ODM2 Elevation Datum Controlled Vocabulary ODM2 Working Group A vocabulary for describing vertical datums. Vertical datums are used in ODM2 to specify the origin for elevations assocated with SamplingFeatures. AHD Australian Height Datum. Height datum used for standardised elevations across Australia. The datum surface is defined as that which passes through mean sea level at the 30 specified tide gauges and through points at zero AHD height vertically below the other basic junction points. See http://www.ga.gov.au/scientific-topics/positioning-navigation/geodesy/geodetic-datums/australian-height-datum-ahd