The definition you seek may not exist.
As a somewhat helpful reference, remeber that for NPAs the OCH is calculated against the field elevation, unless the threshold is lower up by 7 feet or more.
The software that calculates RTOW charts will have a method to slove this, although it may differ among the providers. DERA makes little sense, btw.
On practical terms, in daily life I use THR ELEV as reference zero. The way it comes from the FMS after selecting DEP runway.
Last edited by FlightDetent; 11th May 2021 at 10:10.