Data Sources & Methodology

Data source

All figures on StraitTraffic are derived from AIS (Automatic Identification System) broadcasts. Ships above 300 gross tonnage on international voyages, and all passenger ships, are required to transmit their identity, position, speed and course. StraitTraffic receives these broadcasts through a network of terrestrial receivers via a commercial data feed and processes them on its own servers.

Terrestrial AIS coverage is limited by the range of shore receivers. Coverage inside a strait is usually good; further out at sea it fades. Vessels can also switch AIS off, transmit incorrect data, or be affected by GPS interference, which is a known issue in several of the regions we monitor.

What StraitTraffic calculates itself

• Vessels in zone now — vessels that have transmitted a position inside the monitored strait zone in the last 30 minutes.
• Transits — for each strait we define three areas: the strait zone and an approach area on each side. A transit is counted when a vessel is observed in one approach area, then inside the zone, then in the opposite approach area. A vessel that enters the zone and turns back is not counted. Repeated AIS messages never create additional transits: one physical passage is one transit.
• Direction — determined by which approach area the vessel came from and which one it went to.
• Ship type — taken from the AIS type code (tanker, cargo, passenger). Container ships and LNG/LPG carriers cannot be identified from the code alone, so they are recognised from the vessel’s name and dimensions; when in doubt the vessel stays in its base category. Vessels with unknown or unusual types are shown as “Other / Unknown” rather than being dropped.
• Daily history — completed transits are stored and summed per UTC day from the day monitoring began. We do not import historical data from elsewhere.
• Status — the number of transits in the last 24 hours compared with the strait’s own average: over 30 days when that much history exists, otherwise over 7 days, otherwise over the days available. Within ±25% of the average the status is “Normal traffic”; 25–50% below is “Traffic below normal”; 50–75% below is “Traffic significantly below normal”; more than 75% below is “Significant disruption”; more than 25% above is “Traffic above normal”. With fewer than two full days of history the status reads “Not enough data yet”.

Data quality safeguards

• Position reports that imply an impossible speed (a jump in coordinates) are discarded.
• Messages older than the last accepted one for the same vessel are ignored.
• A vessel that disappears inside the zone for longer than a plausible passage time is recorded as an unconfirmed passage and excluded from all counts.
• Comparisons with weekly or monthly averages are only shown once the history actually covers that period.

Data freshness

Every page shows the state of the AIS feed. “Live” means a message was received within the last two minutes; “Delayed” means the last message is older than that but younger than 15 minutes; “Temporarily unavailable” means the feed has stopped and the positions shown must not be treated as current.

Limitations

StraitTraffic is not an official navigation or vessel-traffic service and must not be used for navigation, safety or legal purposes. Counts can be lower than reality where receiver coverage is poor, and vessel identities depend on what ships broadcast. Treat the figures as an indicator of activity and trend, not as an authoritative record.