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NOAA Current Predictions FAQ

Frequently Asked Questions


  1. Are all stations listed in the Tidal Current Tables available in this application?
  2. Why do some stations have depth information and others do not?
  3. Why do some stations list the depth value as "below chart datum" and others list the depth value as "below surface"?
  4. Why do the latitude and longitude information for some stations appear inaccurate?
  5. Why can I not retrieve time-series prediction values for subordinates stations despite the fact that a prediction curve is displayed?
  6. How often is this product updated?
Q: Are all stations listed in the Tidal Current Tables available in this application?

While most stations listed in the published Tidal Current Tables are available in NOAA Current Predictions, some stations do not have enough information to generate daily predictions with a high degree of accuracy. These stations have been omitted.

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Q: Why do some stations have depth information and others do not?

Due to improvements in technology over the past several decades, currents information can now be collected at discrete depths with a high degree of accuracy. This allows predictions to be representative of a given depth. Predictions based on older data are mean values determined to have been representative of the current at each location. Therefore, no specific depth is listed.

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Q: Why do some stations list the depth value as "below chart datum" and others list the depth value as "below surface"?

Current predictions are traditionally based off of measurements from instruments either mounted at the water surface looking downward or at a fixed depth below the water surface looking upward. Predictions based off of information collected by surface-mounted instrumentation will always be at a fixed depth. These stations have depth values referenced "below surface". For stations where information is collected from instruments at a fixed location below the water surface, the depth of the measurements will vary with the tidal heights above. Predictions for these stations are typically referenced to a depth relative to low tide and have depth values referenced "below chart datum".

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Q: Why do the latitude and longitude information for some stations appear inaccurate?

The latitude and longitude are listed to the exactness recorded in the original survey records, therefore, locations of some stations are presented in varying degrees of accuracy. While new GPS technology has allowed very precise location information to be gathered for more recent data, some older stations have latitude and longitude information available only to the nearest minute and may not indicate the exact position of the station. For those stations with latitude and longitude information available to the nearest minute, the station location may appear to be over land. In such cases, unless the description locates the station elsewhere, reference is made to the current in the center of the channel.

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Q: Why can I not retrieve time-series prediction values for subordinates stations despite the fact that a prediction curve is displayed?

Stations designated as subordinate only have time offset and speed ratio information to a given reference station at times of maximum and slack current. Therefore, only the predicted speeds at times of maximum or slack current can be calculated accurately. The prediction curve is a mathematical interpolation between the known maximum and slack values and should not be treated as real data.

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Q: How often is this product updated?

Updates are applied to NOAA Current Predictions every quarter; during the first two weeks of January, April, July and October. These updates may include adding new stations, updates to the adjustment values and reference station for subordinate stations or removal of stations that have been superseded by a nearby station or because accurate predictions can no longer be calculated based on existing data. The number of stations updated and type of updates applied each quarter will vary depending on data collected and analyzed during the quarter.

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Revised: 10/15/2013
NOAA / National Ocean Service
Web site owner: Center for Operational Oceanographic Products and Services