NOAA CO-OPS tidal-current constants

What it is: Harmonic constants for tidal currents (velocity, not water level) at NOAA current-meter stations, published through NOAA's metadata API.
Publisher: NOAA CO-OPS (US National Oceanic and Atmospheric Administration).
What OTC takes: Official current constants: per depth bin, the major and minor axis amplitude and phase, and the azimuth of the current ellipse. These need their own record type (quantity “current”), separate from water-level constants. Subordinate current stations publish offsets only, with no constants.
Licence: US Government work in the public domain (17 U.S.C. §105). NOAA's disclaimer says the information “may be used freely by the public”.
Attribution line OTC uses: “Contains NOAA CO-OPS data (public domain). Not an official NOAA product.”
Access and watching: Keyless JSON API: a station list, then one constants request per station. The constants are static and change only when NOAA re-analyses a station, so a weekly watcher hashes each station's response, as for the NOAA water-level constants.
Coverage: US coasts and estuaries. NOAA's current-prediction list had 4,439 stations when checked on 2026-10-07, of which 2,221 are harmonic stations and the rest subordinate.
Rank: Official constants (currents). They are ranked only against other current constants.