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Landsat satellite imagery has been a boon to the study of the Earth’s surface for nearly five decades, but researchers at the USGS Earth Resources Observation and Science (EROS) Center have only recently begun to tap into the full potential of its long historical record.
Harnessing the depth of the Landsat archive is at the heart of the Land Change Monitoring, Assessment, and Projection (LCMAP) initiative, which is showing its potential to probe more deeply into pressing landscape questions as production ramps up for a full public release.
LCMAP’s land cover and spectral change map products amount to a time machine of sorts for remote sensing scientists, offering a year-by-year look at changing conditions across the United States since 1985, as well as insights into when those changes occurred and how significant they were.
Every Landsat-based pixel in every LCMAP product equates to a 30-by-30-meter plot of ground, and every change to a pixel feeds into our understanding of our planet.
The LCMAP team recently shared two new webpages that showcase how its data products might help scientists tease out trends in wetland dynamics and measure the impact of economic shock on urban growth.
The first page features information from EROS scientist Jennifer Rover, who leads Applied Science projects for LCMAP. Rover looked at wetlands in an area of east-central North Dakota that’s seen a decades-long uptick in precipitation since it was first mapped by the U.S. Fish and Wildlife Service (FWS) National Wetlands Inventory (NWI).
The NWI is a widely-used dataset for wildlife and wetlands research, but updates come slowly due to a time- and labor-intensive production process.
Rover found that results from LCMAP’s land cover map product aligned closely with the NWI’s classifications – at least in terms of stable, permanent bodies of water. Where the NWI had permanent lakes and ponds, LCMAP had water. But LCMAP also captured the growth of lakes, ponds, and wetlands that had taken place since the NWI borders were drawn for the area in the 1970s. Moreover, LCMAP saw multiple changes to areas tagged by the NWI as “emergent wetlands,” many of which moved from barren land to grassland, grassland to wetland, and wetland to open water and back again multiple times over the 33-year study period.
Rover’s analysis suggests that LCMAP’s land cover product can be used to fill in data gaps for regions of the country where the NWI is outdated, but her analysis didn’t stop there.
LCMAP’s Change Day product allowed Rover to tag each change in wetlands to the time of year in which it occurred. That information can be especially valuable for wildlife managers who aim to bolster specific bird or fish populations across wide geographies.
Shorebirds, for example, need exposed, bare shorelines at certain times of the year. Knowing which areas with cyclical change patterns are most likely to dry out or remain stable and when can help guide wetlands management decisions.
The second LCMAP web release put the spotlight on LCMAP’s product validation.
LCMAP’s reference dataset consists of 25,000 locations across the U.S., each tied to the geographic coordinates of a randomly-selected Landsat pixel. Changes to those 25,000 plots of land are investigated through a variety of sources, such as high-resolution imagery, to verify what type of change actually occurred. Comparing those pixels to LCMAP’s land cover results serves to characterize product accuracy and uncertainty.
Taken independently, however, the reference dataset can be used to see wide-scale changes to the national landscape.
That’s what LCMAP Assessments lead Roger Auch found recently when looking at growth in “urban” reference plots. The U.S. went through a period of tremendous growth in housing in the 1990s and early 2000s, a trendline reflected in the reference dataset. At the height of the boom in 2006, the reference pixels showed the addition of more than 10,000 square kilometers of urban area.
Growth in urban land cover dropped precipitously in 2007, as the U.S. housing sector began to collapse, and growth dropped to a 33-year low as the Great Recession took hold the following year. Growth in urban pixels remained low through 2016, even as the wider economy recovered.
Those pixels, of course, represent satellite observations – the visible impact of a nationwide economic shock and recovery whose story is normally told through numbers such as new home sales or construction spending.
The pending release of LCMAP Collection 1 will offer observations like those for every 30-meter plot of land in the conterminous United States, enabling national and regional assessments of not only annual patterns of urban growth, but patterns of forest harvest and regrowth, recovery from fire or invasive species damage, or shifts in cropland over time.
Ultimately, LCMAP data products are tools that can be used to answer a host of important questions tied to land cover and land use in the U.S. and guide decision-makers on how best to harness the nation’s land resources.
“These examples show how we can take the annual data, and really investigate it and pull out information we have never had before,” said Jesslyn Brown, who leads the LCMAP team at EROS. “That’s really important because it shows how we’re going to work with our partners and develop applications going forward.”
To learn more, visit the LCMAP home page.
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