26. Datum
26-2
26.4 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.
26.5 ECDIS and Datum
The true ECDIS uses ENC material, produced to standards using WGS-84 datum. Positioning devices connected to
the ECDIS must work in the WGS-84 datum. IMO requires that the ECDIS must give an alert if the datum of a
positioning device is not the WGS-84. In practice this is impossible, because the standard used by positioning
devices (IEC 61162, previously NMEA 0183) does not include a datum message. Consequently, the classification
societies only approve a positioning device interfaced with the ECDIS in which datum cannot be changed (i.e., is
always WGS-84).
Note: The ability to check datum of position is a relatively new feature for position receivers. It was introduced in
standard IEC 61162-1 Ed2, published in July 2000. Only an EPFS (for example GPS or DGPS) that has "IEC
61162-1 Ed 2 (2000-7)" indicated in its type approval certificate can support the ECDIS to detect datum mismatch.
26.6 ECDIS and User Selectable Local Datum
The ECDIS allows the operator to change viewed datum. This selection of the datum does not change anything
inside the ECDIS for navigation calculation processes or for electronic sea chart display processes. But it changes
the numerical values of positions displayed on the ECDIS display into the operator-selected datum. This is very
useful, especially if you like to plot positions from the ECDIS on a traditional paper chart.
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