Data Set and Methods
Calculating Operational MHW, MHHW, and MTL
The lidar-derived shoreline is defined as a specific elevation relative to NAVD 88 (Stockdon, 2002). Therefore, the tidal datum elevations tabulated here also need to be referenced to NAVD 88. Three different methods were used to determine the elevations of the tidal datums with respect to NAVD 88, depending on the data that were available at each tide station. For stations with published bench mark sheets and tidal datums or bench marks referenced to NAVD 88 or NGVD 29, either the "Tidal Datum Method" or "Bench Mark Method" was used. For stations chosen from the tidal predictions sheets, and for stations with published bench mark sheets with no data referenced to a fixed vertical datum, the "Mean Tide Level Method" was used. These three methods are explained in detail below.
Tidal Datum Method
The bench mark sheets that were used give values for the tidal datums at their respective sites (Appendix I, "page 4 of 5" PDF Format)). These elevations were referenced to Mean Lower Low Water (MLLW) at all but three sites: Seapoint, Maine, one of the Virginia Beach, Virginia, sites, and Dekle Beach, Florida. Tidal datums at those sites were referenced to MLW, MLW, and MHW, respectively. Many of the bench mark sheets also listed the elevation of either NGVD 29 or NAVD 88 with respect to MLLW.
If the elevation of NAVD 88 with respect to MLLW was listed on a bench mark sheet (along with the elevations of MHW, MHHW, and MTL), the elevations of MHW, MHHW, MTL with respect to NAVD 88 were simply calculated by subtraction. If the elevation of NGVD 29 was given instead of the elevation of NAVD 88, the tidal datums were first calculated with respect to NGVD 29 by subtraction. Then, the elevations of the tidal datums above NGVD 29 were converted to elevations above NAVD 88 using VERTCON, a National Geodetic Survey (NGS) software (obtained from the NGS Web site at http://www.ngs.noaa.gov/PC_PROD/pc_prod.shtml) that calculates the difference in height between NAVD 88 and NGVD 29 at a given latitude and longitude. We refer to this method of determining MHW, MHHW, and MTL with respect to NAVD 88 as the "Tidal Datum Method" simply because the elevations of MHW, MHHW, and MTL above NAVD 88 were calculated directly from the tidal datum information on the bench mark sheets.
Bench Mark Method
If no elevation for either NGVD 29 or NAVD 88 was given along with the tidal datums on a station's bench mark sheet, the elevations of the bench marks were viewed. All of the tide stations have a series of bench marks near them to which the tidal datums are referenced. Many of these tidal bench marks are linked by a Permanent IDentifier (PID#) to datasheets maintained by NGS (http://www.ngs.noaa.gov/cgi-bin/datasheet.prl). Elevations of the bench marks with respect to NAVD 88 can be found on these datasheets (Appendices I and II). If one or more of the tidal bench marks for a station had a link to an NGS datasheet, the "Bench Mark Method" was used to calculate MHW, MHHW, and MTL with respect to NAVD 88 at that station.
Because the NGS datasheets provide information on bench mark heights above NAVD 88 and the CO-OPS bench mark sheets provide information on the elevations of the bench marks above MHW (Appendix I, "page 4 of 5" (PDF Format)), the elevation of MHW above NAVD 88 can be calculated. For each tide station, the height of MHW above NAVD 88 was calculated for all bench marks with links to NGS datasheets. The final elevation for MHW at each tide station was determined by averaging the MHW elevations for all the bench marks at that tide station. MHHW and MTL above NAVD 88 were then calculated using the tidal datum information provided on the CO-OPS bench mark sheets.
For example, for the tide station located at Atlantic Beach, North Carolina, MHW, MHHW, and MTL above NAVD 88 were calculated using the Tidal Datum Method because the elevation of NGVD 29 was given in the Tidal Datum section of the bench mark sheet (Appendix I, "page 4 of 5" PDF Format)). However, if the elevation of NGVD 29 had not been given, the Bench Mark Method would have been used. If this had been the case, MHW above NAVD 88 would have been calculated for each of the six bench marks with PID# links to NGS datasheets (PID#s: AC7539, EA0180, EA0179, EA0177, AC7488, and AC7538). The final elevation for MHW would have been the average of those six values.
The number of bench marks used in the averaging varied among the sites where this method was used. The more bench marks used at a site, the more reliable the resulting elevations of MHW, MHHW, and MTL above NAVD 88. Some sites had as many as nine bench marks with links to NGS datasheets while others had as few as one. Four sites included in this report each had only two bench marks with links to NGS datasheets: Pawleys Island Pier, South Carolina; Isle of Palms Pier, South Carolina; Indian Rocks Beach, Florida; and Westport, Washington. However, the difference in heights of MHW above NAVD 88 for the two bench marks at each of the five sites was less than 1 cm. Three sites (Rockland, Maine, Rockport Harbor, Massachusetts and Cape Arago, Oregon) each had only one bench mark referenced to NAVD 88. These three sites were included in this report because they were needed to fill gaps in the data along the East and West Coasts.
Mean Tide Level Method
Because there were not enough open and near-open coast stations with tidal datums or bench marks referenced to either NAVD 88 or NGVD 29 along the New England and New York coasts, a third method of calculating MHW was employed. This method was used to calculate elevations of MHW above NAVD 88 at the 35 stations chosen from the tidal prediction pages and at the 11 stations with bench marks sheets but no data referenced to a fixed vertical datum.
Nine open and near-open coast stations with tidal datums or bench marks referenced to NAVD 88 or NGVD 29 had been chosen in New England and New York. The data for these 9 stations showed that the elevation of MTL with respect to NAVD 88 is nearly constant in this region (see MTL elevations shown in Figures 3, 5, and 6). It ranges from -0.12 m to -0.17 m with a mean of -0.14 m and a standard deviation of 0.02 m.
For the "Mean Tide Level Method", we assume that the elevation of MTL above NAVD 88 from New England through New York is the mean of the MTL elevations from the 9 stations, or
-0.14 m. For the 11 stations with bench mark sheets with tidal datums not referenced to a fixed vertical datum (and no PID# links to NGS datasheets), we simply calculated the heights of MHW and MHHW above MTL by subtraction. We then added these values to -0.14 m to get the elevations of MHW and MHHW above NAVD 88. For the 35 stations listed on the tidal prediction pages, we took half of the tide range and added that to -0.14 m to get the elevation of MHW above NAVD 88. This method was only used along the coasts of New England and New York.
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