Coastal Classification Atlas

Mapping Methods

A coastal storm-hazard assessment involves two different activities. The first is classification of ground conditions, and the second is translation of that information into a storm-hazard vulnerability index. Present ground morphology and any man-made alterations of the land were first interpreted in the laboratory, using three independent visual sources:

1. airborne color video surveys,
2. 35-mm aerial color slides, and
3. Digital Orthophoto Quarter Quadrangles (DOQQs) obtained from the U.S. Geological Survey EROS Data Center in Sioux Falls, South Dakota (http://edc.usgs.gov/index.html).

The video tapes and slides are oblique low-altitude photographs of the beach obtained during helicopter surveys with horizontal positions determined by a Global Positioning System (GPS). Dates of the various surveys are presented in Table 1.

Table 1. Types of imagery used for mapping and the dates they were acquired. Type of Imagery Date Aerial Oblique Video Survey June, 1996 Digital Orthophoto Quarter Quadrangle Jan., 1995 to Jan., 1996 Lidar Survey January, 1999 Map Verification June, 2005

The video surveys, slides, and DOQQs were used to prepare the coastal classification units (Fig. 2) and urban attributes of each coastal segment. Elevations of the land surface adjacent to the barren backbeach were obtained by processing lidar (light detection and ranging) data. The highest elevations of the foredunes or beach crest (where dunes are absent) are shown as a series of dots. Each dot is color coded to represent a narrow range of elevations. The units of elevation are decimeters, or tenths of meters.

In some local areas, there are substantial differences between the backbeach position observed on the DOQQs and those observed on the lidar surveys. These differences are attributed generally to coastal dynamics and specifically to beach erosion, spit accretion, or river mouth migration. Examples of discrepancies between the DOQQs and lidar surveys are shown on Christmas Point NE/SE and Cedar Lakes East NW. The dates of the surveys (Table 1) show that the lidar survey is more recent than the DOQQ survey.

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