In some official electronic navigational charts (ENCs), isolated shoals, with a depth value less than the depth of the surrounding area, are depicted as spot soundings instead of isolated dangers, as required by the IHO S-57 standard.
According to the IHO Presentation Library, such soundings are not shown on an ECDIS screen when utilising the "Standard" and "Base" Display modes.
To have these soundings shown, the ECDIS display should be switched to "Custom" mode and the "spot soundings" layer should be switched on.
The Transas Navi-Sailor ECDIS software generates automatic anti-grounding alarms in voyage planning (check route) and monitoring modes for (S)ENCs, in spite of the fact that such soundings are not shown in ECDIS standard and base display.
This does not affect Transas TX-97 charts produced from paper sources since such shoals are always depicted as obstructions and are always displayed. However, this could affect TX-97 charts converted from official ENCs.
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