Areal estimates of fragmented land cover: effects of pixel size and model-based corrections

The effects of coarse spatial resolution on the apparent extent of fragmented cover types on digital maps were assessed and a model-based method for correcting for underestimation due to omission of small fragments was tested. Burn scar maps based on Landsat Multispectral Scanner (MSS) imagery and a map of ponds in Arctic tundra based on ERS-1 synthetic aperture radar (SAR) imagery were spatially degraded to simulate EOS Moderate-Resolution Imaging Spectroradiometer (MODIS) and 100m SAR maps. Omission of small fragments on the degraded imagery decreased the apparent extent by as much as 50%. The apparent extent on degraded maps, where various degrees of systematic tendency towards commission were simulated by identification of pixels with less than 50% burn scar or open water, was either close to that measured on the original map (for small classification bias) or overestimated (large classification bias). An area correction procedure based on fitting sizes to a lognormal distribution with censored data was tested. The area correction compensated for 25 to 70% of the underestimation relative to the original maps. Area correction led to inflated areal estimates with biased maps or with a Pareto rather than the lognormal distribution model, indicating that the areal correction procedure must be used judiciously if improved estimates are to be expected.