In GPS surveys, the surveyed coordinates are computed in the World Geodetic System 1984 (WGS84) coordinates which is a geocentric coordinates system, expressed in geodetic coordinates (i.e. latitude, longitude and ellipsoidal height) or Cartesian coordinates (i.e. X, Y and Z). In other words, the height values obtained from GPS surveys are in ellipsoidal height referenced to the surface of the WGS84 ellipsoid but not the orthometric height above the geoid surface.
However, the height values we use in daily survey works are in orthometric height above Hong Kong Principal Datum, HKPD. The following documents outline some suggested procedures on how to convert the GPS surveyed height values (in WGS84 Ellipsoidal Height) to the local height above HKPD within a small local area: