Lets have a go at it then...
First of all, imagine that the compass calibration routine is in effect 'Zeroing the Compass' to that particular place on the earths surface and the associated magnetic interference experienced at that location. Any movement of the quadcopter will result in the compass using the magnetic values it detects and sense checking it against the values it calibrated itself to at the beginning of its flight. So if a flight takes the copter 200m away the values are unlikely to change drastically in normal circumstances. if it were to go 2000 miles away though, like say an airliner does then a reasonable amount of compass drift would be expected, thus why airliners don't rely on compass alone to navigate (they have correction factors and lots of complex **** we don't need to talk about!).
Now to understand why the compass module can go haywire after a calibration, I will use a real life example...
Imagine you pitch up to a marina flying site, the surface you intend to take off from is a concrete quayside pavement, there are cast iron post and chainlink fences in front of you and a large cast iron port entrance marker to the side.
You do your usual compass calibration routine, spin spin spin...
What you would notice if you had an old fashioned compass to hand, is that the compass will have detected all of that ferrous metal surrounding you and be acting up, the reinforced concrete pavement, the fence and the marker all have the potential to interfere. Now, if the compass calibrates itself in a very magnetically active environment like that you will have a big issue. Its really simple to understand too...
As you take-off with your copter, your compass will be constantly checking the values its reading to the values it expects (in this case very high magnetically active values from a quayside). As soon as it realises they are far below what 'you' told it too expect in the calibration its likely to go mental. This might mean you get large drifts or very small inaccuracies. Either way the readings it will be seeing will not be anywhere near what you told it to expect in that area, thus it becomes an unreliable piece of equipment!
Therefore, to avoid compass errors like this, always calibrate your compass in a known area away from possible sources of interference....and....if it aint broke don't fix it!
Just a side note to it though, with my P3 if I tried to take off from a site that is too active from a magnetic perspective I would expect a compass warning...
The worst thing you can do to it is to ignore your surroundings and possible causes of interference and blindly calibrate it, always assess why you are being asked to calibrate. Its a computer at the end of the day, they ain't great at perception!
Does that help? I hope it gives you a little bit of understanding without too much confusion anyway!
