A collection of observable properties. This vocabulary defines terms for observed properties originally used for groundwater, surface water and marine water quality observations. Most PropertyKinds are associated with a Species object via the objectOfInterest property or a real-world Feature via the featureOfInterest property. The sub-class of PropertyKinds that can be measured are ScaledQuantityKinds, which have appropriate units of measure (qudt:unit property). This water quality ontology re-uses the Quantities, Units, Dimensions, Data Types (QUDT) ontology which is developed by TopQuadrant and NASA.