Finding and comparing US historical AIS data

Choose an archive for a historical question

A map of recent vessel positions cannot answer how shipping activity changed across several months. For that, start with an archive and an explicit study period. NOAA's Vessel Traffic catalogue describes AIS location and vessel-characteristic data used for ocean planning, traffic analysis and ecological research. It points to Marine Cadastre resources and lists available data formats. Check the catalogue's actual year and geographic coverage before planning a comparison; the existence of a national catalogue is not evidence that every place has equally complete observations.

Define an area and period before downloading

AccessAIS supports selecting a geographic area and time window, downloading AIS points and viewing traffic patterns and vessel counts. Its recent-year collection changes as data are added and older years leave the tool. Save the selected dates and boundary with your research notes. For a year-to-year comparison near a harbour, reuse the same boundary instead of comparing two convenient screenshots at different zoom levels. Check that both downloads actually cover the requested period before interpreting a difference.

One message is not one ship or one voyage

A count of received records answers a different question from a count of distinct transmitters. One vessel can contribute many observations. A distinct-identity count can avoid counting those messages repeatedly, but it still does not tell you how many separate journeys occurred. A voyage count requires an additional definition of entry, exit and repeated visits. The Coast Guard's AIS explanation is useful background on the broadcast information underlying these records. Treat a coloured traffic layer as a measurement with a definition, not as a self-explanatory census.

Build a comparison someone else can repeat

Keep a short methods note: source, retrieval date, period, boundary, included vessel classes, identity field and counting rule. State how you handled missing coordinates, repeated records and unknown classifications. If you create a grid, record its cell size and what each colour measures. A cell counting messages and a cell counting distinct ships can have the same colour scale while representing very different activity. Do not describe either as a congestion score unless you have separately defined and validated that measure.

Compare like with like, then use the live map for context

Before explaining a change as increased trade, check for differences in source coverage, observation availability, season and the length of the comparison periods. Missing observations are uncertainty, not evidence of an empty sea. PocketWorld's ship explorer offers geographic context from available recent reports; it does not reproduce the Marine Cadastre archive or provide historical replay through this page. Follow the archive links for research data, and keep historical findings labelled with their dates. This separation prevents an old traffic pattern from being mistaken for the current location of a ship.