*2.1. LiDAR Data and GIS*

Beginning in 1998, the National Oceanic and Atmospheric Administration (NOAA) in partnership with the US Army Corps of Engineers (USACE) have conducted light detection and ranging, also known as LiDAR mapping surveys of beaches and nearshore areas on a national scale [8,9]. The data from these surveys have been shown to be effective tools for monitoring the long-term evolution of barrier islands and coastal environments. These datasets are available in the public domain at https://coast.noaa.gov/dataviewer/#/ (accessed on 25 January 2021).

Using NOAA Digital Coast archives, classified LiDAR data were extracted for the years of 1998, 2007, 2010, 2015 and 2018, as well as two additional LiDAR sets for 2004; post-Hurricane Charley and post-Hurricane Ivan [10]. As the data from 1998 were not classified and having low spatial resolution, it has been removed from further processing and analysis. The LiDAR data from each consecutive year were filtered for ground elevation and bathymetry, representing reasonable elevation points down the coastal environment. Using LiDAR data processing tools in ArcGIS, terrain models for each year were generated with a spatial resolution of 2 m and 5 levels of pyramid structure for optimal zooming [11]. From these terrain models, raster DEMs were generated using the 3D Analyst tool in ArcGIS.

To create the DEMs (see Figure 1), LAS datasets for each of the subsequent years were needed [9]. LAS files were extracted from the LiDAR datasets, and the statistics of the LAS datasets were calculated. Next, the dataset was added to ArcMap and the classification codes were edited and filtered so that only the ground and bathymetric elevation data would be incorporated. This was done to exclude data points that may have shown tree canopies, vegetation, and building elevations that would interfere and skew the results.

The next step in this mapping process was to create a terrain model from the LiDAR data. This terrain model is a multiresolution TIN-based surface stored as features, created by importing the ground and bathymetry LiDAR data points to a multipoint feature class. "LAS to Multipoint", a 3D Analyst tool, was used to create this feature. Next, the multipoint feature class was combined with a 2D shapefile of the study area [11].

**Figure 1.** Digital Elevation Models (DEMs) for the North Captiva island using LiDAR data from 2004 and 2018.

Florida Department of Environmental Protection (FDEP) has been collecting shoreline data for the entire state since the early 1970s and they established R-monuments along the coast at an interval of 304.8 m (1000 ft) for periodically measuring the beach topography to assess the long-term erosional/accretional trends.
