Below is a graph of my X8 with APM 2.6 sitting on the floor next to my computer. As you can see, it thinks its slowly rising. When I am in Alt Hold or Loiter, it rises slowing in the air.
I'm trying to understand what is causing this. Could it be the barometer built into the APM? It appears that the reported pressure is directly associated with the Relative Altitude. If so, is there any fix for this, or is this a hardware failure?