sea_floor_depth_below_geopotential_datum

"Depth_below_X" means the vertical distance below the named surface X. The "geopotential datum" is any estimated surface of constant geopotential used as a datum i.e. a reference level; for the geoid as a datum, specific standard names are available. To specify which geoid or geopotential datum is being used as a reference level, a grid_mapping variable should be attached to the data variable as described in Chapter 5.6 of the CF Convention. rdf:langString