Plotting positions from electronic position-fixing systems
Care should also be exercised when taking geographical positions from electronic position-fixing systems like GPS, and plotting these onto charts.
The OOW should bear in mind that:
· if the chart datum differs from the datum (usually WGS84) used by the electronic position-fixing system, a datum shift will have to be applied to the position co-ordinates before they are plotted on the chart: it should be noted that where an appreciable datum shift does exist for a particular chart, a 'satellite-derived position' note providing latitude and longitude datum shift values will appear on the chart;
· on charts whose survey source data is very old, the accuracy of those charts may be poor in certain areas: under these circumstances the OOW should not rely totally on position fixing using electronic systems, and should where possible use visual and radar navigational techniques to maintain safe distances off the land.
Navigation in coastal or restricted waters
As a general rule, navigation should be carried out on the most suitable largescale charts on board, and the position of the ship should be fixed at frequent intervals. All relevant navigation marks should be positively identified by the OOW before they are used. Visual and radar position fixing and monitoring techniques should be used whenever possible.
In coastal waters, the OOW should be aware that ships' routeing schemes and ship reporting systems requiring reports to be made to coast radio and vessel traffic stations may exist.
Knowledge of the ship's draught. stability conditions and manoeuvring characteristics is also important. As the ship enters shallow water, squat may have a critical effect on the manoeuvrability of the ship and cause an increase in draught. Squat effect varies in proportion to the square of the ship's speed, and will therefore reduce as speed is reduced.
The importance of all the bridge team fully understanding the coastal waters phase of the passage plan, as well as understanding their individual roles and those of their crew, cannot be stressed too strongly.
