Neptune Atlas
Position list vs AIS vs port line-up
These three get used interchangeably and they answer completely different questions. Conflating them is the most common and most expensive mistake in a chartering workflow, because two of them say nothing at all about whether a ship is available.
The three, precisely
| What | Answers | Where it comes from |
|---|---|---|
| AIS position | Where she is right now | Automatic transponder feed. Always available, bought from a vendor. |
| Position list | When she is free next, and where | E-mail from owners and competing brokers. Nobody sells it. |
| Port line-up | What is expected at the berth, in what order | The port authority's own publication. |
One ship, three records, three different answers
Take a Supramax at anchor off a discharge port. Each record describes her correctly and none of them agrees with the others about what a broker should do next.
| Record | What it says about her | What it does not say |
|---|---|---|
| AIS position | At anchor, in this position, twenty minutes ago, with a crew-keyed destination in a field | Whether she is waiting for a berth, waiting for orders, or already fixed for her next voyage |
| Position list | Open at that port between the 11th and the 14th, delivery dropping outward pilot | Whether she is still open, since the line was written before the owner took a firm offer this morning |
| Port line-up | Booked at berth 4 behind two other vessels, discharging fertiliser, agent named | Anything at all about what she does afterwards, or who to ask |
Only the middle row is about employment, and it is the only one of the three that a broker can act on. The other two are context: useful for sanity-checking the middle row, useless as a substitute for it.
Why a live fix tells you nothing about availability
A ship idling off Singapore may be under charter for the next six weeks. A ship steaming hard across the Indian Ocean may be open on arrival. Position and availability are simply not the same variable, and no amount of AIS resolution converts one into the other.
AIS carries a crew-keyed destination field, which is where most attempts to derive availability start and end. Presence is not the problem: measured across 94,337 fixes, 91% carry a destination string and 86% carry an ETA. Resolution is. Only about half of those destinations, 47,399, match a real port, because the field is free text and a crew types what it likes. It is a hint, not a schedule.
A list of who has declared for a port is still a real and useful document, as long as it is called what it is. It says a crew typed that port into a field. It does not say the authority has given her a berth, it does not say she will arrive, and it certainly does not say she is available. Presented under its own name it sanity-checks a position; presented as a line-up it is a berthing order nobody published.
Why the position list is the valuable one
It is the only one of the three that answers the question chartering actually asks, and it is 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 been collecting those for two years has an asset no competitor can
purchase at any price. That is also why any tool for it has to keep the owner's original
wording alongside whatever it parses: no structured reading of
OPEN SPORE 11/14 AUG DELY DOP carries everything the line said. Reading those lines
field by field is a skill of its own, and
how to read a tonnage circular takes one apart.
A position is a claim with a date on it
The catch in the valuable record is that it decays, and it decays fastest exactly where it matters. A position is somebody's statement about the future, made at a moment, and it is overtaken by the owner taking a fixture, by a delay at the discharge port, or by a change of plan nobody circulated. The line does not update itself and the sender rarely writes to retract it.
So a position needs three things recorded beside it that a bare list usually loses: who said it, when they said it, and the area it was said about rather than the exact port. Without the source you cannot judge it; without the date you cannot age it; and without the area you cannot search it, because open Singapore and open West Coast India both need to come back for a cargo loading in the Arabian Gulf. The dates themselves have their own vocabulary, which laycan, laydays and cancelling sets out.
What a line-up is, and is not
A line-up is the authority's berthing order: which vessel, which berth, which cargo, which agent, in what sequence. It is not an arrival list derived from AIS, and presenting one as the other gets a berthing order quoted that does not exist.
No port authority publishes an IMO number. Any IMO shown against a line-up entry belongs to whatever hull a matcher attached to the authority's text, which matters because authorities print whatever they call the ship, frequently a name she was sold out of years ago.
There is also no world line-up feed to buy. Every authority publishes independently, in its own format and on its own schedule or not at all, so coverage is quoted port by port and each port is a parser somebody has to write and then keep working. Port line-up data sources goes through what the publications actually look like.
The constraint all three miss
Drydock and special survey dates. A hull committed to a special survey is not open for a voyage that overlaps it, however good her position looks and wherever AIS says she is. That is forward supply, knowable months ahead, and it is the one thing a position list structurally cannot carry.
It is also the reason none of the three records can be treated as complete on its own. The position list is the record to act on, the fix is the record to check it against, the line-up is the record of what the port has agreed to, and the survey date is the constraint that sits outside all of them. Keeping four separate answers separate is more work than blending them into one feed, and it is the only version that is ever right.
Keeping the three apart is a design decision before it is a feature. The position list on this site holds what owners have said, shows an AIS fix beside it as a separate fact with its own age, and makes no claim on the line-up at all.
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
Neptune Atlas
Neptune Atlas keeps the three apart on purpose. A position carries an open range and an area and is what a tonnage search runs against; live fixes are drawn on their own map with the age of the freshest one stated rather than implied; and the arrivals view is named for what it is, a destination a crew declared. There is no port line-up importer in this version and an arrivals list is deliberately not offered as a stand-in for one, because a berthing order the product cannot actually see is the one answer a desk must never be given. Every paid plan starts with 7 free days. A card is needed to start them, and cancelling before they end costs nothing.
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