wing incidence is the angle of the chord line of the wing relative to the "Datum" line or centerlne of the fuselage.

the datum line does not necessarilly have anything to do with the attitude of the aircraft in flight... its sometimes set arbitrarilly by the designer. NORMALLY designers try to set the datum at the level line for the fuselage in normal cruise.

I have seen one home-builder ultralight plan set that had the datum line drawn through the main and tailwheel axles. he specified something like 15 deg wing incidence and 10 deg tailplane incidence. (if you moved the datum line to the more conventional line... you get +4 deg wing and -1 deg tailplane on the same airplane)

Some designers will set the datum as the same as the thrust line... Its just a reference to measure all the other angles of the aircraft from.

The relationship of the upper and lower wing incidences to each other, and to the tailplane (and the engien thrustline) is what matters... not the actual number compared to the datum line.