Wetland-Change Data Derived from Landsat Imagery, Assateague Island to Metompkin Island, Maryland and Virginia, 1984 to 2015: Land-cover Change Analysis

This U.S. Geological Survey (USGS) data release includes geospatial datasets that were created for the analysis of Virginia and Maryland Atlantic coastal wetland changes over time. Wetland change was determined by assessing two metrics: wetland persistence and land-cover switching. Because seasonal water levels, beach width, and vegetation differences can affect change analyses, only images acquired during the spring (March, April, and May) were included in the wetland-change metrics (N=10). Land-cover switching was evaluated using Landsat images for successive spring image-acquisition dates: 1985–1989, 1989–1994, 1994–1999, 1999–2004, 2004-2009, 2009-2011, 2011-2013, 2013-2014, and 2014-2015. To evaluate land-cover switching, land-cover types defined by Bernier and others (2015) were reclassified as 1 (water), 3 (wetland), or 7 (non-wetland). These values were chosen so the results of subtracting two dates will create unique values for each scenario. For example, if a cell in 1994 is classified as land and in 1989 was wetland, the result (1994-1989 or 7-3) is 4. If the cell in 1994 is wetland and in 1989 was water (3-1) the result is 2. With this analysis, each two-date combination results in a raster that identifies wetland-land-water conversions, such that water-to-land is -6, wetland-to-land is -4, water-to-wetland is -2, wetland-to-water equals 2, land-to-wetland is 4, and land-to-water is 6.

本美国地质调查局（U.S. Geological Survey, USGS）发布的数据集包含用于分析弗吉尼亚州与马里兰州大西洋沿岸湿地随时间变化的地理空间数据集。湿地变化通过两项指标进行评估：湿地持久性与土地覆盖转换。由于季节性水位、滩涂宽度与植被差异会对变化分析造成干扰，本次湿地变化指标分析仅纳入春季（3月、4月、5月）获取的影像（共10景）。土地覆盖转换分析采用Landsat影像，对应连续春季影像获取时段分别为：1985–1989年、1989–1994年、1994–1999年、1999–2004年、2004–2009年、2009–2011年、2011–2013年、2013–2014年及2014–2015年。为开展土地覆盖转换分析，将Bernier等人（2015）定义的土地覆盖类型重分类为3类：1（水体）、3（湿地）及7（非湿地）。选取上述编码值的目的是，通过两个时相的编码值相减，可为每种土地覆盖转换场景生成唯一的差值结果。例如，若某像素在1994年被分类为陆地，而1989年为湿地，则差值结果为1994年编码减1989年编码，即7-3=4；若某像素1994年为湿地、1989年为水体，则差值为3-1=2。通过该分析方法，每一组双时相组合均可生成一幅栅格影像，用于识别湿地、陆地与水体间的转换关系：水体转陆地为-6，湿地转陆地为-4，水体转湿地为-2，湿地转水体为2，陆地转湿地为4，陆地转水体为6。