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What is the Landsat 8 Pre-Collection Quality Assessment (QA) band and how is it used?

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Question:

What is the Landsat 8 Pre-Collection Quality Assessment (QA) band and how is it used?

Answer:

The Landsat 8 Pre-Collection Quality Assessment (QA) band is a created file that contains values that represent bit-packed combinations of surface, atmosphere, and sensor conditions that can affect the overall usefulness of a given pixel within a scene.

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.

What are QA Bits?

The bit-packed information in the QA bands is 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 cloud, bit 2 may be used to indicate water, and bit 3 may be used identify snow. If the condition is true, the bit is set to “1,” and “0” if false.

In the example, “0001” means the pixel contains a fill value, and should likely be ignored.

  • Bit 0 = 1 = fill
  • Bit 1 = 0 = no cloud
  • Bit 2 = 0 = land
  • Bit 3 = 0 = no snow

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 8 Pre-Collection Quality Assessment (QA) band page provides details about the QA Band files and associated bits.

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