ADSB out Altitude reporting method

[tomg]

Did an extended cross country this weekend and ran my tail number on both planefinder.net and flight tracker. Both reported my ADSB altitude lower than what I saw on my altimeter (barometric pressure altitude). Autopilot kept altitude within +- 25 ft the entire timeframe at cruise. Barometric pressure was adjusted the entire trip from the closest ASOS.

Eastbound altitude was 5,500, ADSB reported 5,225
Westbound was 4,500, ADSB reported 4,200

So what is the ~300 ft difference caused by?

 

[n456ts]

The transponder always reports your altitude with the standard pressure setting of 29.92 inHg. On the Skyview, you're looking at the altitude based on the pressure setting you gave it.

 

[tomg]

Thanks.