Alt_error between HUD alt and Planning Alt

Hello, everyone:
I had recently found that the actual flight altitude(87 m) was not consistent with the planned flight altitude(100 m) when flying my plane, and that the Alt_error (13 m) parameter was found to be the difference between the actual altitude and the altitude of the plan. How should I fix the actual flight altitude to the plan flying altitude?

ArduPlane 3.7.1


Small Altitude variations are common in plane. 13m seems a little high. Could be that your aircraft is not tuned properly. This should be a good starting point -

Thank you for your reply

This is my one more flight.
I detailed the process of my flight, the altitude on the ground is 0 m when i take off,and the planned Mission with MP altitude is 100 meters, but in the course of the flight altitude gradually decreased, the minimum 80 meters, 20 meters with the target altitude difference between the. The final landing when the altitude is displayed -8 meters, the flight time is about 15 minutes.

Now I want to know which sensors are leading the altitude, thus shielding the sensor in question so that the altitude is not in such a big error.