A small “6” on a chart looks simple. Copy it into a notebook without the chart’s notes and you have removed information needed to interpret it. A number is not a complete account of the water beneath a boat.
Ask what the number represents
NOAA explains soundings as water-depth measurements represented on nautical charts. The chart specifies the units, and depth contours connect depth information to help describe the bottom.
For a study exercise, copy one sounding together with the chart’s stated units and relevant reference information. Keep the chart title and source beside it. Do not assume that a unit used by one chart applies to another.
Read more than the isolated figure
Look around the number. What nearby contours, notes or symbols also require explanation? Make a list of questions rather than deciding that the single figure establishes usable clearance.
An invented worksheet might contain “6, unit not recorded”. The correct next step is to recover the missing context. Multiplying or comparing the figure before doing so only makes the uncertainty less visible.
This article is a reading exercise, not a clearance calculation or route assessment. Use current official charting and competent passage planning for real navigation. Our guide to tide-table labels offers another example of why units and reference information belong beside every measurement.