A quantitative method has been developed to extract the location and elevation of Dhigh from lidar surveys. The method follows a set of fixed rules and processes in order to locate Dhigh. This is an objective process, meaning that if it is repeated multiple times, the answers will always be the same.
The complex algorithm is simplified below in order to illustrate the basic principles of this objective technique:
Step 1: A cross-section of lidar data is extracted from the survey, creating a profile view of the beach and dune. In order to eliminate small variations in elevation, the profile is mathematically smoothed.
Step 2: Slope is calculated between adjacent data points across the width of the cross section. Elevation peaks are identified based on changes in the direction of slope (positive to negative).
Step 3: Using established mathematical methods (Stockdon et al., 2002), the shoreline is located. Dhigh is the first elevation peak landward of the shoreline.
In the absence of a dune, Dhigh is located at the berm crest. The mathematical methods for locating the berm crest are nearly identical.
An alternative, more subjective method for locating the dune crest and toe is based on gridded, digital elevation models of lidar data. In this method, the location of the dune crest and toe are first digitized by a user and then refined using GIS-based algorithms. For more information on that technique, see the following publication:
Elko, N., Sallenger, A., Guy, K., Stockdon, H. and Morgan, K., 2002. Barrier Island Elevations Relevant to Potential Storm Impacts: 1. Techniques. USGS Open File Report 02-287.
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