Dry Tropical and Subtropical Forest and Woodland Monitoring (2000 to 2005)
The estimation of biome-wide forest cover and forest cover loss is based on a probability-based sampling approach employing multi-resolution satellite data. Biome-wide change indicator maps were created using moderate spatial resolution imagery for 2000 to 2005 from the MODerate Resolution Imaging Spectroradiometer sensor (MODIS). These change indicator maps were used to stratify the respective biomes into high, medium and low change likelihood strata. Subsequent samples of 18.5km by 18.5km blocks of high spatial resolution image pairs from the Landsat ETM+ sensor were taken within each stratum and used to determine biome-wide area of forest clearing. The sampling strategy employed the MODIS data in the design to stratify the blocks and also in the analysis via a survey sampling regression estimator of forest clearing. This statistically rigorous sampling strategy provides a biome-level clearing estimate with known uncertainty.
Forest cover monitoring method and products
Biome-wide forest change hotspot maps were created using annual MODIS imagery for 2000 to 2005. MODIS 32-day composites were used as inputs and included data from the 1-7 reflective bands and Land Surface Temperature. In order to produce a more generalized annual feature space that enabled the extension of spectral signatures to regional and inter-annual scales, the 32-day composites were transformed to multi-temporal annual metrics. The classification tree bagging algorithm related the expert interpreted forest cover loss and no loss categories to the MODIS inputs. The classification tree yielded per MODIS pixel 5-year change probability maps. This map and the tree canopy cover data showing forest extent for year 2000 are available for download. These data represent areas of intensive forest cover clearing. However, MODIS data alone are inadequate for accurate change area estimation because most forest clearing occurs at sub-MODIS pixel scales.
The MODIS forest loss hotspots were used to generate percent hotspot values per 18.5 km sample blocks and then used to stratify the biome into regions of high, medium and low likelihood of forest cover loss. Blocks were sampled within each stratum and characterized into forest cover loss using multi-date Landsat ETM+ imagery. Each Landsat sample block was classified using a supervised decision tree classifier to yield 2000 forest cover and 2000 to 2005 forest clearing areas. Forest was defined as greater than 25% canopy cover and change was measured without regard to forest land use. All blocks used in this analysis can be viewed at the Landsat Supplementary Data page.
The Landsat-derived calibration data were used to estimate forest cover and forest cover loss within the biome using a regression estimator employing MODIS change hotspot fraction as auxiliary variable. The final Landsat-calibrated biome-wide dataset with spatial resolution 18.5 km quantifying forest cover for 2000 and forest cover loss from 2000 to 2005 is available for download.
Provided data are available for use for valid scientific, conservation, and educational purposes as long as proper citations are used. We ask that you credit the Global Forest Monitoring data as follows:
Hansen M., Stehman S., Potapov P. (2010) Quantification of global gross forest cover loss.
Proceedings of the National Academy of Sciences of the U.S.A.
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