The difference in relative altitude for ADS-B traffic targets between ForeFlight Mobile and Garmin devices when using a GDL-39 is due to the comparison of different altitude sources: GPS altitude and pressure altitude.
ForeFlight compares the aircraft’s GPS altitude to the pressure altitude of ADS-B traffic targets (ADS-B traffic broadcasts use pressure altitude). It does not use the pressure altitude data supplied by the GDL-39 unless ADS-B Out is installed on the aircraft.
Garmin devices, however, calculate relative altitudes by comparing the pressure altitude from the GDL-39 to the pressure altitude of ADS-B traffic targets.
This difference in the types of altitude data used (GPS altitude in ForeFlight versus pressure altitude in Garmin) explains the variation in displayed traffic altitudes when ADS-B Out is not installed.
When ADS-B Out is installed, both systems use the aircraft’s actual pressure altitude, resolving the discrepancy by comparing the aircraft’s and the traffic’s pressure altitude.
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