Originally Posted by
DaveReidUK
At the risk of stating the obvious, most non-symmetric aerofoils still produce lift at a small negative AoA.
That depends entirely on what you choose for your datum.
A common choice is the so-called "geometric AoA" which uses a line drawn through the TE and the centre of the nose radius (with flaps and slats retracted). This will produce arbitrary numbers because the lift coefficient at "zero" will vary between airfoils and also vary with flap/slat deployment.
Another choice is the so-called "aerodynamic AoA" which is simply a datum where zero degrees AoA is the zero-lift coefficient angle. This could tell you useful things, but it would feel strange because for many airfoils zero aerodynamic AoA will occur at as much as minus 7 or 8 degrees geometric AoA. It would also lose its definition as soon as flaps/slats etc were deployed (unless you recalibrated the datum for each stage of flap, which would be weird).
I suspect the datum is chosen as much as anything to be something which allows for easy rigging and testing of the instrument.