Kartverket (Norway)

What it is: The Norwegian Mapping Authority, which operates Norway's permanent tide gauges and publishes their harmonic constants through its tide API.
What OTC takes: Official constants: amplitude, phase, speed, Doodson code and observation span per station. The API gives phases relative to UTC+1 (utcoffset="+01:00"); OTC converts them to Greenwich and records the conversion.
Licence: CC BY 4.0, as stated on Kartverket's API page.
Attribution line OTC uses: “Contains data from Kartverket (Norwegian Mapping Authority), CC BY 4.0.”
Update cadence and watching: A weekly watcher hashes the response for every station and rebuilds only the stations that changed.
Coverage: Kartverket's station list had 33 permanent tide gauges when checked on 2026-10-07: the Norwegian mainland coast and Ny-Ålesund on Svalbard.
Rank: Official constants.