The data analyst assumes that there is a temporal window during which hay and pasture green up before most other annual or perennial vegetation. Another source of error is the discrepancy between satellite imagery and NAPP photograph acquisition time. Acquisition dates of the NAPP photographs range from the late 1980s to 1997, whereas the satellite data were mostly acquired from 1991 to 1993. Any changes that took place across the landscape over this time period complicate interpretation and comparison between reference and mapped land cover. One class that suffers most is the transitional barren, a class that is designed for conditions such as temporary clearing and regeneration of forest cover.
Similar problems exist within agricultural classes due to crop rotations.
Low accuracy for classes related to land use is understandable. Despite the extensive use of ancillary data, such as the census data, it is very difficult to unambiguously separate high intensity residential from other urban uses, either during the mapping or photo-interpretation process. Accuracy assessments were conducted separately for each region, i.e., each region is regarded as a population for drawing samples. Therefore, all accuracy estimates reported here should be pertinent to the entire region only, not necessarily valid for any given sub-division within the region. Hence the estimates of accuracy parameters for the two regions should be regarded conservative due to inclusion of sample points with only primary land cover label.