Global Land Survey(GLS)
A primary goal of the GLS collection is to provide at least one cloud-free leaf-on image for every vegetated location of the global land area.
The collection is repeated for four epochs - 1975, 1990, 2000 and 2005 to enable researchers and users to study land cover change over time.
To facilitate this process, data from a variety of mostly Landsat sensors are assembled, centered around the four epochs.
It is important to note that imagery used for each epoch is the most appropriate scene available within a range of several years,
centered on the epoch date. Thus, a 1975 epoch image may have been acquired in 1980, but is the closest appropriate image available.
All collection epochs except 2000 include imagery from multiple satellites. The 1975 epoch includes images from the MSS sensors of
Landsat satellites 1-4. The 1990 epoch contains images from mostly the Landsat 5 TM, but also some ETM+ from Landsat 7. For the 2005 GLS collection,
data from a variety of sensors, principally Landsat-5 TM and Landsat-7 ETM+, but also EO-1 ALI are used whenever possible to obtain
near-complete global coverage. Where Landsat-5 or Landsat-7 imagery are not available, EO-1 ALI are used to augment the areal coverage.
The ALI sensor was designed to provide Landsat type panchromatic and multispectral bands to enable continuity of the Landsat series.
ALI coverage is exclusively over oceanic islands.
GLS data processing at GLCF:
GLS data are available for free to the public through the USGS and the GLCF. The GLCF ingests GLS data from USGS and distributes the same imagery
with a few further processing steps. New preview thumbnails and browse images of each GLS scene are generated using two different band composites.
The browse images for each and every GLS scene are checked thoroughly to assess dataset quality and problematic scenes are reported. For instance,
there may be issues with certain scenes having substantial cloud cover over land areas or scenes that may be affected by band striping.
Currently, all GLS collections are made available to the public although certain epochs are not yet complete. New GLS scenes, once added to the
GLS collections, will be distributed as and when available to the GLCF for ingestion and processing. Status maps and lists for each GLS epoch are
provided to show current status of GLS data availability. Updates to the collection will be provided on the Overview page when available.