24-1
24. Datum
24.1 General
Datum is a mathematical model of the earth based on which a sea chart is produced. If the datum of a position
sensor and that of a sea chart are different, a transformation has to be made somewhere in the system. Not doing so
can result in errors of several sea miles. The difference between two datums is never constant, but depends on
position. This means that the difference between WGS 84 and local datum, generally used in paper charts, is not
generally valid with electronic sea charts.
24.2 Paper Charts
Datums used in paper charts have been traditionally national datums for historical reasons. Many paper charts do not
have a marked datum, therefore compatibility with electronic charts may be complicated. In some paper charts, the
correction terms are printed in lieu of datum, for correction of the WGS 84 system satellite locations. The correction
terms are usable but only with the paper chart in question.
24.3 Electronic Sea Charts
Until now, electronic sea charts have generally been based on rasterizing or vectorizing of paper charts, resulting in
the respective transfer of the local datum.
Exceptions:
• The ARCS (raster) material includes polynomials for each chart, making it possible for the ECS system to solve
the difference between the WGS 84 datum and the local datum with an accuracy sufficient for authority
responsibility. In some charts, the mentioned difference is not known with sufficient accuracy, resulting in
displaying a permanent warning window when displayed in ARCS compatible systems.
• ENC vector material which according to S57 standard has to be produced by a National Hydrographic Office in
the WGS-84 datum.
24.4 Positioning Devices and Datum
In early days of electronic positioning devices, datums received little attention because the commonly used systems
utilised special charts (like Decca charts). Later on, data output was added to these systems, but still no attention was
paid to datums and the position errors were considered as an inaccuracy of the system. With the spread of the GPS,
however, datum has become better known. An accurate position is of no value if co-ordinates are in a wrong datum.
GPS satellites utilize the WGS-84 datum. However, the WGS-84 datum is not a general solution for all positioning
systems, due to differences between electronic sea chart system and the chart material displayed.
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