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U.S. Geological Survey Fact Sheet 2018–3053, August 2018.
U.S. Landsat Analysis Ready Data (ARD) has transformed how Landsat imagery is processed. By taking most of the preprocessing off users' hands, it makes it easy to produce Landsat-based maps of land cover and land-cover change, and other geophysical and biophysical products.
What it is
ARD is made of Level-2 products derived from Landsat Collections Level-1 scenes, delivered as tiled, georegistered data — both top-of-atmosphere and atmospherically corrected — in a common equal-area projection, ready to use for monitoring change on the land.
It covers the conterminous United States, Alaska and Hawaii, from these Level-1 products:
- Landsat 8 Operational Land Imager (OLI) / Thermal Infrared Sensor (TIRS), Tier 1 and Tier 2;
- Landsat 7 Enhanced Thematic Mapper Plus (ETM+), Tier 1;
- Landsat 4–5 Thematic Mapper (TM), Tier 1.
Products
| Product | What it is |
|---|---|
| Top of Atmosphere reflectance (TA) | reflectance as measured at the sensor, not corrected for the atmosphere |
| Top of Atmosphere Brightness Temperature (BT) | radiance at the sensor, not atmospherically corrected, in kelvin |
| Surface Reflectance (SR) | atmospherically corrected |
| Surface Temperature (ST) | the temperature of the Earth's surface, in kelvin |
| Pixel Quality Assessment (QA) | common and sensor-specific quality bands |
Each comes with pixel-by-pixel quality information — recording sensor, sun, atmosphere and terrain conditions — and a trace back to the Level-1 scenes used. Products are generated in the Albers Equal-Area Conic projection on the WGS 84 datum, straight from Level-1 scenes through the Level-2 processing system.
The tile grid
ARD tiles follow a grid adapted from the Web Enabled Landsat Data (WELD) system. It resembles the National Land Cover Database grid, but WELD uses WGS 84 and NLCD uses NAD 83 — a difference that shifts the two grids by about half a pixel in each direction.
Projection parameters
| Parameter | Conterminous U.S. | Alaska | Hawaii |
|---|---|---|---|
| First standard parallel | 29.5° | 55.0° | 8.0° |
| Second standard parallel | 45.5° | 65.0° | 18.0° |
| Central meridian | −96.0° | −154.0° | −157.0° |
| Latitude of origin | 23.0° | 50.0° | 3.0° |
| False easting and northing | 0 m | 0 m | 0 m |
Output format is GeoTIFF.
- Each tile is 5,000 × 5,000 pixels of 30 metres, all acquired on one day — smaller than a traditional Landsat scene (Landsat 8 about 6,300 × 6,000 pixels; Landsat 7 about 6,166 × 5,666).
- So every pixel in a tile marks the same spot on Earth throughout the ARD record, from 1982 to the present.
- 422 tiles cover the conterminous United States, 110 Alaska and 8 the Hawaiian Islands.
ARD tile h05v02 over a Landsat 8 scene (path 44, row 27), for illustration. USGS.
Getting the data
Users can tailor requests by region, spectral band and date. Products come bundled as .tar files, which vary widely in size and can be much larger than traditional scene-based Level-1 products. ARD is free to download from EarthExplorer: https://earthexplorer.usgs.gov/
Questions go to Landsat User Services at the USGS Earth Resources Observation and Science (EROS) Center in Sioux Falls, South Dakota (custserv@usgs.gov).
Sources
- U.S. Geological Survey, 2018, U.S. Landsat Analysis Ready Data: U.S. Geological Survey Fact Sheet 2018–3053. https://doi.org/10.3133/fs20183053 · https://pubs.usgs.gov/publication/fs20183053
- The projection parameters and the tile figure come from the fact sheet's PDF. Its other figure is drawn over commercial background imagery and is not reproduced here.
- Rewritten in hubnx's own words.
Лицензия: CC0 1.0 (общественное достояние) · По материалам pubs.usgs.gov
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