The workhorse order for coastal waters, fairways and approaches. Horizontal uncertainty is 5 m plus 5 percent of depth and vertical uncertainty uses a = 0.5 m and b = 0.013. Cubic features over 2 m must be found in depths to 40 m, and over 10 percent of depth beyond, with 100 percent feature search. Order 1b keeps the same uncertainty limits but drops feature detection, and Order 2 relaxes horizontal uncertainty to 20 m plus 10 percent of depth for deep, general-description areas.
Outside the sea buoy the spec relaxes to Order 1a, so we can open the line spacing up a bit.
Order 1a demands a complete feature search but allows less than full bathymetric coverage, a distinction often lost when people quote coverage percentages.
Related terms
Special Order
A survey order for harbours and channels where under-keel clearance is critical, with full coverage and 1 m feature detection.
S-44
The international minimum standard for hydrographic surveys, setting uncertainty, detection and coverage limits by order.
Feature detection
The ability of a survey system to find seabed objects of a stated minimum size, usually given as a cube edge length.
THU
Total horizontal uncertainty: how far a sounding's true position could be from where it was recorded, at 95 percent confidence.
TVU
Total vertical uncertainty: the depth part of a sounding's uncertainty, judged against a depth-dependent limit.