Altitude Question

The FlightAware FAQ states that “FlightAware displays flight levels as if they were true altitudes” and “Altitudes displayed are the airplane’s altitude above sea level (MSL)”. What that means then is that immediately after runway departure from, for example, DEN (an airport that is ~5,000ft above MSL) an aircraft’s FlightAware altitude will always be something greater than ~5,000ft, correct? What would the FlightAware altitude be if the airport was under MSL? Thanks.

Correct, rounded to the nearest 100 ft. Aircraft taking off or landing at one of the few airports below sea level would just generate a - sign, as far as I know.