Creating the World's First Global Land Cover Database

This installment of "Imaging by ERDAS" features an update on the world's first medium-resolution global land cover database, still under construction by Earth Satellite Corporation (EarthSat). When completed in mid-2003, this data set will serve scientists and others involved in land cover change detection studies.

Have you ever wondered how scientists know how fast rain forest is being clear-cut or how far urban sprawl has spread into rural areas? Changes such as these are determined through "change detection" studies, measuring "before" and "after" amounts (often from aerial photographs or satellite imagery) to determine how a feature, such as a shoreline or a city, has been changed over time.

The New Look

By mid-2003, independent scientific researchers, government agencies, and commercial entities will have an easier way to compare land cover changes using their GIS software to view images from GeoCover-LC 1.0, a digital database of land cover imagery and vectors that will include the major landmasses of the entire world.

GeoCover-LC will be the first worldwide land cover database prepared using moderate resolution Landsat Thematic Mapper (TM) imagery. A focus on utility is provided through the availability of data in raster, ARC GRID, and vector formats. Images and vectors from this data set will be available on a scene-by-scene basis, on a regional basis, or even on the basis of latitude and longitude or other map corner coordinates. They will be fully compatible with Esri software, requiring no further massaging or rectification prior to use.

GeoCover-LC is being developed as an EarthSat proprietary product. A total of over 7,600 TM scenes will be interpreted over the life of the program. The National Imagery and Mapping Agency (NIMA) has purchased an unlimited Government Use license for the first 1,200 TM scenes, covering the Middle East and eastern and northern Africa. While the original impetus of the GeoCover-LC project was to respond to the need of the scientific community for standardized land cover data both to understand the current environmental conditions as well as to monitor change, these needs coincided with NIMA's directive to create land cover data to support geospatial applications for the military.

GeoCover-LC is the logical follow-on to another program under way at EarthSat for NASA, called GeoCover Ortho. In this program, archived TM data from circa 1990 and Multi-spectral Scanner (MSS) data from circa 1977 are being acquired and orthorectified (matched to a terrain model and projected into a map coordinate system) using EarthSat proprietary software. GeoCover Ortho produces individual satellite images in orthorectified format (TM and MSS) as well as mosaics covering entire regions of the globe in one image. GeoCover Ortho data will be archived both at EarthSat and at NASA, and eventually at the USGS EROS Data Center (EDC) at Sioux Falls, South Dakota, for resale to scientists and policy makers interested in global conditions over time.

Creating the New World

To create GeoCover-LC, the orthorectified TM scenes created for GeoCover Ortho are imported into ERDAS IMAGINE geographic imaging software for categorization into 13 informational categories. EarthSat's analysts use ERDAS IMAGINE to categorize each scene independently. Completed scenes are then edgematched to reduce the impact of differing seasons and years in the data set. The categorized scenes are mosaicked together by the Universal Transverse Mercator (UTM) zone and are imported into ArcInfo.

An ArcInfo application custom written by EarthSat vectorizes the mosaic into one- by one-degree (latitude/longitude) cells. The functionality of ARC GRID and the GRIDPOLY command permits the transfer of raster attributes to vector topology. Another application divides the 13 classes in these files into subcoverages (hydrology, urban features, and vegetation). This data set will serve NIMA and other U.S. government agencies as a baseline for monitoring global change. Against this baseline, analysts of the future may conduct change detection analyses using similar or dissimilar satellite platforms in order to track global land cover change. The recent successful launch of Landsat 7 in April 1999 (see sidebar "Land Cover Change Detection Studies Get Huge Boost from Space") has ensured that this vital tool for tracking global change will have validity well into the future.

NIMA has purchased an unlimited use license to GeoCover-LC, allowing distribution of the data to any U.S. government agency. EarthSat retains the license rights to GeoCover-LC for all other uses and will sell the product to private and nongovernmental organization (NGO) users from its archive.

Ready to Use by 2003

The project team started building GeoCover-LC 1.0 with orthorectified imagery of the Middle East, northern Africa, and eastern Africa in July 1999. These files are nearing completion and will be available starting in mid-2000. The data set for the entire world should be available by 2003. The minimum mapping unit (MMU) applied to GeoCover LC data is 1.4 hectares for all categories except water, with water displayed at full 28.5 x 28.5-meter resolution.

EarthSat's plan for GeoCover-LC 1.0 allows for users of differing experience levels to purchase data in several stages including a rough categorization ("preclumped") that will permit high-end users to group and edit the spectral data according to their own classification legends, as final raster ERDAS IMAGINE categorized files, and as ARC GRID or vector coverages. Users may purchase licenses to final categorized files or merely to interim "clustered" data sets (grouped into areas of similar spectral reflectance but not assigned an informational meaning) if they wish to recategorize the land cover according to a different legend. For interested users who do not have an image processing capability, EarthSat can recompile or reclassify GeoCover-LC data to a larger MMU or a different land cover legend under a separate contract.

Looking into the Future

The purpose of GeoCover-LC 1.0 is to provide a baseline of land cover derived from spectral analysis. This data will show the earth as it appeared in 1990. The existence of two new data sets, GeoCover Ortho and GeoCover-LC 1.0, will ensure that land cover updates may be generated quickly and efficiently, and they will provide decision makers with hard data about global change trends in the 1990s and beyond.

In the future, imagery from Landsat 7 may be used to build GeoCover-LC 2.0. With both GeoCovers, scientists, researchers, and others will have access to two complete sets of data, one created from 1990 satellite data and the other during the early 2000s. These two data sets—captured in decades when so many changes, from the felling of Brazil's rain forests to urban sprawl in the western United States, were at their zenith—will be a monumental tool for land cover change detection studies in the coming century.

EarthSat's Program Manager Thom Jones sees a virtually unlimited potential for the use of GeoCover-LC data for global change applications. "This program is the equivalent of a snapshot of the earth's land cover in time. We have utilized the strengths of computers and people to identify patterns in the imagery—spectral patterns by computer and spatial patterns by the interpreters. The inclusion of interim categorized data in the available product list means that other scientists with different analytical criteria can still benefit from what we've done. GeoCover-LC will allow scientists and policy makers to work more efficiently when considering global change issues."

For more information, contact Roger A. Mitchell, vice president, Business Development (tel.: 301-231-0660, e-mail: rmitchell@earthsat.com).

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