The Landsat Collection 1 Level-1 Quality Assessment (QA) band allows users to apply per pixel filters to the Landsat 4-5 Thematic Mapper (TM), Landsat 7 Enhanced Thematic Mapper Plus (ETM+), and Landsat 8 Operational Land Imager (OLI)-only and Landsat 8 OLI/Thermal Infrared Sensor (OLI/TIRS)- combined Collection 1 Level-1 data products. Note: The Landsat 8 TIRS-only QA band only contains fill information.
The Pre-Collection Landsat 8 QA band page provides information on Pre-Collection Landsat 8 data.
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.
The tables below display the Landsat 4-5 TM, Landsat 7 ETM+, Landsat 8 OLI-only, and Landsat 8 OLI/TIRS combined Collection 1 Level-1 Quality Band attributes.
What are QA Bits?
The bit-packed information in the QA bands is a translation of binary strings. As a simple 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 may be used to identify fill values, Bit 1 may be used to identify a dropped pixel (Landsat 4-5 TM, 7 ETM+) or terrain occlusion (Landsat 8 OLI, OLI/TIRS), Bits 2 and 3 together may be used to indicate the level of radiometric saturation present. If the condition is true, the bit is set to “1,” and “0” if false.
Used effectively, QA bits improve the integrity of science investigations by indicating which pixels might be affected by instrument artifacts or subject to cloud contamination. 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 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. A worse consequence would be that the reported reduction of vegetation growth would be taken as an indicator of environmental change, potentially prompting unnecessary land management policies or practices.
The Landsat QA Tools can be used to extract bits from Landsat QA bands.
The Landsat Collection 1 Level-1 QA band is included in the “Level-1 GeoTIFF Data Product” download option for each result on EarthExplorer.
Rigorous science applications seeking to optimize the value of pixels used in a study will find QA bits useful as a first level indicator of certain conditions.
The Collection 1 Level-1 QA image can be stretched to emphasize the light and dark pixels for a quick look at general quality conditions. In the image below, the lighter pixels are likely to be affected by a quality condition, in this case, clouds and cloud shadows. The dark areas indicate clear conditions. Although this enables a quick “qualitative” assessment, users are encouraged to use the common pixel values (table below) to interpret the values, or use the Landsat QA Tools to extract specific QA bits that may impact quantitative image analyses for their particular studies.
Landsat Collection 1 Level-1 Quality Assessment Band (BQA.TIF) displayed as .jpg for reference only. Landsat 5 Thematic Mapper (TM) Path 39 Row 37, acquired April 26, 2012.
The pixel values in the QA file must be translated to 16-bit binary form to be used effectively.
The tables below show the bits currently populated in the Landsat 4-5 TM, Landsat 7 ETM+, and Landsat 8 OLI and OLI/TIRS Collection 1 Level-1 QA bands.
For the single bits (0, 1, and 4):
1 = Yes, this condition exists
For radiometric saturation bits (2-3), read from left to right, represent how many 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:
11 = “Yes” = Algorithm has high confidence that this condition exists (67-100 percent confidence
Some common pixel values and their meanings are included in the table below.
For assistance with unpacking the Quality Assessment band, check out the Landsat QA Tools
The Collection 1 LandsatLook 8-bit Quality Image (.jpg) provides a quick view of the quality of the pixels within the scene to determine if a particular scene would work best for the user's application. LandsatLook Collection 1 Quality images can be downloaded individually from the Download Options for each Landsat Level-1 result using EarthExplorer (EE).
The tables below display the values and colors associated with the Collection 1 LandsatLook 8-bit Quality Images.