A field guide to the edges of the map
Where can this map
hear ships?
MarineActivity reads shore-based AIS receivers. It can show a vessel when one of those receivers hears its VHF broadcast. Coverage follows the receivers, not a neat line around the coast. There is effectively no mid-ocean coverage here.
The short answer
A radio horizon, not an ocean-wide net
AIS is a VHF radio broadcast. A receiver on shore can hear vessels within radio range, subject to antenna height, terrain, interference and whether that receiver is operating. Beyond its horizon, the ship may keep transmitting while this site receives nothing.
Our network has worldwide reach along some coasts. It is uneven: dense around Europe, North America and East Asia, thinner elsewhere, and effectively absent in mid-ocean. We do not use satellite AIS.
For the range calculation and examples, read the technical coverage guide. The underlying radio system is described in ITU-R M.1371.
Schematic · not a measured range map
A missing dot does not establish that a vessel stopped transmitting.
Read the map
What the Coverage layer actually shows
01 · Observed
Where a feed heard traffic
The coloured layer groups vessel positions from the AISStream snapshot into 2° latitude by 2° longitude cells. If that snapshot is unavailable or empty, it uses the live Digitraffic feed instead. It shows up to 400 cells with the most vessels, so it is evidence of observed traffic, not a map of every receiver's range.
02 · Unobserved
Blank does not mean empty
A blank area can mean no receiver, no vessels in range, a quiet reporting period, or an unavailable feed. A less busy cell can also be omitted by the 400-cell limit even when receivers heard vessels there. Blank is not a verified boundary of reception.
03 · Current view
Use the status and timestamp
The vessel plot and Coverage layer can draw from different requests and feeds. The Coverage layer uses a nonempty AISStream snapshot and otherwise falls back to Digitraffic. Check the live map's feed status and each vessel's received time before drawing a conclusion.
On the map, the Coverage button is in the map tools beside Measure. The link above opens it switched on. Use the same button to hide it.
A practical check
Why is a vessel missing?
No single sign proves the cause. These checks narrow the possibilities without treating an absent dot as an incident.
- 1
Check the feed first
Open the live map. If its status says the feed is unavailable, the map cannot tell you whether any vessel is present. Try again when service returns.
- 2
Read the Received field
On a vessel page, Received shows the age of the latest position in our store, not when the ship last transmitted. An old position must not be treated as its present location.
- 3
Look at the surroundings
If the last position is offshore or other nearby ships have also vanished, receiver range is a plausible explanation. If nearby vessels still update, an individual equipment, reporting or data issue is also possible. Read the fuller diagnosis.