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Supercomputing to Go Back in Time: Reconstruction of the History of Canada’s Forests with Satellite Data
Canada is a vast nation, dominated by forested ecosystems. From a total area approaching 1 billion hectares, Canada has about 350 million hectares of land with trees. These treed areas are found in forested ecosystems, interspersed among wetlands and lakes.
At the Canadian Forest Service (CFS), Senior Research Scientist Mike Wulder and his colleagues Joanne White and Geordie Hobart have been working with imagery from the Landsat series of satellites for decades. Previously they were limited by the capacity for data transfer, computing software and hardware, and the degree of automation possible due to the state of the science and the nature of the data. Over time, in addition to improvements in computing capacity, government agencies worldwide are increasingly offering free and open access to analysis-ready data products. This has been a game changer in the remote sensing community, however if data is not provided in an analysis-ready form, users are still required to build and implement custom approaches and tools prior to any value-added processing.
For instance, in 2008 the United States Geological Survey made all archival and new data collected by the Landsat series of satellites free and open access, as well as analysis ready, which greatly reduces the amount of pre-processing required before the Landsat data can be used to derive information. From a forest monitoring point-of-view, data from the Landsat series of satellites have many qualities that make them a data source of choice, the most important of which is a spatial resolution (30 m) that can capture human impacts over large areas.
Given the CFS mandate to monitor and report on Canada’s forested ecosystems, Wulder was interested in how to best use the vast data archive of Landsat to re-create the recent history of Canada’s forests. In the early 2000s, his team used Landsat to generate a national land cover map of the forested area of Canada but the production of that land cover map took several years and in some areas of the country it was difficult to acquire suitable cloud-free imagery. Although it was the most detailed map of Canada’s forests produced to date, it represented only one point in time (circa 2000) and did not capture change. Forests are dynamic ecosystems, with alterations to area and composition made by temporary changes such as wildfire and forest harvest, as well as more permanent changes such as expansion of urban or agricultural areas. As such, the time series aspect of the open access archive allowed for the capture and labeling of change.
So, Wulder and his colleagues had a goal. But how were they going to get there? Issues to address included the impacts of cloud, shadow, smoke, haze, seasonality, satellite revisit, etcetera, on the data.
“We have been able to port our processing algorithms to WestGrid, enabling more rapid processing: scripts that used to take weeks to complete now take days or hours,” says Wulder. “We are able to collaborate in a shared environment, working from the same base data and ensuring consistent outcomes. The gains in computing power offered by WestGrid has allowed for increasingly ambitious analyses.”
When problems are big, it is often fruitful to create a team with complementary skills. From a government perspective, universities offer additional brain power to help address scientific and technical questions, such as how to deal with billions of pixels over space and time. In the case of this research, Wulder's team partnered with the Integrated Remote Sensing Studio from the Faculty of Forestry at the University of British Columbia, led by Professor Nicholas Coops, a Canada Research Chair in Remote Sensing. Coops brought his insights and team to help address many of the obstacles that stood in the project’s way. Critically, Dr. Txomin Hermosilla joined the team as a Post-Doctoral Fellow and has been instrumental in leading the analysis and providing scientific insight. Through this collaboration, and with the assistance of UBC's Advanced Research Computing team, the researchers developed an approach to generate consistent, time series informed, image composites, which captured forest dynamics on the landscape. Now, the missing part was the capacity to consistently, transparently, and with some flexibility, to process the data. Enter WestGrid.
Using WestGrid computing resources, they have been able to undertake research to capture and label three decades of forest change over Canada’s forests (see http://forests.foundryspatial.com/) and are now generating annual maps of land cover for the same 30-year period. The expert computing support offered by one of WestGrid’s support team analysts, Belaid Moa, has been crucial to Wulder’s team’s success.
By way of scope, recall that Canada is approaching 1 billion hectares in size, which is in turn represented with about 10 billion Landsat pixels. This spatial coverage of ~10 billion pixels is populated with a number of spectral channels and indices on an annual basis. These annual coverages are replicated for each year in the time series.
“WestGrid offered the capacity for dealing with such spatially large and temporally dense data sets,” says Wulder. “No longer limited by computing considerations, the algorithms we are developing are increasingly sophisticated, providing new and otherwise unavailable information on Canada’s forested land cover and the disturbances and recovery occurring over the forested land base. Canadian science, monitoring, and reporting activities are strengthened by the computing capacity offered by WestGrid, as well as by the cross-institutional collaborative work environment.”
In addition to ensuring changes in forest cover are considered in the land cover mapping processes, this research will improve the CFS’s ability to refine the assignment of land cover by leveraging knowledge of forest successional processes with the time series of data.
Going forward, Wulder and his collaborators will continue working on algorithm refinements to improve the science though inclusion of new knowledge, novel spatial data sets, as well as extending the suite of forest conditions being estimated. For instance, the team aims to use a national sample of airborne laser-based estimates of forest characteristics to map forest height, canopy cover, and biomass at the national level. The long baseline offered by the science outcomes supported by WestGrid to date are especially informative to investigations related to climate change.
For more information on Wulder's team's work, please see: http://bit.ly/2e1FvBk.
- Multi-year proxy BAP time series animation