Data Processing

Orthoimages of the undeveloped areas of the New Jersey barrier islands were acquired in digital format from the U.S. Department of Agriculture (USDA), U.S. Geological Survey (USGS), New Jersey Geographic Information Network (NJGIN), and National Oceanic and Atmospheric Administration (NOAA). They were used to extract the data included in this report.

Variability in image quality from one date to another, between images taken on the same date, and between segments of the same image exists due to many factors, including variations in the sun’s angle, atmospheric conditions, the season, the reflectivity of the earth’s surface, the camera angle, the type of camera lens, the camera’s settings, film sensitivity, water turbidity, water depth, tide level, waves, and errors in image rectification.  No adjustments were made to account for tide levels. Standard errors of linear regression for all areas are provided:

• For the back-island shoreline: 13.19-m average, 0.11-m minimum, and 364.89-m maximum;
• For the sand line: 29.13-m average, 0.51-m minimum, and 456.92-m maximum;
• For the open-ocean shoreline: 28.86-m average, 1.62-m minimum, and 219.95-m maximum.

Open-Ocean Shorelines: A line was hand-digitized at the approximate water line by using ArcMap 10.2 and a scale of approximately 1:2,000.  The line placement represents the still-water level, such as tide level, at the time of acquisition and was fit through the average position of waves and swash apparent on the beach.  No adjustment was made for varying tide levels.

Back-Island Shoreline Points: Shore-normal transect lines spaced 20-m apart were laid over the orthoimages by using ArcMap 10.2 and a scale of approximately 1:2,000. Points were hand-digitized at the intersections of the apparent back-island shoreline and the transect lines. Only one back-island shoreline/transect point was digitized per transect line.

Back-Island Shorelines: These lines are the back-island shoreline points connected to form a line. The lines falling between the points or transect lines do not follow the natural shoreline.  They have been included for use with software that requires data in line format.

Sand Polygons: The orthoimagery was trimmed to the study areas by using ERDAS Imagine 9.3 and the 0.3-m resolution imagery was resampled to 1-m and smoothed using a 3-m x 3-m focal mean. Forty image classes were generated by using the unsupervised classification method with the following settings: principal axis initializing, 1 standard deviation, 10 iterations maximum, 0.95 convergence threshold, skip factor of 1 for both x and y (no skipping), and zeroes not classified.  The resultant classes were visually examined and coded as either sand or not sand. Occasionally, coding differed within a single image due to varied image quality and surface conditions, such as the color of the sand in overwash and scoured beach areas, and in areas adjacent to inlets. The coded raster datasets were converted to polygons by using ArcGIS 10.2. Polygons outside the study areas were deleted. Non-sand polygons within the main beach area smaller than 25 square meters (m²) were merged with the sand area. Sand polygons with areas less than 100-m² were deleted. The open-ocean shoreline edge of the polygons was trimmed and filled to the open-ocean shorelines digitized from the same image to simplify the irregular and misclassified areas caused by wet sand and waves. 

Sand Lines: The main (beach) polygons, selected from the previously described sand polygons, were converted to lines using ArcGIS 10.2. The lines that represented the inland extent of the main sand area were kept and primarily compose a continuous line. Some lines surrounding islands of vegetation within the main sand polygon were kept based on an editing rule. The vegetation island had to be 100-m² or larger, had to be the closest vegetation island of that size; in the shore-normal direction, to the continuous sand line; and the distance between the continuous sand line and the vegetation island had to be less than or equal to the shore-normal width of the vegetation island for 50 percent or more of its longshore length. The remaining lines were deleted.

Base Lines and Transects: The base lines are off-shore and approximate the trend of the of the open-ocean shorelines of the study areas. The transects originate from the base lines at 20-m intervals and run perpendicular to them. Both were used for various USGS studies and are included for convenience (Hapke and others, 2010).