Chart No. 1 · I 31
Approximate depth contour
Depths & contours
A dashed contour line means the depth line is only approximate — survey data there is thin.
What it means
A depth contour drawn as a dashed line is approximate: the surveys in that area were too sparse or too old for the line to be charted firmly.
Why it matters
Treat dashed contours with extra margin — the bottom between old survey lines can hold surprises. The source diagram on the chart shows how well each area was surveyed.