When the value is greater in 1975 than 1990, we interpret this as vegetation loss during that 15-year period. When the value in 1975 is less than 1990, we interpret this as vegetation gain. Total vegetation change is taken to be the sum of loss and gain on an area basis. Comparison of temporal changes in reflectance measures from satellites, such as NDVI, can be useful for gaining insight into land cover changes when land cover maps from two different dates are not available. Interpreting the measurements relative to land cover change is not simple, because some changes in reflectance are not changes in land cover.
Crop rotation is a good example.
Change in NDVI measurements may be the result of seeing a field in production on one date and fallow on the other. Interpretation of these measurements for actual land cover change requires a lot of additional work beyond calculating their difference over time. Please be aware that some of the pages may take up to 1 minute to fully load using a 28.8k modem due to the amount of graphics in the pages.