On: Planes overhead

[slipperyp]

Where do they get data like speed, bearing, and altitude (at what looks like finer than 1 minute granularity)?

[DigitalBison → slipperyp]

Not sure where Wolfram Alpha gets it from, but that sort of data is available from, for example: https://www.flightstats.com/developers/bin/view/Main/

[amandle → slipperyp]

It says under the table "locations based on projections of delayed data" I would guess that they look at where a flight plan is going from and to and interpolate in between.

[jser → amandle]

I think you're right -- it doesn't match FlightAware's FAA data at at all (at least when comparing in downtown Chicago). FlightAware had many more flights, including non-commercial and cargo.