The order applied to harbours, berthing areas and critical channels where every shoal matters to deep-draught ships. Limits are 2 m horizontal uncertainty, vertical uncertainty from a = 0.25 m and b = 0.0075, detection of cubic features larger than 1 m, and 100 percent feature search with 100 percent bathymetric coverage. In practice it means full-coverage multibeam, careful sound speed control and a tight tide or ellipsoid solution.
We're Special Order inside the 20 metre contour, so any holiday bigger than a node gets an infill line.
In shallow water the fixed 0.25 m term dominates the vertical limit, so tide and draft errors, not the sonar, usually decide whether Special Order is met.
Also heard as
- IHO Special Order
Related terms
Exclusive Order
The strictest survey order, used for berths and critical channels where under-keel clearance is minimal.
Order 1a
A survey order for coastal waters and fairways where features matter but under-keel clearance is less critical.
S-44
The international minimum standard for hydrographic surveys, setting uncertainty, detection and coverage limits by order.
Holiday
A gap in survey coverage where no data was collected, which must be filled before the survey is complete.
TVU
Total vertical uncertainty: the depth part of a sounding's uncertainty, judged against a depth-dependent limit.