How can a coastal navigator use a bathymetric contour (veril de sonda, e.g., the 20-meter depth line) as a safety line of position during reduced visibility?

1. By maintaining continuous soundings and recognizing when crossing or paralleling the charted depth contour, providing a physical boundary against grounding
2. By assuming the depth contour replaces all COLREGs steering rules
3. By matching engine RPM to the sounding depth in meters
4. By calculating the vessel's speed based on the color of the sea bottom