Coastal Elevation Data

Figure CE1. Coastal elevation datasets can be used to approximate the land area that can be affected by a given sea-level rise.  A key concern with this approach involves the uncertainty associated with the elevation data. Here, light blue shading designates regions that are in the uncertainty (or error) range of the data.  Panel ‘A’ shows data from a 30-m digital elevation model compiled from topographic maps and the region at or below 1 m. Here the vertical error includes a region up to 2.2 m above the 1 m scenario. Panel ‘B’ exhibits the lower uncertainty associated with higher-resolution lidar data. In this case the vertical error includes a smaller region up to 0.3 m above the 1 m scenario. (Dean Gesch, USGS)

Coastal changes are driven by complex and interrelated processes (CCSP, 2009). Inundation will be the primary response to sea-level rise in some coastal locations, but there are other possible responses to sea-level rise,  such as erosion on sandy shores or marsh accretion in coastal wetland systems. A goal is to quantify the various effects of sea-level rise and to identify the areas and settings along the coast where inundation will be the dominant coastal change process. In some locations, the extent of inundation is controlled largely by the slope of the land, with a greater degree of inundation occurring in areas with more gentle gradients.

The present land elevation is a critical factor in assessing potential impacts in areas that are vulnerable to an inundation response to rising seas.   Accurate delineations of potential inundation zones are essential to determine the potential socioeconomic and environmental impacts of predicted sea-level rise. A common approach that has been used to identify and quantify the extent of land vulnerable to sea-level rise relies upon the use of elevation data such as topographic maps or digital elevation models.  Typically, researchers use these data to identify and estimate the land areas that occur below an elevation that coincides with a particular sea-level rise scenario.  This analysis requires high-quality elevation data.  The USGS is responsible for maintaining the National Elevation Dataset (NED; http://ned.usgs.gov/) which is a compilation of the best available elevation data for the United States.  This dataset provides elevation data that have been processed and reviewed to ensure that they are consistent and meet current standards.  The NED is updated regularly as newer and better data become available.

One aspect of elevation-based sea-level rise studies that is often neglected involves the accuracy of elevation data.  Because the landscape in many coastal regions is made up of a gently sloping land surface, such as along the East and Gulf coasts of the U.S., understanding the uncertainty ranges of these data is important to identify accurately regions that could be affected.  As data with better resolution and accuracy are incorporated into the NED, the uncertainty ranges of the elevation data have improved substantially (figure CE1).