The international standard that defines the survey orders (Exclusive, Special, 1a, 1b and 2) and the minimum quality each must reach. For every order it sets maximum horizontal and vertical uncertainty at 95 percent confidence, the size of cubic feature that must be detected, and the required feature search and bathymetric coverage. Contracts and national specifications cite it as the baseline, then usually tighten it.
Spec says S-44 Order 1a for the fairway and Special Order inside the breakwater.
When a hydrographer says a survey met Special, the claim covers a whole package of uncertainty, detection and coverage, not just a depth accuracy figure.
Also heard as
- IHO S-44
Related terms
Exclusive Order
The strictest survey order, used for berths and critical channels where under-keel clearance is minimal.
Special Order
A survey order for harbours and channels where under-keel clearance is critical, with full coverage and 1 m feature detection.
Order 1a
A survey order for coastal waters and fairways where features matter but under-keel clearance is less critical.
TVU
Total vertical uncertainty: the depth part of a sounding's uncertainty, judged against a depth-dependent limit.
THU
Total horizontal uncertainty: how far a sounding's true position could be from where it was recorded, at 95 percent confidence.