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396 · Datum
ECDIS and Datum
The real ECDIS uses ENC material, produced to standards using WGS-84 Datum. Positioning devices connected to
ECDIS must work in the WGS-84 Datum. The IMO resolution requires that the ECDIS must give an alarm, if the
datum of positioning device is not the WGS-84. In practice this is impossible, because the standard used by
positioning devices (IEC-1162, previously NMEA-183) does not include a Datum message. Consequently, the
classification societies only approve positioning device in which Datum cannot be changed (i.e. is always WGS-84)
to be interfaced to the ECDIS.
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 EPFS (for example GPS or DGPS) ,which has "IEC
61162-1 Ed 2 (2000-7)" indicated in their type approval certificate can support the ECDIS to detect Datum
mismatch.
ECDIS and user selectable local Datum
The ECDIS provides for user a possibility to change viewed datum. See chapter "How to select Datum" on page
398. 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
screen into the user selected Datum. This is very useful, if you like to plot positions from the ECDIS on traditional
paper chart.