IHO S-44 Order 1a vs Order 2: what changes between them

Every time a new area of interest lands on your desk, one of the first calls you make is which S-44 order it needs to be surveyed to. That call drives everything downstream: line spacing, whether you need full bottom coverage, what vessel and sensor combination you can justify, and how much boat time the budget has to absorb. Get the order wrong and you either oversurvey a stretch of open water nobody will thank you for, or undersurvey an approach channel and end up back out there next season.

IHO S-44 lays out five categories, from tightest to loosest: Exclusive Order, Special Order, Order 1a, Order 1b, and Order 2. Exclusive and Special cover berths and critical channels, anywhere a few centimetres of underkeel clearance matters. The loosest category, 2, covers open water where a general picture of the seafloor is enough because nothing transits close to the bottom. 1a and 1b sit in the middle, and in practice most planners spend their time weighing 1a against 2, since that's where the real tradeoff in survey effort shows up.

What each order classification describes

The standard isn't a label for how careful you were. Each category is defined by a combination of things: the horizontal and vertical accuracy you're expected to achieve, whether you need to search for and report features down to a minimum size, and whether full seafloor coverage is required or a sounding line pattern with appropriate spacing is acceptable.

That last point is the one that changes your field plan the most. Order 1a calls for full seafloor search, meaning your lines, or your multibeam swath overlap, have to be tight enough that nothing on the bottom goes unseen. Order 2 doesn't carry that requirement. You run representative line spacing and accept that smaller features between lines may not get caught, on the reasoning that nothing operating in that zone is at risk from them.

Order 1a vs Order 2: the practical difference

When someone asks what separates 1a from 2, the honest answer skips the accuracy numbers and goes straight to what the water is used for. Order 1a is the default for harbor approaches, recommended tracks, and anywhere depth under the keel is a live constraint for the vessels using it. Order 2 is the default for offshore areas, general reconnaissance, and places where a change in depth of a metre or two doesn't threaten anyone's passage plan.

If you're scoping a job and the client's traffic genuinely transits close to the bottom there, you plan for 1a and budget the boat time that full coverage demands. If the area is a buffer zone, an outer approach nobody's using for close navigation, or a stretch you're surveying mainly to fill a gap in the chart, Order 2 is defensible and considerably cheaper to run.

Where this gets decided before the boat leaves the dock

The order you pick is a survey decision, but the boundary you draw around each order zone is a planning decision, and that's usually the harder one. Charts go out of date, shoals migrate, and the shallow fringe of an AOI is rarely where the old chart says it is. Before committing vessel days to full 1a-standard coverage along a whole approach, it helps to know roughly where the shallow water sits this year, including the parts too thin, too remote, or too exposed for a survey boat to reach safely in the first place.

That's the gap Satellite Bathymetry is built for: an annual depth estimate from satellite imagery, covering your area of interest so you can see where the shallow fringe has shifted before you draw your order boundaries and send the vessel. It doesn't replace the sounding an Order 1a or Order 2 survey is going to run. It just gives you a better starting picture for deciding where that boat needs to go first.

A quick answer, if you only needed one

Order 1a means full seafloor search, tighter accuracy, used where underkeel clearance matters. Order 2 means representative line coverage, looser accuracy, used where it doesn't. The rest of the standard is detail on how those two requirements get specified and checked.

If your next AOI has a shallow margin you're not confident about, pull a depth estimate for it before you lock in the survey order.