A datum line is an arbitrary line. It can be anywhere the person using it wants it to be and in any direction. It is simply defined as a point or line to which all other measurements are referred to. and for convenience, is usually aligned with the desired attitude of the fuselage when the plane is in flight. Back before aircraft, this was also commonly referred to a "water line" as use in nautical drafting and generally meant the line at which the depth in the water would be consistent when the ship was afloat. This terminology was carried over into the aircraft drafting effort but has slowly been called other things including , in some case, the datum line. Bottom line, you can specify any reference you want as long as all other points are in relation to that arbitrary definition.