An area-based accuracy assessment methodology for digital change maps

A methodology was developed to evaluate the accuracy of maps of land-cover change produced from multi-temporal Thematic Mapper images for Australia. T he methodology is based on 500-pixel-by-500-pixel areal sample units. Sampl es units were selected in a two-stage procedure that first ensured that a c ertain number were selected for which change was apparent on the change map being evaluated. The remaining sample units were then selected according t o a spatially distributed random sample without replacement. An independent estimate of change was obtained for each sample unit by a human operator u sing one or more image enhancement techniques to determine subjectively how much change occurred. Confidence intervals for an increase and a decrease in woody vegetation were calculated on the independent estimate of change a nd the amount of change on the original map compared against them. To obtai n stable confidence intervals, the method requires that 33 samples per chan ge map/TM image are examined for each map of land-cover change, something t hat requires approximately two person-days per change map. The methodology was found to be robust for change maps on which change was relatively rare (about 0.13% of the total change map) and also for those on which change wa s relatively common (about 2.5% of the total change map).