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- Special Order: Areas where under-keel clearance is critical, i.e. the depth is less than 40 meters.
- Order 1a: Areas shallower than 100 meters where under-keel clearance is less critical, but features of concern to surface shipping may exist.
- Order 1b: Areas shallower than 100 meters where under-keel clearance is not considered to be an issue for the surface shipping expected to transit the area
- Order 2: Areas generally deeper than 100 meters where a general description of the sea floor is considered adequate.
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