Most of the drawings for airplanes and model airplanes have a datum line, to which all angles are referred.
That datum line is just a reference line and it may be defined by the designer in an arbitrary manner.
Most of the times, the central line of the fuselage is selected and it is represented as a perfectly horizontal line.
That central line of the fuselage is normally the central line of the aerodynamic shape (side of fuselage) that will produce the minimum drag during normal horizontal flights.

Read this how-to:

http://www.robart.com/how_to/incidence_meter.aspx

Regards!