How is airport pattern altitude on the Airports page determined?

The Airports page depicts the Traffic Pattern Altitude (TPA) as published by various sources.  

Our first source for pattern altitude information in the US is the FAA.  ForeFlight pulls the data directly from FAA's eNASR database.   

Our next source examined is Jeppesen's database.  The Jeppesen database is the primary reference for international airport TPA data.  

The third source examined in the US to determine the TPA is AIM 4-3-3.     

Standard traffic pattern altitude is defined by AIM 4-3-3 as: 

1. 1,000 feet above ground level (AGL) for Propeller−driven aircraft. 

2. Large and turbine−powered aircraft enter the traffic pattern at an altitude of not less than 1,500 feet AGL or 500 feet above the established pattern altitude.

3. Helicopters operating in the traffic pattern may fly a pattern similar to the fixed−wing aircraft
pattern, but at a lower altitude (500 AGL) and closer to the runway. This pattern may be on the opposite side of the runway from fixed−wing traffic when airspeed requires or for practice power−off landings (autorotation) and if local policy permits. Landings not to the runway must avoid the flow of fixed-wing traffic.

When standard rules are applied, the notation of (est.) is included next to the altitude.  

For Canadian airports, ForeFlight references Transport Canada AIM RAC 4.5.2 Note 1.  This is a standard rule, so the TPA is displayed as an estimated altitude and includes the notation (est.).