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	By Eugene Fosnight, U.S. Geological Survey Earth Resources Observation and Science Center (http://eros.usgs.gov), Sioux Falls, S.D.
	Satellite imagery is a valuable tool for studying changes to Earth's land surface over time. Images collected for decades are used to compare and evaluate the effects of natural and man-made alterations. Now TerraLook, a joint initiative between the U.S. Geological Survey and NASA's Jet Propulsion Laboratory, has made free satellite imagery available to a wide user community for monitoring change. TerraLook was developed to serve the traditional science communities that have used satellite data for analyzing and monitoring change, as well as science writers, field scientists, resource managers, policy makers, members of the education community and the general public—groups that have a growing need for information but don't have the technical expertise, time to prepare and organize images for visualization, or access to specialized scientific image processing software. TerraLook images are designed for visual interpretation rather than digital analysis. Image Sources TerraLook comprises images derived from two satellite image programs: 1. Landsat images from the USGS Earth Resources Observations and Science Center (EROS) 2. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images from the NASA Land Processes Distributed Active Archive Center (LPDAAC) Landsat data have been collected since 1972. The most recent TerraLook release uses Global Landsat Survey (GLS) data as the Landsat data source. The GLS images were selected from 2.5 million images in the Landsat archive to provide global coverage of Earth's surface during 1975, 1990, 2000 and 2005. Images from each year were selected for best cloud-free coverage during the local growing season. The nominal resolution is 60 meters for the 1975 images and 30 meters for the 1990, 2000 and 2005 images. More than 1.5 million images have been collected by ASTER, one of five sensors aboard the NASA Terra spacecraft, since it was launched in 1999. TerraLook images can be derived from any image in the LPDAAC ASTER archive since launch. ASTER images have twice the resolution (15 meters) of the more recent Landsat acquisitions. The TerraLook images are bundled with image footprint and metadata files that describe the images and the collection. The image footprint files delineate the image boundaries so the image location can be seen easily, and the metadata files contain information that describe the sensor, acquisition time and name of each image file. The image footprints are stored as standard shapefiles. The image and collection metadata are stored in standards-compliant XML files. The metadata can be viewed easily in any Web browser. TerraLook products consisting of images, footprints and metadata are bundled into zip files.

	A TerraLook collection can consist of any combination of ASTER and Landsat GLS images. A common use is to study Earth over time; however, many users simply want an image for their area of interest. A typical choice is a collection through time for one or more cities or parks, or for a region or country, but a collection can also consist of images from one sensor for a larger area within the same time period. The selection of images is completely up to the user. Two additional features make the TerraLook data even more accessible: The data can be downloaded for free in a JPEG format, and the images are rendered in a "simulated natural color." Simulated natural color makes the imagery easy to understand by a wide range of users. The algorithm used to process TerraLook images permits the creation of a visually consistent time series from the earliest Landsat sensor launched in 1972 to the currently orbiting ASTER and Landsat sensors. Color Enhancement TerraLook images are a simulated natural color rather than a "true" or "natural" color image, which are created from blue, green and red light. Most satellite sensors don't collect data in the blue spectral band of the electromagnetic spectrum. Even when a blue band is available, the blue light, as seen from space, is scattered by atmospheric moisture, which is why the sky appears blue. This effect creates a noisy blue band, particularly in humid areas. Most satellite sensors collect near-infrared (NIR) data, which are sensitive to the health of vegetation. In response to the lack of a blue band and the availability of an information-rich NIR band, most satellite images are viewed using some combination of visible and infrared data. TerraLook images use the bands derived from the red, green and NIR bands collected by the Landsat and ASTER sensors with the algorithm described below to form a simulated natural-color image: Red = Red Green = 2/3 Green + 1/3 NIR Blue = 2/3 Green – 1/3 NIR The green band is enhanced through the addition of information from the vegetation-sensitive NIR band to create a synthetic green band. The synthetic blue band is created from the green band, which is spectrally most similar to blue, with the vegetation information suppressed.

	TerraLook images are georeferenced with industry standard world files, which makes them suitable for use in most geographic information system (GIS), image processing and Web mapping applications. Just as important, the images can be viewed in any software that can read and display JPEG images; this includes most Web browsers, word processors and graphics packages. Additionally, a TerraLook viewer lets users view, compare, analyze and annotate TerraLook images. The multilingual (Spanish, French and English) open source TerraLook viewer is available from Source Forge (http://terralook.sourceforge.net) and allows images to be displayed by selecting an image footprint. The TerraLook viewer has special tools that make it easy to study Earth through time and construct virtual mosaics. Layers can be created interactively to store annotation and map data, such as park or urban boundaries, or new information, such as the location of sample sites. TerraLook builds on the success of NASA JPL's Protected Area Archive project (http://asterweb.jpl.nasa.gov/TerraLook.asp), with additional support from numerous international conservation programs. Feedback from the conservation community guided the development of the TerraLook product and viewer, which will continue to evolve as the project reaches out to an even wider user community.

	Publisher's Note: To search and order TerraLook data, visit the USGS Global Visualization Viewer at http://glovis.usgs.gov. Additional information on TerraLook is available at http://terralook.cr.usgs.gov.


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