Vertical Datum Type standard values The zero value of the associated vertical coordinate system axis is defined to approximate a constant potential surface, usually the geoid. Such a reference surface is usually determined by a national or scientific authority, and is then a well-known, named datum. geoidal The zero point of the vertical axis is defined by a surface that has meaning for the purpose which the associated vertical measurements are used for. For hydrographic charts, this is often a predicted nominal sea surface (i.e., without waves or other wind and current effects) that occurs at low tide. depth Atmospheric pressure is the basis for the definition of the origin of the associated vertical coordinate system axis. barometric In some cases, e.g. oil exploration and production, a geological feature, such as the top or bottom of a geologically identifiable and meaningful subsurface layer, is used as a vertical datum. Other variations to the above three vertical datum types may exist and are all included in this type. otherSurface