Fundamentals

Position list vs AIS vs port line-up

Published 9 August 2026 · updated 17 October 2026

People use these three terms as if they meant the same thing, but they answer completely different questions. Mixing them up is the most common and most expensive mistake in chartering work, because two of them say nothing at all about whether a ship is available.

The three, defined

One ship, three records, three different answers

Take a Supramax at anchor off a discharge port. Each record describes it correctly. But each one points a broker to a different next step.

Only the middle row is about the ship’s next job, and it is the only one a broker can act on. The other two are background: they help you check the middle row, but they cannot replace it.

Why a live fix tells you nothing about availability

A ship waiting off Singapore may be on charter for the next six weeks. A ship sailing fast across the Indian Ocean may be free when it arrives. Position and availability are different things, and better AIS data cannot turn one into the other.

AIS carries a destination field that the crew types, and most attempts to work out availability from AIS use only this field. The field is usually filled in: measured across 94,337 fixes, 91% carry a destination and 86% carry an ETA. The problem is matching it. Only about half of those destinations, 47,399, match a real port, because the field is free text and the crew can type anything. It is a hint, not a schedule.

A list of ships whose crews have entered a port as the destination is still a real and useful document, as long as you call it what it is. It says a crew typed that port into a field. It does not say the authority has given the ship a berth, it does not say the ship will arrive, and it certainly does not say the ship is available. Under its own name, it helps you check a position. Presented as a line-up, it becomes a berthing order that nobody published.

Why the position list is the valuable one

It is the only one of the three that answers the question chartering asks, and the only one that cannot be bought. It arrives as lines of e-mail:

JOGELA           OPEN SPORE 11/14 AUG DELY DOP
MERIDIAN EXPRESS OPN AG 12/16 AUG
ALBA             OPN ECSA END AUG

A desk that has collected these for two years has information that no competitor can buy. Any tool for it must keep the owner's original wording next to whatever it reads from it. The reason is that no structured version of OPEN SPORE 11/14 AUG DELY DOP keeps everything the line said. Reading these lines field by field is a skill of its own, and how to read a tonnage circular explains one line by line.

A position is a claim with a date on it

The problem with the valuable record is that it goes out of date, and the most useful positions go out of date fastest. A position is someone’s statement about the future, made at one moment. It stops being true when the owner fixes the ship, when the discharge port is delayed, or when the plan changes and nobody tells anyone. The line does not update itself, and the sender rarely writes to withdraw it.

So a position needs three things recorded next to it, and a plain list usually loses them: who said it, when they said it, and the area it covers rather than the exact port. Without the source you cannot judge it. Without the date you cannot tell how old it is. Without the area you cannot search it, because open Singapore and open West Coast India must both appear for a cargo loading in the Arabian Gulf. The dates have their own terms, which laycan, laydays and cancelling explains.

What a line-up is, and is not

A line-up is the port authority's berthing order: which vessel, which berth, which cargo, which agent, in what order. It is not an arrival list made from AIS. If you present one as the other, you quote a berthing order that does not exist.

No port authority publishes an IMO number. Any IMO number shown next to a line-up entry belongs to the hull that a matching tool linked to the authority's text. This matters because authorities print whatever name they use for the ship, often a name it stopped using when it was sold years ago.

There is also no world line-up feed to buy. Every authority publishes separately, in its own format and on its own schedule, or not at all. So coverage is offered port by port. Each port needs a parser (a program that reads the publication) that someone has to write and then keep working. Port line-up data sources shows what the publications actually look like.

The constraint all three miss

Drydock and special survey dates. A ship booked for a special survey is not free for a voyage that overlaps it, however good its position looks and wherever AIS says it is. These dates are known months ahead. But a position list, by its nature, cannot hold them.

This is also why none of the three records is complete on its own. The position list is the record to act on. The AIS fix is the record to check it against. The line-up records what the port has agreed to. The survey date is a limit outside all of them. Keeping four separate answers apart is more work than mixing them into one feed, but it is the only way to get the right answer.

Keeping the three apart is, first of all, a design decision. The position list on this site holds what owners have said, shows an AIS fix next to it as a separate fact with its own age, and makes no claim about the line-up at all.

An open position can also be offered on a public board.

References

• IMO, AIS transponders, on the carriage requirement in SOLAS regulation V/19. imo.org
• IMO, International Convention for the Safety of Life at Sea (SOLAS), 1974. imo.org
• ITU, Recommendation ITU-R M.1371: Technical characteristics for a VHF automatic identification system using time division multiple access in the maritime mobile service. itu.int
• IMO, IMO identification number schemes, resolutions A.600(15) and A.1078(28). imo.org

Related