NASA's Global Orthorectified Landsat Data
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02-112.qxd 2/4/04 11:36 AM Page 313 NASA’s Global Orthorectified Landsat Data Set Compton J. Tucker, Denelle M. Grant, and Jon D. Dykstra Abstract tens of meters spanning the last 30 years. They constitute an NASA has sponsored the creation of an orthorectified and indispensable history of land-surface state. Data at these spa- geodetically accurate global land data set of Landsat Multi- tial resolutions can provide a high potential mapping accuracy spectral Scanner, Thematic Mapper, and Enhanced Thematic of natural vegetation and alterations to it, if and only if highly Mapper data, from the 1970s, circa 1990, and circa 2000, re- accurate scene-to-scene within- and among-date registration is spectively, to support a variety of scientific studies and educa- achieved. Otherwise, misregistration errors between or among tional purposes. This is the first time a geodetically accurate dates are confused with land-cover change and resulting inter- global compendium of orthorectified multi-epoch digital satel- pretations are meaningless (Townshend et al., 1992). lite data at the 30- to 80-m spatial scale spanning 30 years has The best solution to eliminate or minimize misregistra- been produced for use by the international scientific and edu- tion errors is to precision orthorectify each scene within each cational communities. We describe data selection, orthorectifi- of the three epochs. The process of orthorectification removes cation, accuracy, access, and other aspects of these data. erroneous image displacements caused by the interaction be- tween terrain relief or local elevation changes and sensor ori- entation variations. The orthorectification process results in Background remotely sensed image products that possess both the image- Spatial variability exists for our planet’s land surface at di- based information of the original satellite data and the geo- mensions of meters to tens of meters, due to local terrain vari- metric information of a geodetically accurate map. ability and associated microclimatic influences upon vegeta- When Landsat data are assembled into a mosaic to cover tion types and associations. Accordingly, spatial data at tens an area of interest, an underlying assumption is that the data of meters are required to accurately map many areas, because have a consistent geometry throughout the image. This has of the low spatial autocorrelation of land-surface features been demonstrated for the MS, TM, and ETM+ instruments with (Townshend and Justice, 1988; Townshend and Justice, 1990). increasing accuracy, respectively (Desachy et al., 1985; Welch In addition, a variety of natural and human land-use changes, et al., 1985; Malaret et al., 1985; Bryant et al., 1985; Storey and such as wild fires, deforestation, wetland conversion, and ur- Choate, 2000). If the Earth’s surface were the same elevation banization, represent alterations of landscapes, which also over the geographical area of interest and the satellites in ques- occur at spatial scales of tens of meters. These are important tion were in identical orbits for the period of interest, a Land- perturbations of the global environment and require similar sat data mosaic could be “tied” together using a linear or affine spatial scale data for quantification. Currently, we lack infor- mapping, and the resulting mosaic would be an accurate rep- mation regarding where environmental change is occurring, resentation of the surface with accurate distances among all what the changes are, and what the post-change properties of surface features. When the topography is irregular, as is nor- the altered areas are (Townshend et al., 1991). mally the case, it is necessary to correct localized horizontal Understanding environmental or land-cover dynamics displacements created by perspective view distortions around represents an important challenge in the study of the global areas of local relief. This process, called orthorectification, environment, because many land-cover changes take place at combines knowledge of the elevation of each image point with fine scales of resolution, requiring Landsat-type imagery for the precise viewing geometry at that point to calculate a hori- accurate measurement. Uses for such data range from biodi- zontal correction to the satellite data. The result is an image versity and habitat mapping for localized areas, to specifying product that appears as if the satellite or the viewer is looking parameters for large-scale numerical models simulating bio- normal to the Earth at every location. In such orthogonal geochemical cycling, hydrological processes, and ecosystem views, the horizontal position of any feature directly beneath functioning. These needs have been recognized in the Interna- the viewer would not be effected by local terrain variations. tional Geosphere Biosphere Programme, the World Climate Correcting satellite imagery for variations in topography Research Programme, and the International Satellite Land Sur- and/or satellite viewing perspectives is best accomplished by face Climatologic Project, among others (Becker et al., 1988; using ground coordinates, also known as ground control IGBP, 1990; WMO, 1992). Responding to these needs, the Sci- points. This enables digital elevation data to be associated with entific Data Purchase Program of NASA’s Stennis Space Center the respective satellite data by matching coordinates. Accurate (http://www.esa.ssc.nasa.gov/datapurchase) has directed the association of the digital elevation data with the satellite image production of global orthorectified and co-registered Landsat data is necessary to compensate for topography and/or viewing Multispectral Scanner (MS), Thematic Mapper (TM), and En- perspective variations. When the satellite data have been cor- hanced Thematic Mapper (ETM+) data for three periods: the rected for terrain and/or satellite viewing perspectives, these late 1970s, circa 1990, and circa 2000, respectively. data are referred to as having been “orthorectified.” In this Landsat 80-, 30-, and 15-m satellite data are the only paper we use orthorectified to mean the satellite data have record of global land-surface conditions at a spatial scale of been corrected for terrain displacements, corrected for any C.J. Tucker and D.M. Grant are with the Laboratory for Photogrammetric Engineering & Remote Sensing Terrestrial Physics, NASA/Goddard Space Flight Center, Vol. 70, No. 3, March 2004, pp. 313–322. Greenbelt, MD 20771 ([email protected]). 0099-1112/04/7003–0313/$3.00/0 J.D. Dykstra is with Earth Satellite Corporation, 6011 © 2004 American Society for Photogrammetry Executive Blvd., Suite 40, Rockville, MD 20852. and Remote Sensing PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING March 2004 313 02-112.qxd 2/4/04 5:10 PM Page 314 satellite viewing variations, and have accurate geodetic coordi- were acquired over the circa 1990 epoch data, to cover most nates associated with the data. islands and parts of Antarctica, in addition to covering the The benefits of common orthorectification of global Land- same areas as the circa 1990 epoch (Plate 1c). sat data for three epochs are a very high geodetic mapping ac- curacy for all epochs, minimizing between- and among-scene Data Selection Criteria registration errors within each epoch, and minimizing The absence of catalogues of Landsat 1, 2, 3, 4, and 5 data between- and among-date registration errors. The latter two from all ground receiving stations made data availability de- considerations minimize classification errors due to misregis- termination a major task, because many receiving stations are tration; the former consideration provides the basis for a the best, and frequently the only, source of Landsat data from highly accurate geodetic global mapping standard. The data their reception regions. Lists of all possible Landsat scenes set we describe achieves not only highly accurate within- and available from the Earth Resources Observation Systems among-scene common orthorectification globally, but has also (EROS) Data Center (EDC) and foreign ground receiving stations been “tied” to ground control points to result in a high ab- were compiled and reviewed. Thirteen receiving stations, in- solute geodetic accuracy. These data represent a potentially cluding EDC, were utilized for the circa 1990 data (Figure 1 valuable data source for the study of the terrestrial environ- and Plate 2). MSS data for the 1970s epoch were only avail- ment which has heretofore not been available. able from the Canadian, European, and American archives, thus limiting the total number and quality of late 1970s im- Orthorectification Basis ages available for orthorectification. Initially, the Landsat data set we describe herein included as- For each available scene from all epochs, an image ana- sembling and orthorectifying Landsat MSS data from the 1970s lyst examined the satellite data for sensor problems, missing and TM data for circa 1990. In the third year (2000) of the pro- scan lines, clouds and haze, and any other manifestation of ject, NASA decided to add circa 2000 data from the ETM+ in- poor data quality. These image characteristics were weighed strument. We refer to these three periods hence forth as against the year and the season the data were acquired. Prefer- “epochs.” A consequence of the evolution of the project is ence for scene selection was based on the following: that Landsats 4 and 5 data became the orthorectification basis • Year of Acquisition. (1) MSS scenes were acquired largely for for the three epochs. In retrospect, it would have