A relatively new technology for acquiring shoreline data is lidar (LIght Detection And Ranging), a scanning laser system mounted on an aircraft. One such system, National Aeronautics and Space Administration's (NASA) Airborne Topographic Mapper (ATM), can collect hundreds of kilometers of three-dimensional (3-D) beach topographic data in a single day with a vertical accuracy of approximately 15 cm (Sallenger and others., 2003). Since the ATM collects large expanses of data in a short amount of time, and since shorelines (and error bars associated with the shorelines) can be objectively derived from these data, lidar is an efficient technique for studying large-scale coastal change. Together with NASA and the National Oceanic and Atmospheric Administration (NOAA), the U. S. Geological Survey is currently using lidar to acquire topographic data for the Nation's beaches. Shoreline position will be extracted from the data, and these present-day shorelines, along with historical shoreline data, will be used to assess both long-term (over the last 100 years) and short-term (storm-induced) shoreline change.
|Figure 1. Lidar beach profile from September 15, 1997 at Assateague Island, Virginia. Click for larger image.|
Stockdon and others. (2002) calculate shoreline position from ATM lidar data. Equally-spaced cross-shore beach profiles are first extracted from the topographic data. For each profile, a linear regression through a range of data points on the foreshore is used to calculate the horizontal position of the shoreline at a given vertical datum (Figure 1). Error bars on the horizontal position of each shoreline point are calculated, and beach slope is calculated as well.
In order to extract shoreline position from the lidar data, the elevation of the vertical datum that will define the shoreline must be known. For the National Assessment of Coastal Change Hazards Project, the shoreline is defined as the operational Mean High Water (MHW) elevation contour. NOAA tide station data were used to compile elevations of MHW for open coast (and near-open coast) environments along the East, Gulf, and West Coasts of the United States. Two other tidal datums, Mean Higher High Water (MHHW), and Mean Tide Level (MTL, the average of MHW and Mean Low Water), were also tabulated as possible alternatives to MHW as the shoreline definition.
Since the lidar data used for extracting shoreline position are referenced to the North American Vertical Datum of 1988 (NAVD 88), the NOAA tidal datums were computed with respect to this fixed vertical datum. In this report, we present the methods used to determine which tide stations were acceptable for this project and the techniques that were used to calculate the tidal datums with respect to NAVD 88. We then present the elevations of MHW, MHHW, and MTL along the East, Gulf, and West Coasts. Finally, we describe the way this set of "raw" MHW data was used to create "MHW Zones", sections of coast to which a single MHW elevation is assigned. This operational MHW elevation will be the height at which lidar shorelines are extracted.