Data Set and Methods
MHW, MHHW, and MTL elevations for the East, West, and Gulf Coasts were obtained from NOAA's National Ocean Service (NOS) Center for Operational Oceanographic Products and Services' (CO-OPS) National Tidal Benchmark System (NTBMS). CO-OPS maintains tide stations around the country for tidal datum control. These tide stations are classified into three categories: primary (or control) tide stations, secondary stations, and tertiary stations. Primary tide stations have been continually operating over at least the nineteen years of the National Tidal Datum Epoch (NTDE), which is from 1960 - 1978 (NOAA, 2001). Secondary tide stations have been in constant operation for at least one year, but for less than 19 years, and tertiary tide stations have been in constant operation for at least 30 days, but for less than a year (NOAA, 2001).
Tidal datums calculated at the tide stations are referenced to tidal bench marks. Each tide station has a network of bench marks installed near it: primary stations have a set of 10 bench marks, secondary and tertiary stations each have a network of 5 bench marks, and stations that are in operation for less than thirty days have at least 3 bench marks installed near them (NOAA, 2001). These tidal bench marks make up the NTBMS. Bench mark descriptions and elevations, tidal datum elevations, the length of time over which the tidal datums were computed, and other information for each tide station and its network of bench marks are included on CO-OPS published bench mark sheets. These bench mark sheets used in this report are available on the CO-OPS Web site at http://co-ops.nos.noaa.gov/bench.html. See Appendix I (PDF Format) for an example bench mark sheet.
NOS defines a new NTDE every 20 - 25 years to take into account changes in mean sea level along the coast, and is in the process of updating the Nation's tidal datums to a NTDE spanning 1983 - 2001 (http://co-ops.nos.noaa.gov/datum_update.shtml). At the time this report was being written, just over half of the tide stations chosen for the report had been updated to the new epoch. Therefore, this report uses the tidal datums based on the 1960 - 1978 NTDE.
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