High Normalized Difference Vegetation Index Values Research Articles

Spatial analysis of the distribution of Lyme disease in Wisconsin.

Surveillance measures for human cases of Lyme disease in Wisconsin were compared and associated with tick distribution and vegetation coverage. During 1991-1994, 1,759 confirmed human cases of Lyme disease reported to the Wisconsin Division of Health were assigned a county of residence, but only 329 (19%) could be assigned with certainty a county of exposure. Distributions of cases by county of exposure and residence were often consistent from year to year. Tick distribution in 46 of 72 Wisconsin counties was mapped based on collections by researchers, statewide surveys of infested deer, and submissions from the public. Satellite data were used to calculate a normalized difference vegetation index (NDVI) for each county. A geographic information system (GIS) was used to map distributions of human Lyme disease cases, ticks, and degree of vegetation cover. Human case distribution by county of exposure was significantly correlated with tick distribution; both were positively correlated with high NDVI values in spring and fall, when wooded vegetation could be distinguished from agricultural crops in the satellite image. Statistical analysis of spatial patterns using a measure of spatial autocorrelation indicated that counties with most human cases and ticks were clustered in parts of western Wisconsin. A map delineating the counties with highest risk for Lyme disease transmission was generated based on numbers of exposed human cases and tick concentrations.

The effect of microphytes on the spectral reflectance of vegetation in semiarid regions

The normalized difference vegetation index (NDVI), which is derived from satellite sensor images, is widely used as a measure of vegetation and ecosystem dynamics, change in land use, desertification, and climatic change processes on a regional or global scale. Surprisingly, in semiarid regions, relatively high values of NDVI were measured in landscapes where little, if any, photosynthetic activity of higher plants exists. We tested the hypothesis that the high NDVI values may be caused by the photosynthetic activity of microphytes (lower plants), consisting of mosses, lichens, algae, and cyanobacteria, which cover most of the rock and soil surfaces in semiarid regions. We found that the spectral reflectance curves of lower plants can be similar to those of the higher ones and their derived NDVI values can be as high as 0.30 units. We conclude that, in semiarid environments, the reflectance of lower plant communities may lead to misinterpretation of the vegetation dynamics and overestimation of ecosystem productivity.

Population densities of sheep related to Landsat Thematic Mapper radiance

Abstract The mean annual population densities of free-ranging sheep in upland North Wales were significantly correlated with near-infrared (0.76–0.90 μm) radiance recorded by the Landsat Thematic Mapper. The relationship between sheep density and the normalized difference vegetation index (NDVI) indicated that high sheep densities always occurred on vegetation with high NDVI values, but low sheep densities may occur on vegetation with either low or high NDVI values.