How do I gauge aircraft altitude?

Altitude levels of aircraft in flight are difficult to gauge by people standing on the ground. Large size aircraft may appear to be lower than smaller aircraft because their size makes them look closer to us. Smaller aircraft may seem higher when following a larger size aircraft at the same altitude.

The MAC FlightTracker tool takes the guesswork out of gauging aircraft altitude because the animated flight track replay offers an option to display altitudes for each aircraft as it navigates through the airspace above us.  

Updated 10/3/2016