Chart No. 1 · I 21
Dredged channel / area
Depths & contours
Dashed limits with a legend give the depth a channel is dredged and maintained to.
What it means
Dashed lines mark the limits of a dredged channel or basin, with a legend such as '7.3 m' (sometimes with the word 'maintained' or the year of the latest survey) giving its controlling depth.
Why it matters
If the legend shows a survey year rather than 'maintained', silting may have reduced the depth since. Big ships need the whole dredged depth — small craft should stay clear of their track anyway.