Scientists use data collected by remote sensing - typically via satellites, radar or aerial photography - as an additional source of drought indicators. Many of the remote sensing maps and analyses are based on data gathered by the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometry (AVHRR) satellite instrumentation. AVHRR data allow researchers to compute the Normalized Difference Vegetation Index (NDVI), a measure of plant health. Drought is only one influence on plant health, so sparse vegetation does not automatically imply drought.