Use the generic gml:DefinitionType for basic Phenomenon definitions,
gml:description may be used for a more extensive description of the semantics, with
a link to a definitive version (if available). gml:name should be used for the
"short name" or label.
SamplingSite implements a characterisation of the Feature of Interest,
through a set of observations. The key implication is that the location of the site
and the observation procedures provides for a representative sample of the actual
feature, within the scope of the sampling regime (e.g. a time
period)
This is a view of the sampling feature that contains no observations, and
can be used simply to map the spatial location, access identify and metadata
etc.
time-instant coverage which samples a property of the
feature of interest at different times
A view of the sampling feature with some subset of the constrained
observations, where the result set is a trend over the sampled time envelope.