flight level datum on a 747

[googolplex]

Does anybody out there know where the datum point is for the Mode C transponder on a 747-400? (or any other aircraft for that matter) i.e. Does the flight level readout relate to the underside of the airframe, the location of the static pipe or some other fixed point on the aircraft?

[Notso Fantastic]

The static ports are on either side of the front fuselage just below main deck floor level if my memory serves. I would imagine that that is the 'aircraft altitude. Bearing in mind instrument accuracy limitations, I would not think any artificial corrections were applied.