Reference datum height

In aviation, reference datum height (RDH) is the height of a point, located above the intersection of the runway centre line and the threshold, through which the downward extended straight portion of the glide path passes.[http://www.eurocontrol.int/lexicon/lexicon/en/index.php/Instrument_Landing_System_Reference_Datum_Height Instrument Landing System Reference Datum Height] Eurocontrol.int It is usually found on the aerodrome instrument approach charts.