Marine Glossary

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safety contour

The safety contour is a crucial ENC setting that separates waters considered safe for your vessel from those that are not guaranteed to be safe. It is marked on the chart as a bolder contour so it stands out from other contours. There will also be a water color change at this contour.

There are important subtleties, however, in setting up this contour. The navigation program has a place where you type in the safety contour you want, but the program can only draw contours at the native contours of the ENC. On international charts, the contours are usually at 10m intervals. If you want your safety contour at 6m, and you enter that value, you find that the safety contour is drawn at 10m as that is the nearest contour above what you want.

The native contours of a US chart are based on the contours of the RNC it was made from, which could have been in feet or fathoms. In practice, you enter the safety contour you want (usually equal to your safety depth) and then double check with a cursor pick to see what you actually got. In principle you could look at the contours that are available and choose one of those. You may have to enter a tenth of meter below one you see in order to have it be selected.

When the safety contour ends up rather deeper than you want, you can rely on the safety depth setting to change the safe water soundings to gray, leaving the dangerous ones in black. The safety depth is read and applied digitally, as you entered it, so you know that value, regardless of what chart you are on.

In some navigation programs there is not a separate input for safety depth and safety contour, but just "safety depth" is entered. Such programs treat this value as the normal safety depth (being the depth where soundings change color) and then the safety contour will either be that one, if available, or the next one up. That arrangement covers most situations, and indeed the likely best starting point on systems with both inputs is to set them equal... and then just keep an eye on the chart to see what the effective value is.

The problem with just entering one of the closest contours you see on the chart rather than using your computed safety depth, is the next chart you sail onto may have different native contours.

If a rock, wreck, or other obstruction outside of your safety contour has a known sounding that is less than your active safety contour value, or if the obstruction sounding is unknown, then that symbol will be replaced with the danger symbol. In some areas, your choice of safety contours will affect how many of these danger symbols you see.

See also safety depth and depth contour.