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Accessings BQA values

The Landsat Collection 1 Level-1 Quality Assessment (QA) 16-bit 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 following conditions are available:

  • Cloud (cloud presence / cloud mask)
  • Designated fill
  • Terrain occlusion
  • Radiometric saturation
  • Cloud confidence (confidence in cloud presence)
  • Cloud-shadow confidence (confidence in cloud shadow presence)
  • Snow-ice confidence (confidence in snow or ice presence)
  • Cirrus confidence (confidence in cirrus cloud presence)

Each of these values can be obtained using our utility function named decodeLs8Qa. The function returns a dictionary of all conditions, where each condition contains an extracted value. Here is a working example script for extracting cloud values of the BQA band, with the other options commented below:

return [decodeLs8Qa(BQA).cloud]

//return [decodeLs8Qa(BQA).designatedFill]
//return [decodeLs8Qa(BQA).occlusion]
//return [decodeLs8Qa(BQA).radiometricSaturation * 0.33]
//return [decodeLs8Qa(BQA).cloudConfidence * 0.33]
//return [decodeLs8Qa(BQA).cloudShadowConfidence * 0.33]
//return [decodeLs8Qa(BQA).snowIceConfidence * 0.33]
//return [decodeLs8Qa(BQA).cirrusConfidence * 0.33]

Visit EO Browser to see the script for:

Interpreting BQA values

Example interpretation: when using evalscript for Cloud, value 0 means clouds are not present, and value 1 means clouds are present. In EO Browser, we can observe clouds in white, and all other pixels in black. Below are value interpretations for each of the available conditions.

Note: in the following tables, the color column represents a color legend for the EO Browser visualization, using the scripts listed above. The values could be visualized with different colors, if a different visualization was used.


Designated fill, Terrain occlusion & Cloud


ValueDescriptionColor

0

This condition does not exist

1

This condition exists

BQA Cloud over Italy, 2022-01-26


Radiometric saturation


ValueDescriptionColor

0

No bands contain saturation

1

1-2 bands contain saturation

2

3-4 bands contain saturation

3

5 or more bands contain saturation

BQA Radiometric saturation over Italy, 2022-01-26


Cloud confidence, Cloud shadow confidence, Snow ice confidence & Cirrus confidence


ValueDescriptionColor

0

Not Determined (algorithm did not determine the status of this condition) / No (this condition does not exist)

1

Low - algorithm has low to no confidence that this condition exists (0-33 percent confidence)

2

Medium - algorithm has medium confidence that this condition exists (34-66 percent confidence)

3

High - algorithm has high confidence that this condition exists (67-100 percent confidence)

BQA Cloud confidence over Italy, 2022-01-26