The Land Cover Trends (LCT) research project examines the rates, causes, and consequences of contemporary U.S. land-use and land-cover change. The strategy involves classification of sample blocks of Landsat imagery into eleven land-cover classes. The blocks are chosen from a random sampling of each of the eighty-four Level III ecoregions in the conterminous U.S. The classification is performed on the same spatial blocks for five time periods: 1973, 1980, 1986, 1992, and 2000. The temporal series of land-cover classifications is then analyzed for each block to identify and quantify any land-cover changes occurring over time. Change statistics that result from this analysis form the basis of work documenting current change and projecting future land-cover changes.
Data for Trends analysis come from change grids previously generated from the land cover classification products for the study years. This derived product consists of categorical data where each cell contains a value between 1 and 121. These categories represent the “from-to” land cover conversions that occurred for each cell. The study is based on a total of 2688 sample blocks analyzed for the four change periods: 1973 to 1980, 1980 to 1986, 1986 to 1992, and 1992 to 2000. This results in approximately 11,000 change grids, and each grid is approximately 15k bytes in size. Trends also uses about 200 polygon shapefiles that define the boundaries of blocks and ecoregions. The Trends analysis will utilize the GeoData Portal (GDP) to retrieve the change grids and shapefiles needed for the statistics. In addition, access through the GeoData Portal to a selection of shapefiles of regions, states, watersheds, etc. would provide a range of choice for the researcher interested in defining a study area.
A researcher who is interested in obtaining land-cover change statistics for a given study area accesses the Trends analysis through a web-based portal that provides simple GIS capabilities. The LCT portal will use GDP functionality according to the following use-case.
One possible view of user getting per-study area (in shapefile) summary of land cover changes using methods and data provided by the Land Cover Trends project.
The tables and graphs returned to the researcher at the end of the analysis by the LCT portal. The portal would provide change statistics on land cover within the study area. For each of the “from-to” land cover conversions such as “from agriculture to developed” or “from forest to wetland”, the researcher can view estimates of the total change and variance and the change and variance as percent of study area. Relative error and confidence intervals provide additional information on the estimates.
Current Trends analysis uses a stratified sampling approach, utilizing full sample blocks in the statistics even if some percentage of the block falls outside the study area. A future enhancement would clip or mask the change grids that fall on study area boundaries and perform a recount of conversions based on the masked grid. The analysis function would request the full grid for the sample block from the GeoData Portal and perform the masking and recalculation unless the Portal provided this functionality. Also, eventually a Trends web-based GIS would provide access to the complete Trends data set, comprised of roughly 150,000 satellite images, land cover and conversion grids, aerial photographs, videos, and reports. The Trends database, acting as a metadata catalog for the archive, would be available for searches and would provide the names of the files to the GeoData Portal for retrieval. These files could either be displayed at the Trends GIS website or made available for download. Jeanne Jones & Roland Viger