The map opens on a marine chart carrying the AIS traffic AIS Fleet has received, and it keeps itself up to date while you watch. This page is about reading it honestly: what it is showing, and what its silence means.
Reading the chart
- Vessels are coloured by type. The Types button in the top-right is the filter and the legend at once: turn a category off to quieten the chart, and the swatches tell you what each colour means. The categories are Sailing, Pleasure, Passenger, Cargo, Tanker, Fishing, Tug and workboat, Law Enforcement and Other, which is as fine as colours on a chart can usefully get.
- A moving vessel is drawn as an arrow on her course over ground, which is the direction she is actually making rather than the way her bow points. A stopped one sits quietly and is not turned at all, and the selected one carries a ring.
- Light and dark themes are in the account menu, behind the person icon.
The Fleets switch
Fleets, in the same corner, is a switch rather than a list: turn it on and the boats from your fleets join the map in their own colour, wherever in the world they are, and turn it off and you have your plain traffic chart back. It covers every fleet your plan has active at once, and the button only appears once you are in one.
It is off until you ask for it, and it stays as you left it, so a switch you set before a rally is still on next time you open the map.
Turning it on usually changes nothing you can see, because fleets are spread across the world. So the switch says what it did: how many fleet boats it put on the map, and how many of them are in the view you are looking at. If they are elsewhere, Show all frames them for you. Nothing moves your map unless you ask it to.
The filter can never hide one of yours
A vessel you follow, a fleet mate while the Fleets switch is on, your own boat and the vessel you have selected keep their name and their marker whatever else you switch off.
A vessel you follow goes further: she is on the map whether the Fleets switch is on or off, and she is put there wherever she is, so you do not have to be looking at the right piece of water to see her. Set the map on your own stretch of coast and a boat you follow still appears, mid-ocean if that is where she is.
A boat can carry only one colour, so a fleet mate you also follow shows as one you follow. That is the stronger link of the two.
Zoom decides how much is drawn
At ocean scale the map draws the larger ships only. Zoom in and the smaller vessels join, and from coastal and harbour zooms it draws everything it has. If you cannot find a 20 metre boat while looking at half an ocean, zoom in before concluding she is not there.
Where a view holds more traffic than the map will draw, it says the view was truncated rather than dropping the rest without telling you. Zooming in narrows the view and brings the rest back.
Where the positions come from
AIS is a radio signal that vessels broadcast to say who they are and where they are. It is received by stations ashore and by receivers aboard other boats, and AIS Fleet draws what has been reported to it through the networks it takes part in, including the AIS receivers aboard Saillogger boats.
Two consequences are worth knowing before you trust an empty patch of sea:
- Coverage is good along busy coasts and in cruising areas, and thin far out. AIS is a line-of-sight radio system, so a vessel well offshore usually goes quiet until she is back within range of somebody listening. That silence is normal and says nothing about her.
- AIS Fleet shows the traffic its own network has heard, not every vessel afloat. A vessel with no AIS transponder is not here at all, and one that has never been heard by a receiver we take reports from has nothing to draw.
The exception is a boat that keeps her log with Saillogger and shares it: she can report her own position from aboard whenever she has an Internet connection, which is how she stays on the map where no receiver can hear her. Find and follow a vessel covers what else she brings.
Why a vessel disappears
Running through the ordinary explanations, in the order they are likely:
- She is out of range. Offshore, or along a coast with nobody listening. She comes back when she does.
- You are zoomed too far out for a vessel her size to be drawn.
- You have her type switched off in the Types filter. Following her makes that impossible, and so does the Fleets switch while she is a fleet mate.
- Her transponder is off, in port or otherwise. Nothing is being broadcast to hear.
- She has never been heard. We know some vessels by MMSI without ever having had a position for them. There is nothing to draw, so she appears in your Following list rather than on the chart.
If she is a boat you follow, her row in Following tells you which of these it is faster than the chart will.