OptionalstateClassification vs. the mean low/high water marks (water levels only).
OptionalstateClassification vs. the lowest/highest navigable water marks (water levels only).
The reading, in the unit of its timeseries (TimeseriesInfo.unit — not always cm
for W). null when the gauge reported no value: the client maps the sentinel
99999 (NO_VALUE_SENTINEL) the API relays for that to null.
A measurement value plus the API's state classifications.