Users may encounter artifacts in the U.S. Landsat Analysis Ready Data (ARD); already known issues are listed below. Additional artifacts will be added to this page as product analysis continues.
Quality Band Discontinuity
Discontinuities have been discovered in the U.S. Landsat ARD PIXELQA and SRAEROSOLQA optical thickness bands. The diagonal line in the image below indicates a discontinuity along the north/south boundary of the Worldwide Reference System 2 (WRS-2) Landsat scenes used in this example tile.
Figure 1. Example of the Quality Assessment Band (Aerosol Bits) Discontinuity Issue in a U.S. Landsat Analysis Ready Tile (h05v02_20131203)
Cloud detection and aerosol retrieval are performed at the Landsat scene level, before ARD tile processing begins. The discontinuities in PIXELQA are likely due to temperature differences between the northern and southern Landsat scenes, resulting in the cloud confidence to be identified differently.
A possible resolution of the quality assessment band discontinuity issue will be investigated and considered for a future version of U.S. Landsat ARD.
Surface Reflectance Cloud Quality Assessment Anomaly (Landsat 4-5 TM, Landsat 7 ETM+)
The false-positive identification of cloudy pixels has been discovered in the Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS) algorithm’s internal detection of clouds, which is output as the SRCLOUDQA band in Landsat 4-5 TM and Landsat 7 ETM+ Surface Reflectance data products. An algorithmic calculation is incorrectly identifying clear irrigated field and mountain shadows as clouds in high desert regions. Analysis has determined that compared to the surrounding ambient temperature, these areas meet the difference criteria used by LEDAPS in its cloud cover assessment, and are therefore incorrectly marked as clouds.
Below is an example of an SRCLOUDQA band (left window) compared to a surface reflectance natural color composite image (bands 3,2,1) (right window). The red rectangle indicates the same region in both windows. In the SRCLOUDQA band, LEDAPS is indicating clouds (dark grey polygons), adjacent cloud (white boundary lines), and cloud shadows (light grey polygons) are present in the scene. As evident in the surface reflectance natural color composite, no clouds are visible.
Figure 2. Example of a SRCLOUDQA Band (left window) Compared to a Surface Reflectance Natural Color Composite Image (bands 3,2,1) (right window)
The misidentified clouds in high desert regions in LEDAPS-based U.S. Landsat ARD products does not significantly impact their scientific integrity but a possible resolution of the issue will be investigated and considered for a future version of U.S. Landsat ARD.
Unused Landsat Scene Listed in U.S. Landsat ARD Tile XML Metadata File
The Lineage Quality band (LINEAGEQA) identifies which Landsat Level-2 Albers scene was the source for each pixel in a U.S. Landsat ARD tile by referencing scene identifiers listed in an associated XML metadata file. However, a small number of Landsat 8 and Landsat 7 ARD XML metadata files have been found to list an extra scene as source input when it is not in fact included when generating that particular ARD tile.
Below is an example of three scenes listed in the XML metadata file as input for ARD tile LC08_CU_024016_20130715_20171016_C01_V01 (outlined in red). It is noticeable that Scene 1 is beyond the boundary of ARD tile h024v016 and does not appear in the LINEAGEQA image, which correctly includes only Scenes 2 and 3 as input sources. The XML is in error stating Scene 1 was used in generating ARD tile h024v016.
Figure 3. US. Landsat ARD Tile LC08_CU_024016_20130715_20171016_C01_V01 outlined in red, displayed with scenes noted as input.
A possible resolution of the unused Landsat scene listed in Landsat ARD tile XML metadata file issue will be investigated and considered for a future version of U.S. Landsat ARD.
Incorrect Precision of WGS84 Axis Values
An error that defines map projection geotag values in Landsat Level-2 Albers scenes results in assigning inaccurate values to the semi-major and semi-minor axes, lengthening them by ~3-meters (m). This error is carried through only in the Landsat Science Product Albers production stream, but subsequently does appear in all U.S. Landsat ARD tiles currently available on EarthExplorer.
This geotagging issue does not result in any resampling or impact the validity of the ARD pixel values. The error manifests itself in a sub-2-m offset in each ARD tile. The offset values range geographically across the United States from about 0.75-m in Chesapeake Bay to about 1.5-m in Portland, Oregon.
Minimal impacts on data analysis have been observed, but there is a potential for issues that involve high-resolution sensors (i.e., LIDAR) or vector-based analyses. Any attempts to reproject other data to match an ARD tile using the definitions for WGS84 CONUS Albers will not be successful due to this issue. Any tool, application or software package that utilizes projection information and conducts cross-projection operations and analyses may also be affected by this issue.
A correction for the precision of WGS84 axis values will be included in a future version of U.S. Landsat ARD.
Cloud Shadow Placement Shift Between Projections
Cloud shadows are identified in Landsat Level-1 and Level-2 products by the C Function of Mask (CFMask) algorithm, and then projecting cloudy pixels onto where their shadows should fall on the earth's surface. In different geometric projections (e.g., Albers vs. UTM), cloud shadows will be projected into slightly different positions on the earth's surface. This has the effect of changing the estimate of cloud height, which can then cause a large shift in the position of the final cloud shadow identification. This happens most often over high-altitude, partially transparent clouds such as cirrus. Because of this behavior, the position of cloud shadows in UTM and Albers products may differ. The identification of cloudy pixels themselves is not affected.
Figure 4. False color image (top left) and pixel quality assessment (bottom left) panes for a Landsat scene (path 15/row 35) in the Albers Equal Area Conic projection are shown above. The top right and bottom right panes show the same products for the same scene in the Universal Transverse Mercator (UTM) Zone 18N projection. In the pixel QA images purple and brown represent clouds, beige represents cloud shadows, and cyan represents clear land. The yellow ellipsoid is an example of a region in the scene where the known cloud shadow placement shift between two projections appears.
This issue does not significantly impact the scientific integrity of Landsat products but a possible resolution of the issue will be considered for a future version of U.S. Landsat ARD.