OpenTideConstants

Sources

This page lists every dataset OTC incorporates, what OTC takes from each, and how each one is credited.

How sources rank at one location

When more than one source covers the same location, the station's recommended set is chosen in this order:

  1. Official constants: harmonic constants published by the agency responsible for the gauge (for example NOAA, Kartverket).
  2. Gauge fit: constants OTC fits from a gauge record (GESLA).
  3. Model constants: constants from a global or regional tide model, labelled as model constants. EOT20 and FES2022 are the model sources.

Every set stays in the release files, not only the recommended one, so you can choose another. A source must also pass the per-source convention check; a source that fails is not used in that release. The validation table shows how each set scores against official predictions.

The sources

OTC combines thirteen open sources. For each one, the licence was read and the access path was checked on 2026-10-07. The coverage figures below are from those checks; the number of published stations is known only after a full build.

Most give gauge records, from which OTC fits its own constants. NOAA CO-OPS and Kartverket publish official constants. EOT20 and FES2022 are global tide models: their constants are model constants, interpolated to station points, labelled as model constants in every file, and ranked below official constants and gauge fits.

GESLA (Global Extreme Sea Level Analysis)

What it is
A collection of high-frequency sea-level records from tide gauges worldwide, assembled from many national and regional agencies by the GESLA project. OTC uses the latest release (GESLA-4.1, July 2026, today).
What OTC takes
Gauge records that OTC fits. OTC fits its own constants from the records, after the time-base and broken-record checks in Method. OTC publishes the derived constants only, never the raw series.
Licence
The records are covered by several licences set by their originators; the GESLA licence page lists them. OTC follows TICON-4's practice: derived constants under CC BY 4.0, with per-record provider attribution and the GESLA citations. Every fitted set records its provider and licence class.
Attribution
The GESLA downloads page asks: “Any papers or reports that make use of GESLA data should include an acknowledgement to this website and cite the papers by Haigh et al. (2023) and Woodworth et al. (2017)”, and Caldwell et al. (2015) for UHSLC records. Each set also names the agency that provided the record.
Update cadence and watching
GESLA-4.0 was released in July 2025 and GESLA-4.1 in July 2026. A weekly watcher checks the version on the downloads page, the SHA-256 of the metadata file, and a hash of the licence page. A new release triggers a full rebuild and a new time-base audit of every provider.
Coverage
GESLA-4.1 has 6,682 records worldwide. How many become published stations depends on the checks, and is known only after the first full build.
Rank
Gauge fit: used where no official constants exist for the location, or where official constants fail the convention check.

NOAA CO-OPS

What it is
The Center for Operational Oceanographic Products and Services of the US National Oceanic and Atmospheric Administration, which publishes tide predictions and harmonic constants for US stations through a public API.
What OTC takes
Official constants for reference stations (Greenwich phases), and subordinate-station offsets (time and height corrections to a reference station). OTC publishes them unmodified and marks them as NOAA data.
Licence
US Government work in the public domain (LicenseRef-PublicDomain-USGov). NOAA's disclaimer says the data “may be used freely by the public” and “shall not be modified in content and then presented as official government material”. OTC never presents its files as an official NOAA product.
Attribution line OTC uses
“Contains NOAA CO-OPS data (public domain). Not an official NOAA product.”
Update cadence and watching
The API sends no ETag or Last-Modified headers, so a weekly watcher hashes the normalised response for every station and rebuilds only the stations that changed.
Coverage
NOAA's tide-prediction station list had 3,502 stations when checked on 2026-10-07: 1,260 reference stations and 2,242 subordinate stations. Mostly the United States and its territories.
Rank
Official constants.

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
Norway's permanent tide gauges. The station count will be published with the first release.
Rank
Official constants.

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.

PEGELONLINE (Germany)

What it is
Germany's online water-level service, with 1-minute water levels from coastal, tidal-river and inland gauges.
Publisher
Wasserstraßen- und Schifffahrtsverwaltung des Bundes (WSV).
What OTC takes
Gauge records that OTC fits. PEGELONLINE publishes no harmonic constants. It keeps only about 31 days of history, so OTC will harvest the records into its own archive and fit constants when enough data has accumulated: at least 30 days for a minimal fit, one year for a full fit.
Licence
Data licence Germany – Zero – Version 2.0 (DL-DE->Zero-2.0), with no conditions, as stated in the PEGELONLINE terms of use.
Attribution line OTC uses
“Contains data from PEGELONLINE (WSV), DL-DE->Zero-2.0.” The licence does not require it; OTC credits the source anyway.
Access and watching
Keyless REST API (version 2). A daily harvest, because older records are not kept upstream.
Coverage
The German North Sea and Baltic coasts and the tidal rivers (Elbe, Weser, Ems), plus inland gauges.
Rank
Gauge fit.

Rijkswaterstaat WaterWebservices (Netherlands)

What it is
The Dutch water-data service, with a multi-decade archive of 10-minute water levels.
Publisher
Rijkswaterstaat.
What OTC takes
Gauge records that OTC fits. No harmonic constants are published. OTC fits constants from the long observed records. Rijkswaterstaat also publishes an astronomical (predicted) water level, which OTC can use to cross-check its fit.
Licence
CC0 (Creative Commons Zero), as stated on the Rijkswaterstaat water-data page.
Attribution line OTC uses
“Contains data from Rijkswaterstaat (CC0).” The licence does not require it; OTC credits the source anyway.
Access and watching
Keyless JSON web services (DDAPI 2.0), requested in pages by period. A weekly fetch of new records.
Coverage
The Dutch coast, the Wadden Sea and the estuaries. Full-year records are available for backfill.
Rank
Gauge fit.

Marine Institute (Ireland)

What it is
The Irish National Tide Gauge Network, published through the Marine Institute's ERDDAP server, with records from 2000.
Publisher
Marine Institute, Ireland.
What OTC takes
Gauge records that OTC fits, using the published quality flags. No harmonic constants are published. The Marine Institute's tide predictions can serve as a cross-check.
Licence
CC BY 4.0, as stated in the dataset metadata.
Attribution line OTC uses
“Contains data from the Marine Institute (Ireland), CC BY 4.0.”
Access and watching
Keyless ERDDAP queries (CSV), near real time. A weekly fetch of new records.
Coverage
About 20 or more Irish coastal ports.
Rank
Gauge fit.

DMI ocean observations (Denmark)

What it is
Sea-level observations at 10-minute intervals from Danish stations, with a historical archive, published through DMI's open-data API.
Publisher
DMI (Danish Meteorological Institute).
What OTC takes
Gauge records that OTC fits. No harmonic constants are published. DMI's own tide predictions can serve as a cross-check. Inner Danish waters have very small tides, so the fits are most useful on the North Sea and Wadden Sea coast.
Licence
CC BY 4.0, as stated in DMI's terms for its free data.
Attribution line OTC uses
“Contains data from DMI (Danish Meteorological Institute), CC BY 4.0.”
Access and watching
Keyless OGC API Features (version 2). DMI limits requests per IP address. A weekly fetch of new records.
Coverage
The station list returned 201 stations: the Danish coast, and some in Greenland and the Faroe Islands.
Rank
Gauge fit.

SMHI oceanographic observations (Sweden)

What it is
Hourly and minute sea levels from Swedish stations, with a corrected hourly archive back to the start of each station.
Publisher
SMHI (Swedish Meteorological and Hydrological Institute).
What OTC takes
Gauge records that OTC fits. No harmonic constants are published. Tides are small except in the Skagerrak and Kattegat, so these sets mainly add completeness and Baltic datums.
Licence
Creative Commons Attribution 4.0 SE (CC BY 4.0 SE), as stated in SMHI's terms of use. Commercial use is allowed. Users must name SMHI as the source and say whether the material was changed.
Attribution line OTC uses
“Contains data from SMHI, CC BY 4.0 SE. Changed: OTC fitted harmonic constants from the sea-level records.”
Access and watching
Keyless REST API (CSV). SMHI asks users to avoid mass downloads and repeated fetches, so OTC fetches the corrected archive once and then only new records, weekly.
Coverage
71 Swedish sea-level stations.
Rank
Gauge fit.

FMI mareographs (Finland)

What it is
Water levels from Finnish mareographs on the Gulf of Bothnia and the Gulf of Finland, published through FMI's open-data service.
Publisher
FMI (Finnish Meteorological Institute).
What OTC takes
Gauge records that OTC fits. No harmonic constants are published. The tidal signal here is a few millimetres, so these sets will have low confidence scores and are mainly useful for datum and seiche context.
Licence
CC BY 4.0, as stated in the FMI open-data licence.
Attribution line OTC uses
“Contains data from the Finnish Meteorological Institute (FMI), CC BY 4.0.”
Access and watching
Keyless WFS 2.0 queries, paged by week. FMI sets request quotas per IP address. A weekly fetch of new records.
Coverage
About 14 Finnish mareographs.
Rank
Gauge fit.

UHSLC Fast Delivery

What it is
Hourly sea levels from tide gauges worldwide, contributed by national agencies and delivered within about one to two months. Fast Delivery data are later replaced by the yearly Research Quality data.
Publisher
University of Hawaii Sea Level Center (UHSLC).
What OTC takes
Gauge records that OTC fits. Stations that GESLA already supplies are matched, so that each gauge is fitted once. Each set records the agency that provided the record.
Licence
The dataset licence says the data “may be used and redistributed for free”, with no warranty and not for legal use. UHSLC asks users to cite the dataset.
Attribution line OTC uses
“Contains UHSLC Fast Delivery data”, with the UHSLC dataset citation and the name of the agency that provided each record.
Access and watching
Keyless static CSV and NetCDF files, one per station. A weekly check for new data.
Coverage
Global, about 300 or more stations, many in places with no other open source: Pacific islands, the Indian Ocean, Africa and South America.
Rank
Gauge fit.

EOT20 global ocean tide model

What it is
An empirical global ocean tide model derived from satellite altimetry, with ocean-tide and load-tide grids for 17 constituents at 1/8° resolution.
Publisher
DGFI-TUM, published on SEANOE.
What OTC takes
Model constants: amplitude and phase interpolated from the grids to station points. Every such set is labelled as model constants and scored like every other set. OTC does not re-host the grids.
Licence
CC BY 4.0, as stated on the SEANOE dataset page.
Attribution line OTC uses
“Contains model constants from EOT20 (DGFI-TUM), CC BY 4.0. Hart-Davis et al. (2021), doi:10.17882/79489.”
Access and watching
One keyless download (about 2 GB of NetCDF). The release is static, so a watcher checks the file checksum.
Coverage
The global ocean.
Rank
Model constants: used only where no official constants or gauge fit exist for the location.

FES2022 global ocean tide model

What it is
A global ocean tide model with 34 constituents at 1/30° resolution.
Publisher
CNES, LEGOS, Noveltis and CLS, distributed through AVISO+.
What OTC takes
Model constants: elevation amplitude and phase interpolated from the grids to station points. Every such set is labelled as model constants. OTC never re-hosts the grids and does not use the FES2022 currents.
Licence
The AVISO License Agreement (Issue 20), based on CC BY 4.0. It allows derived products, such as constants at station points, but not the onward distribution of the original grids without written authorization from CNES. The FES currents are for non-commercial use only.
Attribution line OTC uses
“Contains model constants derived from FES2022 (Cnes/Legos/Noveltis/CLS), doi:10.24400/527896/a01-2024.004.”
Access and watching
Download with free AVISO+ account credentials (about 2.2 GB of elevation grids). The release is static, so a watcher checks the file checksum.
Coverage
The global ocean.
Rank
Model constants: used only where no official constants or gauge fit exist for the location.

Used only as cross-checks

These are compared with OTC on every build but are not published as OTC sources: TICON-4 (constants fitted from GESLA-4.0), the XTide harmonics files, and the Slackwater database.

Considered, not added

These sources were checked and are not added for now. Some of their gauges reach OTC through other sources, such as GESLA or UHSLC.