Definition
Using a linear feature, most often a charted depth contour such as the 10 metre line, to bound or guide the track. The echo sounder then gives continuous confirmation: while the sounding stays deeper than the chosen contour, corrected for height of tide, the vessel remains in safe water and any shoaling is an immediate warning. The same principle covers keeping a shoreline, a wall or a line of buoys at a fixed distance to one side.
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