has been whether the
nautical chart should be digitized in raster or vector format.

Raster chart data is a digitized “picture” of a chart. All data is in one layer
and one format. The video display simply reproduces the picture from its
digitized data file. With raster data, it is difficult to change individual
elements of the chart since they are not separated in the data file. Raster
data files tend to be large, since a data point must be entered for every
picture element (pixel) on the chart.
The advantages of raster charts can be summarized as follows:
1. User familiarity since they use the same symbols and colours as
paper charts.
2. They are exact copies of the paper charts with the same reliability and
integrity.
3. The user cannot inadvertently omit any navigational information from
the display.
4. Cost of production is less than their vector counterpart.
5. Wide availability of official raster charts. ARCS charts, for example,
have near worldwide coverage.
6. By using vector overlays together with appropriate software, raster
charts can be used for all standard navigational tasks normally
undertaken using paper charts. They can also emulate some of the
functions of an electronic display and information system (ECDIS).

Disadvantages of raster charts can be summarized as follows.
1. The user cannot customize the display.
2. When using vector overlays the display may appear cluttered.

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