BARLOVENTO
Contact
What IHO S-44 means for a resort project in the Maldives
Survey orders, uncertainty and datum: what IHO S-44 requires, which order a resort project needs at each stage, and what to ask of a bathymetric deliverable.
IHO S-44 (Edition 6.1.0, 2022) defines hydrographic survey orders — Exclusive, Special, 1a, 1b and 2 — by the horizontal and vertical uncertainty and seabed coverage a survey must achieve, not by the equipment used. For a Maldivian resort project, Order 1b serves feasibility, Order 1a serves design and consenting, and Special Order serves dredge payment and critical structures. A bathymetric deliverable should state the order, the achieved uncertainty, the datum and how it was established, the survey control and the calibration records.
What S-44 is
IHO S-44, Standards for Hydrographic Surveys, is published by the International Hydrographic Organization. The current edition is the sixth (Edition 6.1.0, 2022). It does not prescribe equipment or method; it defines what a survey must achieve — in horizontal and vertical uncertainty, feature detection and seabed coverage — for a stated purpose. A survey is "aligned to S-44" when its specification, execution and deliverables are written against those requirements and the achieved uncertainty is documented.
S-44 was written for safety of navigation, but it has become the reference by which engineering, dredging and legal users judge whether a bathymetric dataset is fit for purpose, because it forces the surveyor to declare what was achieved rather than what was intended.
The survey orders
| Order | Vertical uncertainty (95%) | Horizontal uncertainty (95%) | Seabed coverage | Typical resort use |
|---|---|---|---|---|
| Exclusive | a = 0.15 m, b = 0.0075 | 1 m | 200% with feature detection | Berth pockets, quay walls, critical clearance |
| Special | a = 0.25 m, b = 0.0075 | 2 m | 100% with feature detection | Channels, jetty approaches, dredge payment surveys |
| 1a | a = 0.5 m, b = 0.013 | 5 m + 5% of depth | ≤100%, feature detection | Lagoon dredging and reclamation design, masterplanning |
| 1b | a = 0.5 m, b = 0.013 | 5 m + 5% of depth | Recommended but not required | Reconnaissance, feasibility |
| 2 | a = 1.0 m, b = 0.023 | 20 m + 10% of depth | Recommended but not required | Deep water, not typical for resort sites |
The vertical limit is the total vertical uncertainty, computed as √(a² + (b × d)²) where d is depth — in the shallow water of a Maldivian lagoon the fixed term a dominates. The "typical resort use" column is Barlovento's engineering reading, not part of the standard: S-44 leaves the choice of order to the user of the data.
Choosing the order for a resort project
- Feasibility and acquisition. Order 1b singlebeam or reconnaissance multibeam is usually sufficient to establish depths, reef extents and rough volumes.
- Design and consenting. Order 1a multibeam over the lagoon and works footprint, merged with RTK topography of the island into a seamless land–sea model. This dataset serves the EIA baseline, the design and the enquiry package.
- Dredge payment and acceptance. Special Order over the borrow area and reclamation footprint, pre- and post-dredge, with identical control, datum and processing so that the volume difference is defensible (our Survey and Marine works practices). Payment disputes are almost always disputes about survey uncertainty and datum.
- Structures. Special or Exclusive Order where clearance is critical: seaplane docks, quay faces, berth pockets.
What to ask of a bathymetric deliverable
- The survey order, stated, with the achieved total vertical and horizontal uncertainty documented in the report — not just the order targeted.
- The datum — chart datum, mean sea level or ellipsoid — and how it was established or transferred. In the Maldives, where tidal benchmarks are sparse outside Malé, datum transfer by tide gauge and GNSS should be described.
- Survey control: the reference stations, their coordinates and how they tie to the national or a documented local system.
- Coverage: line spacing or full-coverage statement, with a coverage plot.
- Calibration and QC records: patch test for multibeam, bar checks or sound-velocity casts, tide file applied.
- Deliverable formats: XYZ or gridded dataset, CAD and GIS plans, and the metadata that lets a future surveyor reproduce the result.
Not "is it multibeam?" but "what uncertainty did you achieve, to which datum, with what control?" A singlebeam survey with declared control can be worth more than a multibeam survey with none.
Why it matters commercially
Lenders, insurers and arbitral tribunals do not judge a survey on its equipment. They judge it on whether the uncertainty was declared and achieved, whether the datum is traceable and whether a second surveyor could reproduce the result. Writing the specification against S-44 — and insisting on the documentation it implies — is the cheapest insurance a project buys.
Questions
Is IHO S-44 a certification?
No. S-44 is a standard that surveys are specified and executed against. A surveyor or firm is not "certified" to S-44; a survey is aligned to a stated order and documents the uncertainty achieved.
Which order do I need for dredge volumes?
Special Order over the borrow area and placement footprint, with identical control, datum and processing for the pre- and post-dredge surveys. The order matters less than the consistency between the two surveys.
What is chart datum in the Maldives?
A low-water reference established from tidal observation at a benchmark. Outside Malé the benchmark network is sparse, so project chart datum is usually established locally with a tide gauge and transferred by GNSS, and the relationship to mean sea level is documented.
Sources & references
- International Hydrographic Organization, S-44 Standards for Hydrographic Surveys, Edition 6.1.0, September 2022 — iho.int.
- IHO S-44 Edition 6.0.0 (2020) — introduction of the Exclusive Order and the matrix approach to survey specification.
- Barlovento Survey practice — engineering interpretation of survey orders for resort development in the Maldives (the "typical resort use" column is Barlovento's reading, not part of the standard).
This note is general technical information for project planning in the Maldives. It is not legal advice; regulations and fees change and should be confirmed with the relevant authority at the time of application.
Discuss a project with the firm.
Tell us where the project stands and which practices it requires.
Contact Barlovento