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| USGS Earth Resources Observation and Science Center (EROS) |
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This document provides information on an interim basis until a detailed Guide document becomes available.
Introduction. Data were compiled through NASA's Commercial Remote Sensing Program; this program is a cooperative effort between NASA and the commercial and the commercial remote sensing community to provide users with access to quality-screened, high-resolution satellite images with global coverage over the Earth's land masses. The resulting Landsat Enhanced Thematic Mapper Plus (ETM+) data set contains orthorectified ETM+ sensor data from the Landsat-7 satellite. Only the ETM+ images most closely meeting a specific set of criteria, including acquisition date, cloud percentage, data quality parameters, and best available phenology, were considered for this collection. The Landsat ETM+ data were provided by the USGS and orthorectified through a NASA contract with Earth Satellite Corporation, Rockville, Maryland, and are part of NASA's Scientific Data Purchase program, developed in response to the President's Space Policy. The Landsat Orthorectified Pansharpened ETM+ data set possesses additional processing via pansharpening. This process, also known as resolution merging, fuses together the high-resolution panchromatic and lower-resolution multispectral imagery to create a high-resolution color image. Pansharpened data retain as much of the spectral information as is possible from the multiband raster image. The resulting high-resolution color image preserves the original color fidelity and allows for better visualization, interpretation, and clarity.
Data Set Characteristics. Approximately 8,500 Landsat ETM+ images were selected. These circa year 2000 scenes were acquired between 1999 and 2001, and have been orthorectified so as to tightly co-register with the earlier circa 1990 global Landsat TM coverage. NOTE: In both the circa 1990 and circa 2000 data sets, the temporal dates of individual scenes may vary between 1-3 years of the 1990/2000 baseline.) These images were either cloud-free or contained minimal cloud cover. In addition, only ETM+ images with a high quality ranking in regards to the possible presence of errors such as missing scans or saturated bands were selected. It was important to collect data during periods of peak greenness so NASA adopted an approach for image selection that was based upon a data set containing global 1-kilometer advanced very high-resolution radiometer Normalized Difference Vegetation Index data. It was sometimes difficult for NASA to obtain clear images during time frames of peak greenness, resulting in some image substitution from other times of the year.
Processing. Landsat ETM+ data are orthorectified using existing orthorectified TM data as horizontal control. Pixel cross-correlation is used to pass the control from the earlier orthorectified TM data to the Landsat ETM+. A patented Landsat large block triangulation procedure is used to perform a photogrammetric adjustment of each individual Landsat ETM+ scene; data are then placed into a UTM / WGS84 map projection. All bands are individually re-sampled, using a nearest neighbor algorithm. The result is a final image map product with a Root Mean Square Error (RMSE) of better than 50 meters in positional accuracy.
The Landsat ETM+ scenes were then pansharpened using a resolution merging process allowing the high-resolution panchromatic and lower-resolution multispectral imagery to fuse together to create a single high-resolution color image. This process allows the image to retain as the greatest value of the spectral information as possible from the multiband raster image. The resulting high-resolution color image preserves the original color fidelity and allows for better visualization, interpretation, and clarity.
For more information, see:
EarthSat's GeoCover Web Site
Kalluri, Satya, Grant, Denelle, Tucker, Compton, Policelli, Fritz, Dykstra, Jon, and Bearden, Phil, 2000, NASA creates global archive of ortho-rectified Landsat data: EOS, Transactions, American Geophysical Union, v. 81, no. 50, p. 609, 617-618.
Dykstra, Jon D., Jan/Feb 2001, The GeoCover-Ortho Program: Covering the Entire Earth's Landmass with 14,000 Landsat Images; GEO INFORMATICS, p. 26-29. Reference is available in PDF format.
Disclaimer: Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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