An inventory and assessment of significant topographic changes in the United States
The effects of human actions upon the environment have been well documented, and the topic continues to be a primary one in geography and earth science research. Many studies have focused on human induced changes in land cover. Some studies have estimated the total effects of human activity on the landforms and shape of the Earth's surface, but these studies have not emphasized the spatial component of the changes. The primary issue addressed by the research reported here is the need for more comprehensive information on the nature and extent of recent human geomorphic activity. In this case, the geomorphic activity is evidenced by significant changes to the topographic surface throughout the conterminous United States. To meet the requirement for locational and quantitative information about significant topographic changes, the study makes use of seamless multi-temporal elevation data and land cover data. The National Elevation Dataset (NED) and the Shuttle Radar Topography Mission (SRTM) data form a unique pair of elevation datasets that can be used to detect and analyze 20th century topographic surface changes in the United States. The National Land Cover Dataset (NLCD) provided land cover information at a 30-meter resolution that matched the NED and SRTM data. The NED supplied the historical elevation information that was subtracted from the recently collected SRTM data to create a difference grid that provided information about where topographic changes have taken place. The difference grid was processed with a vertical accuracy based threshold to delineate areas of significant elevation change. The delineated areas were subject to a filtering process to eliminate errors of commission, resulting in a dataset of polygon features that outline the areas of significant topographic changes in the conterminous United States. The polygons are attributed with numerous terrain parameters calculated from the NED and SRTM data, the difference grid, and the NLCD. The primary types of topographic changes resulting from human geomorphic activity include surface mining, road construction, urban development, dam construction, and landfills. Notable concentrations of topographic change polygons are found in the mining areas of central Appalachia, northern Minnesota, eastern Wyoming, and southern Arizona, and in the expanding urban areas in coastal southern California. Summary statistics of the distribution of change polygons were accumulated and tabulated for different reporting units, including states, counties, and ecoregions. The environmental effects of topographic changes can be quantified in a number of ways, including hydrologic effects, visual impacts, and comparisons among regions for the effects of mining on topographic relief. Topographic changes are often accompanied by land cover changes, and an example in Appalachia shows the direct correspondence between mining expansion and reduction in forest cover. Although some unique dataset characteristics and data quality issues presented a challenge to development of reliable topographic change maps, the inventory was successfully completed, and it represents a first ever accounting of topographic change across the United States. The results provide locational and quantitative information for individual change features, and thus complement well the existing summary reports of the total effect of human geomorphic activity. Future work in topographic change analysis over broad areas may benefit by investigating alternatives for extraction of significant features, and by adding additional elevation data over finer time intervals to move from simple change detection to monitoring.
0768: Environmental science