Indicator interface for a horizontal or vertical.
A vertical coordinate system has two types. It can either be gravity-related, which requires a VerticalDatum; or ellipsoid/spheroid-based which requires a horizontal Datum. A horizontal Datum is the same as one used for a geographic coordinate system. The IHVDatum interface supports both datum types, so the IVerticalCoordinateSystem::Define method uses IHVDatum, rather than IVerticalDatum or IDatum.
|Datum||Creates a datum.|
|VerticalDatum||Creates a vertical datum.|