Furuno FMD-3100 Manual page 17

The Navigator can further change how soundings are depicted by entering a safety depth in metres (all
soundings with a shallower depth than the safety depth entered are shown in a bold font). This can be clearly
seen in Fig 4 and 5 where the safety depth has been set to 14 metres.
The choice of safety contour is of great importance as it is used to trigger alarms and is also used to decide
how and where on the chart isolated dangers (small shoals, rocks, wrecks, obstructions) are shown. This is in
accordance with IMO ECDIS Performance Standards and IHO S52 Ed 6.1.1, where it is defined that isolated
dangers of depth equal to or less than the own-ship safety contour must always be displayed in 'safe waters'
(waters deeper than the safety contour). Systems must also provide the navigators with the option to decide if
they want the isolated dangers displayed within 'unsafe waters' (waters between the safety contour and the
zero metres contour). This can be seen in Fig 4 and 5. The latter option is given because, as we will see further
on in the chapter, the mariner might be forced to navigate in such 'unsafe' waters.
In Fig 4 the safety contour is set to 10 metres and isolated dangers which result in depths of less than 10
metres are shown within the 'safe' waters area but not in the 'unsafe' waters area as the function "show
isolated danger in shallow area" is not activated.
In Fig 5 the safety contour has been set to 20 metres so the isolated dangers which were visible with the
previous setting of 10 metres are not visible anymore since the area enclosed by the safety contour is considered
an 'unsafe' area as a whole and the "show isolated danger in shallow area" is not activated.
Picture 5
Fig 5: Safety Depth 14 metres, Safety contour 20 metres, Isolated dangers with a depth equal to or below 20m
not shown, compare with Fig 4
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