Observing Earth from Space: Landsat Data and Applications
More than two thousand satellites are currently in orbit. They measure the earth's surface characteristics, ocean currents, clouds and the gaseous content of the atmosphere. One of the oldest programs is Landsat. Run by both the National Aeronautics and Space Administration (NASA) and the United States Geological Survey (USGS), Landsat is still widely used today because it provides its data to researchers free of charge. The Landsat program—a series of satellites that continuously record information about the earth's surface—has contributed to research on an enormous variety of land-use, ecosystem and development issues. Landsat data have been used to study mitigation of deforestation and eutrophication; prediction of the effects of climate change; quantification of urban sprawl; and early-warning and response to extreme weather, natural disasters, and vector-borne diseases. While earth-observing satellites record a range of data types, Landsat technology, in particular, produces pictures of earth’s surface through remote sensing. When solar radiation reaches the earth surface it can be either reflected or absorbed and re-emitted. Remote sensing satellites measure specific wavelength ranges of reflected or re-emitted radiation from earth’s surface and create a picture based on these measurements. When the first Landsat satellite was launched in the early 1970s, the pixel resolution of Landsat data was only eighty square meters, meaning each pixel of data represented an 80m x 80m square. Since then the technology has improved, and sensors can now more finely distinguish among wavelengths of radiation from earth’s surface. Landsat-5 and Landsat-7—-the two satellites currently in operation—-can measure wavelengths at a fine enough scale to create images with a pixel resolution of thirty square meters. These more finely rendered images allow scientists to distinguish slight differences that represent, for instance, healthy and unhealthy vegetation. Though a pixel resolution of thirty square meters isn’t enough to identify the individual houses visible in some parts of Google Earth, it does allow for the identification of both natural and anthropogenic changes in the environment. It is also produces data files on a global scale that are small enough to be manageable. Landsat data has recently been used to map rift valley fever outbreak risk areas in the Arabian Peninsula and to monitor rain forest disturbance in the Amazon. These two projects are described below:
Mapping Rift Valley Fever Outbreak Risk Areas
Rift Valley Fever (RVF) is a mosquito-borne virus that can affect both humans and livestock, resulting in illness, economic losses, and even death. The disease is prevalent in East Africa and the Arabian Penninsula, particularly during rainy periods that allow mosquitos to breed in heavily vegetated areas.
Between August and September of 2000, widespread RVF in both humans and livestock was reported in the western coastal plain of Yemen. In order to prevent future outbreaks, Yemeni and international epidemiological experts wished to pinpoint the areas where mosquitoes were breeding.
High resolution Landsat-7 images from May and September of 2000 were acquired and compared to find regions of intensive vegetation growth during the epidemic (Figure 1). Using these images in addition to aerial surveys, a team of experts were able to identify locations most conducive to growth of mosquito larvae and the transmission of RVF. The outcome of this study allowed for disease control operations to be sent to regions with highest risk for the virus.
