Landsat Collection 1 Level-1 Quality Assessment (QA) provides useful information to users wanting to optimize the value of pixels as a first level indicator of certain conditions within Landsat Level-1 data. An 8-bit LandsatLook Quality Image and a 16-bit Level-1 Quality Assessment Band are available for all Landsat Collection 1 Level-1 products.
The Landsat Collection 1 LandsatLook 8-bit Quality Image provides a description of the quality of the pixels within a scene that may help the user determine its suitability for application. LandsatLook Collection 1 Quality images (.jpg) can be downloaded individually from the download options window for the results of each Landsat Collection 1 Level-1 dataset in EarthExplorer. The image below displays the designations and associated colors for LandsatLook 8-bit Quality Images.
The Landsat Collection 1 Level-1 Quality Assessment (QA) 16-bit band allows users to apply per pixel filters to all Landsat Collection 1 Level-1 data products. Each pixel in the QA band contains unsigned integers that represent bit-packed combinations of surface, atmospheric, and sensor conditions that can affect the overall usefulness of a given pixel. Landsat Collection 1 Level-1 QA bands (.TIF) are included in the Landsat Level-1 GeoTIFF Data Product downloaded from EarthExplorer.* The Collection 1 Level-1 QA band can be stretched to emphasize the light and dark pixels for an immediate survey of a scene's general quality conditions.
*Landsat 8 OLI-only products contain all QA bits; Landsat 8 TIRS-only products contain only fill information.
Although this enables a quick “qualitative” assessment of a scene, users are encouraged to use the common pixel values (table below) to interpret the bit values, or use the Landsat QA Tools to extract specific QA bits that may impact quantitative image analyses for their particular studies, or to classify, visualize, and interpret Level-1 QA bit values.
What are QA Bits?
The bit-packed information in the QA bands is a translation of binary strings. For example, the integer value “1” translates to the binary value “0001.” The binary value “0001” has 4 bits, written right to left as bits 0 (“1”), 1 (“0”), 2 (“0”), and 3 (“0”). Each of the bits 0-3 represents a condition that can affect the calculation of a physical value. Bit 0 is used to identify fill values, Bit 1 is used to identify a dropped pixel (Landsat 1-5 MSS, Landsat 4-5 TM, Landsat 7 ETM+) or terrain occlusion (Landsat 8 OLI-only, Landsat 8 OLI/TIRS), Bits 2 and 3 together are used to indicate the level of radiometric saturation present. If the condition is true, the bit is set to “1,” or “0” if false.
Used effectively, QA bits help improve the integrity of Landsat-derived science investigations by indicating which pixels might be affected by surface conditions, cloud contamination, or sensor conditions. For example, NDVI calculated over pixels containing clouds will show anomalous values. If such pixels were included in a phenology study, the results might not show the true surface characteristics of seasonal vegetation growth. Cloud contaminated pixels will lower NDVI values, and measures like the timing of ‘green up’ or peak maturity would appear later than they actually occurred.
The pixel values in the Level-1 QA band must be translated into 16-bit binary form to be used effectively. The image below shows the bits currently populated in the Landsat 8 OLI and OLI/TIRS, Landsat 1-5 MSS, Landsat 7 ETM+, and Landsat 4-5 TM Collection 1 Level-1 QA bands. The images below display the Landsat 1-8 Collection 1 Level-1 Quality Bit Designations.
For the single bits (0, 1, and 4):
- 0 = "No" = This condition does not exist
- 1 = "Yes" = This condition exists
For radiometric saturation bits (2-3), read from left to right, represent how many bands contain saturation:
- 00 - No bands contain saturation
- 01 - 1-2 bands contain saturation
- 10 - 3-4 bands contain saturation
- 11 - 5 or more bands contain saturation
For the remaining double bits (5-6, 7-8, 9-10, 11-12), read from left to right, represent levels of confidence that a condition exists:
- 00 = “Not Determined” = Algorithm did not determine the status of this condition / "No" = This condition does not exist
- 01 = “Low” = Algorithm has low to no confidence that this condition exists (0-33 percent confidence)
- 10 = “Medium” = Algorithm has medium confidence that this condition exists (34-66 percent confidence)
- 11 = “High” = Algorithm has high confidence that this condition exists (67-100 percent confidence
Landsat 8 OLI/ OLI-TIRS Level-1 Possible Attributes, Pixel Values, and Pixel Value Interpretations
The tables below display the attributes and pixel values,and examples from the full list of possible QA values and interpretations for Landsat 8 OLI and OLI/TIRS Level-1 data.
Landsat 4-5 TM, Landsat 7 ETM+ Level-1 Possible Attributes, Pixel Values, and Pixel Value Interpretations
Landsat 1-5 MSS Collection 1 Level-1 Possible Attributes,Pixel Values, and Pixel Value Interpretations