The USGS has begun to reprocess all of the Landsat images in their archive. These images will be released as Collection 1 data and be designated as either Tier 1 or Tier 2. All Collection 1 data will share common radiometric and geometric parameters. In the future, if these need to be adjusted, then all archived images will be reprocessed and a new Collection will be released.
The USGS provides on-demand processing of Landsat 4, 5, and 7 images to create Climate Data Records (CDR)s and Essential Climate Variables (ECV)s. The CDRs are Landsat scenes that have been atmospherically corrected and converted to Surface Reflectance. ECVs are spectral indices derived from the CDRs and include vegetation, moisture and burn ratio indices.
The Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) sensors acquire Thermal InfraRed (TIR) data and store this information as a digital number (DN) with a range between 0 and 255. The Landsat 8 OLI sensor stores these data as DNS with a range from 0 to 65536. ENVI Standard can easily convert these DNs to degrees Kelvin.
The Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) sensors capture reflected solar energy, convert these data to radiance, then rescale this data into an 8-bit digital number (DN) with a range between 0 and 255. It is possible to manually convert these DNs to ToA Reflectance using a two-step process. The first step is to convert the DNs to radiance values using the bias and gain values specific to the individual scene you are working with. The second step converts the radiance data to ToA reflectance.
Albedo is an important property of the Earth surface heat budget. A simple definition of albedo (a) is the average reflectance of the sun’s spectrum. This unitless quantity has values ranging from 0 to 1.0 and will vary based on the land cover. For example snow would have a high value and coniferous forests a low value.
Landsat data provided by the USGS are distributed as a single file in an archived and zipped “.TAR.GZ” format. These files must be extracted and uncompressed before you can use them.
There are many sites that you can use to locate and obtain Landsat satellite imagery. Two recommended sites are EarthExplorer and GLOVIS by the USGS. You will find a broad collection of Landsat data spanning the entire time of the program, beginning in the early 1970’s. The user interface and download processes are a bit different for each site. More information about each is listed below.
On 31 May 2003 the Landsat 7 Enhanced Thematic Mapper (ETM) sensor had a failure of the Scan Line Corrector (SLC). Since that time all Landsat ETM images have had wedge-shaped gaps on both sides of each scene, resulting in approximately 22% data loss. These images are available for free download from the USGS GloVis website and are found in the L7 SLC-off collection.