What the map shows
APRS is the position and short message network of radio amateurs. Vehicles, hikers, weather stations and digipeaters send their beacon on 144.800 MHz, iGates pass it on to the internet, and there everything comes together in APRS-IS. This map reads the full stream, around a hundred packets a second, and keeps every position for 48 hours. It records weather stations and telemetry (battery voltage, temperature of a digipeater) for a week, one value every five minutes per station.
Zoomed far out, you see every station as a coloured dot, the colour tells you the type. From zoom level 8 the symbols and vehicle tracks appear, from 9 the callsigns. Whatever moves is updated straight away without reloading the map. Tap a station to see its route over the last 24 hours, device, comment, weather and which iGate or digipeater heard it, plus the latest packets word for word. For weather and telemetry you also get curves over a day or a week, also as CSV.
The map is fast because it doesn't load every station one by one: the server turns its data into vector tiles, like the xOTA map uses, and the browser draws them on the graphics card. New positions arrive over a standing connection, only for the area you're looking at.
Under View › Heard by you can limit the map to one or more receivers: then you only see what, say, your local club's iGate heard or a digi passed on, with its coverage. If you run a receiver, you can feed it in directly, over TCP as to an APRS-IS server (iGate or radiosonde software, with the usual passcode, no account needed) or with an account over AXUDP. Then nothing is missing that APRS-IS drops as a duplicate. afu.tools doesn't pass any of it on.
Everything here was transmitted publicly. If you switch off your beacon, you disappear by yourself after 48 hours. The map doesn't show messages between stations or telemetry.