The clearest evidence of operational passage is several distinct vessels completing continuous crossings in both directions, along plausible routes and within a recent time window. An empty AIS map does not prove closure: coverage, delays, filtering, switched-off equipment or vessels without carriage obligations can all create gaps. A general disruption requires official notices, incidents, cancellations or suspensions and sustained evidence that ships cannot complete the passage.
1. Define what the investigation is trying to prove
“There is traffic” can mean very different things. A single tug moving near the coast does not demonstrate that the international corridor is functioning normally. Likewise, two tankers passing do not prove the absence of restrictions, queues, convoys or a severe fall in volume.
The useful question is narrower: are commercial vessels completing transit between the Gulf of Oman and the Persian Gulf, on coherent tracks and within a recent interval? The next step is to assess intensity, regularity, vessel types and any operational limitations.
2. What AIS is and what it can reveal
The Automatic Identification System exchanges identity, position, course, speed, navigational status and other data between ships and shore stations. The International Maritime Organization requires AIS on ships of 300 gross tonnage and above engaged on international voyages, certain cargo ships of 500 gross tonnage and above, and passenger ships, subject to possible exemptions.
Class A units can transmit position every few seconds while under way. This makes it possible to reconstruct a track and distinguish a vessel crossing the Strait from one at anchor, manoeuvring near a port or appearing only briefly.
References: IMO, AIS transponders and U.S. Coast Guard NAVCEN, Class A position reports.
3. AIS is not a perfect census of the sea
AIS uses maritime VHF radio channels. Terrestrial networks receive signals within range; satellite services expand coverage but can introduce delays, gaps or differences between providers. A public map shows what its network received and chose to publish, not necessarily every vessel present.
Not every craft is required to carry AIS. Ships may be exempt, equipment can fail, data can be entered incorrectly, and transmission may cease in circumstances allowed by the rules to protect navigational information. Therefore, AIS presence is positive evidence of traffic, while AIS absence is only an absence of that signal.
Dynamic data—position, speed and course—should also be distinguished from manually entered voyage data such as destination or ETA. A physically coherent track is usually more reliable than an outdated destination field.
IMO requires AIS to remain in operation subject to defined exceptions, and its guidance identifies inherent limitations. See IMO Resolution A.1106(29) and IALA Guideline 1082.
4. The unit of analysis should be a completed crossing, not a dot
A static screenshot can mislead. Operational transit should be verified as a time sequence: approach from one side, entry into the Strait, passage through the traffic area and exit on the other side.
- Stable identity. The same MMSI or identifier persists throughout the track.
- Compatible speed. The vessel is making way rather than merely drifting or remaining at anchor.
- Coherent course. The track advances toward the opposite side without impossible jumps.
- Temporal continuity. Reports connect entry and exit within a reasonable interval.
- Plausible geography. The route follows the recognised corridor or officially communicated alternatives.
One completed crossing is stronger evidence than ten icons without history. It also avoids double-counting a vessel or confusing the anchorages near Fujairah, Bandar Abbas and Musandam with international transit.
5. Metrics that produce an operational picture
Unique vessels
Count distinct ships, not AIS messages. Thousands of reports may come from one vessel.
Measures real diversity.Completed crossings
Record verified entry and exit between both sides of the Strait.
Measures effective transit.Direction
Separate Gulf-bound traffic from outbound traffic toward the Arabian Sea.
Detects asymmetry.Vessel type
Tankers, LNG carriers, container ships, general cargo, tugs and others.
Distinguishes economic function.Median speed, waiting time before transit, the number of anchored ships and deviation from a comparable historical baseline are also useful. No single figure defines status; the combination and direction of change matter.
6. Without a baseline, “many” and “few” mean little
Maritime traffic changes by hour, weather, port scheduling, maintenance, holidays and cargo availability. Comparing a quiet night-time hour with a daily average can create a false alarm.
A cautious assessment uses equivalent windows: the latest 6 or 12 hours compared with similar previous periods; tanker volume compared with its usual pattern; speeds and queues compared with their historical distribution. The aim is not perfect normality, but a large and sustained deviation that agrees with other evidence.
A partial fall may indicate restriction or logistical adaptation. An effective closure requires broad, persistent disruption rather than a short oscillation.
7. Official warnings explain what the map cannot
Navigational warnings and maritime authority communications report mines, restricted zones, closed channels, GNSS interference, route changes, exercises, security threats and other conditions affecting passage.
The Strait has an IMO-adopted traffic separation scheme. During a crisis, vessels may be instructed to use an alternative corridor; judging them only against the normal geometry would be wrong. Current warnings must therefore be checked for temporary suspension, alternative routing or restrictions by vessel type.
The IMO describes the Hormuz routeing scheme and centralises maritime resources. NAVAREA IX publishes regional navigational warnings. Sources: IMO, Strait of Hormuz shipping route and NAVAREA IX Warnings.
8. Ports, terminals and commercial operations complete the picture
A few ships can be moving while much of the trade remains suspended. Economic operation should also consider whether ports and terminals announce calls, tankers load or discharge, departures are confirmed and shipping lines maintain services.
Port signals distinguish an evacuation corridor, military movement or exceptional transit from sustained commercial activity. They also reveal lag: a vessel may cross today even though its cargo was scheduled several days earlier.
A single company statement is not universal proof. Confidence improves when several terminals, operators, authorities and independent tracks point in the same direction.
9. A practical confidence matrix
Positive evidence can support an “open” classification even when volume is reduced; “open with restrictions” may be more accurate. A closure claim needs a higher standard: sustained absence of commercial crossings, notices or instructions suspending transit, and independent confirmation that vessels cannot complete passage.
10. Common mistakes when reading an AIS map
- Counting dots instead of tracks. An icon does not prove a crossing.
- Using one screenshot. Operational status requires a time window.
- Confusing anchorage with transit. Vessels may wait for hours or days.
- Interpreting a gap as a blockade. Coverage or reception may have failed.
- Ignoring craft without AIS obligations. The map does not show all local traffic.
- Treating declared destination as fact. It can be wrong or outdated.
- Using the wrong baseline. Different hours and days have different patterns.
11. What would count as evidence of effective closure
An effective closure is not inferred because traffic “looks low”. It requires convergence: a prolonged interruption of commercial crossings in both directions; official warnings preventing or suspending passage; incidents or obstacles incompatible with navigation; ports and operators confirming cancellations; and no functioning alternative route.
If verified crossings continue, “closed” is difficult to support even when danger is extreme. Categories such as open with restrictions, severely reduced transit or partial interruption may be more accurate.
This asymmetric standard is deliberate: an extraordinary conclusion should require stronger evidence than a cautious intermediate classification.
Frequently asked questions
Does one tanker crossing prove the Strait is open?
It proves that vessel completed a passage at that time. It is important positive evidence, but it does not prove normal conditions or the absence of restrictions for other ships.
Does an empty map mean closure?
No. Delays, filters, coverage failures, switched-off equipment or a temporary absence of vessels can all produce an empty view.
How long should traffic be observed?
It depends on the normal pattern and traffic intensity. Several hours are more useful than a screenshot; a closure conclusion requires persistence and external confirmation.
Can AIS data be manipulated?
It can contain errors, anomalous positions, misconfigured identities or interference. That is why coherent tracks and independent confirmation are essential.
Sources and editorial notes
- International Maritime Organization — AIS transponders and carriage requirements.
- IMO Resolution A.1106(29) — operational use and limitations of shipborne AIS.
- U.S. Coast Guard NAVCEN — AIS overview.
- IALA Guideline 1082 — An overview of AIS.
- International Maritime Organization — Strait of Hormuz route and maritime resources.
- NAVAREA IX — navigational warnings.
Sources accessed 14 July 2026. This guide describes a public verification method and does not replace professional navigation systems or warnings received through authorised maritime channels. Corrections: correcciones@estrechoormuz.com.