Herluf. .. .Altitudes are measured above mean sea level and read from the altimeter when set to the correct QNH. Therefore DA and OCA are referenced AMSL.. .. .However heights (DH or OCH) are measured from another datum such as the aerodrome reference point or the threshold of the applicable runway to which the chart relates. If the surface was dead flat then I suppose the Radar Altimeter may serve as the reference instrument.. .. .In that context DA or DH is the altitude above MSL or the height above the reference datum at which the pilot must decide to land or miss.. .. .OCA (reference MSL) or OCH (reference the applicable datum) is used to eatablish compliance with the appropriate obstacle clearance criteria.