I start up all the time by setting my unit on grass. Doesn't concern me one bit if it's level or not. That really has nothing to do with sensors or calibration. The baramoter is for holding altitude and the GPS is for location. They work in conjunction to maintain location and altitude. They really don't have anything to do where you are taking off on level or inclined surface to begin with.
Barametric noise is mainly due to wide band and electronic noise, including quantization noise. Normally the onboard filter clears or filters out that noise. If to much noise is seen by the quad you get that error. It will clear if you do a restart and the filter does its job. Kind of like flooding a carburetor. Wait a few and restart and usually all is good. Altimeter data tends to be very noisy and accuracy is rather poor but with proper filtering and several calculations of averages the quad does a pretty good job of holding altitude. It is normally set from sea level so when you take off you may see a deviation in its calculated height and where you see you height at. In summary real time accuracy is not totally there, but with a deviation, pressure, temperature and lapse rate as well as water vapor content in the atmosphere they all effect the baramoter. This is all handles by the onboard MC and sensors. All we need to do is fly. And In case of a error get a clean fresh start up.