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Landsat Missions

Landsat 8 Quality Assessment Band

The Quality Assessment (QA) band is an important addition to Landsat 8 data files. Each pixel in the QA band contains a decimal value that represents bit-packed combinations of surface, atmosphere, and sensor conditions that can affect the overall usefulness of a given pixel.

What are QA Bits?

The bit-packed information in the QA bands is decimal translation of binary strings. As a simple example, the decimal 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.

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.

A no-cost tool is available for user download that will extract the bit-packed information in the OLI QA band for easy interpretation. Details are provided on http://landsat.usgs.gov/L-LDOPE_Toolbelt.php.

Level 1 QA Band File (BQA.TIF)

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. Otherwise, users are advised that this file contains information that can be easily misinterpreted and it is not recommended for general use.

Landsat 8 Level 1 data products include a 16-bit QA file (.TIF). Robust image processing software capable of handling 16-bit data is necessary to compute statistics of the number of pixels containing each of the designated bits.

The pixel values in the QA file must be translated to 16-bit binary form to be used effectively. The gray shaded areas in the table below show the bits that are currently being populated in the Level 1 QA Band, and the conditions each describe. None of the currently populated bits are expected to exceed 80% accuracy in their reported assessment at this time.

16-bit Landsat 8 QA Band

For the single bits (0, 1, 2, and 3):

The double bits (4-5, 6-7, 8-9, 10-11, 12-13, and 14-15) represent levels of confidence that a condition exists:

For example, a pixel with a value "58384" translates to the 16-bit binary string "1110 0100 0001 0000." Reading the binary string from right to left and using the table above as an interpretation legend, this pixel is:

Certain decimal values occur regularly and can be interpreted without unpacking them into 16-bit strings and using the table above as a reference. Some common pixel values and their meanings are included in the table below.

Pixel Value
(Decimal)

Cloud

Cirrus

Snow/Ice

Veg

Water

Terrain
Occlusion

Dropped
Frame

Fill

61440

Yes

Yes

Not
Determined

Not Determined

Not
Determined

No

No

No

59424

Yes

Maybe

Maybe

Not
Determined

Maybe

No

No

No

57344

Yes

Maybe

Not
Determined

Not
Determined

Not
Determined

No

No

No

56320

Yes

No

Yes

Not
Determined

Not
Determined

No

No

No

53248

Yes

No

Not
Determined

Not Determined

Not
Determined

No

No

No

52256

Yes

Not
Determined

Yes

Not
Determined

Maybe

No

No

No

52224

Yes

Not
Determined

Yes

Not
Determined

Not
Determined

No

No

No

49184

Yes

Not
Determined

Not
Determined

Not
Determined

Maybe

No

No

No

49152

Yes

Not Determined

Not
Determined

Not
Determined

Not
Determined

No

No

No

48128

Maybe

Yes

Yes

Not
Determined

Not
Determined

No

No

No

45056

Maybe

Yes

Not
Determined

Not
Determined

Not
Determined

No

No

No

43040

Maybe

Maybe

Maybe

Not
Determined

Maybe

No

No

No

39936

Maybe

No

Yes

Not
Determined

Not
Determined

No

No

No

36896

Maybe

No

Not
Determined

Not
Determined

Maybe

No

No

No

36864

Maybe

No

Not
Determined

Not
Determined

Not
Determined

No

No

No

32768

Maybe

Not
Determined

Not
Determined

Not
Determined

Not Determined

No

No

No

31744

No

Yes

Yes

Not
Determined

Not
Determined

No

No

No

28672

No

Yes

Not
Determined

Not
Determined

Not
Determined

No

No

No

28590

No

Maybe

Yes

Yes

Maybe

Yes

Yes

No

26656

No

Maybe

Maybe

Not
Determined

Maybe

No

No

No

24576

No

Maybe

Not
Determined

Not
Determined

Not
Determined

No

No

No

23552

No

No

Yes

Not
Determined

Not
Determined

No

No

No

20516

No

No

Not
Determined

Not
Determined

Maybe

Yes

No

No

20512

No

No

Not
Determined

Not
Determined

Maybe

No

No

No

20480

No

No

Not
Determined

Not
Determined

Not
Determined

No

No

No

19456

No

Not
Determined

Yes

Not
Determined

Not
Determined

No

No

No

16416

No

Not
Determined

Not
Determined

Not
Determined

Maybe

No

No

No

16384

No

Not
Determined

Not
Determined

Not
Determined

Not
Determined

No

No

No

16380

Not
Determined

Yes

Yes

Yes

Yes

Yes

No

No

13246

Not
Determined

Yes

Not
Determined

Yes

Yes

Yes

Yes

No

6176

Not
Determined

No

Maybe

Not
Determined

Maybe

No

No

No

6148

Not
Determined

No

Maybe

Not
Determined

Not
Determined

Yes

No

No

2592

Not Determined

Not
Determined

Maybe

Maybe

Maybe

No

No

No

2592

Not
Determined

Not
Determined

Maybe

Maybe

Maybe

No

No

No

2308

Not
Determined

Not
Determined

Maybe

No

Not
Determined

Yes

No

No

2144

Not
Determined

Not
Determined

Maybe

Not
Determined

Maybe

No

No

No

2112

Not
Determined

Not
Determined

Maybe

Not
Determined

Not
Determined

No

No

No

2080

Not
Determined

Not
Determined

Maybe

Not
Determined

Maybe

No

No

No

2052

Not
Determined

Not
Determined

Maybe

Not
Determined

Not
Determined

Yes

No

No

2048

Not Determined

Not
Determined

Maybe

Not
Determined

Not
Determined

No

No

No

515

Not
Determined

Not
Determined

Not
Determined

Maybe

Not
Determined

No

Yes

Yes

64

Not
Determined

Not
Determined

Not
Determined

Not
Determined

Not
Determined

No

No

No

32

Not Determined

Not
Determined

Not
Determined

Not
Determined

Maybe

No

No

No

32

Not
Determined

Not
Determined

Not
Determined

Not
Determined

Maybe

No

No

No

4

Not
Determined

Not
Determined

Not
Determined

Not
Determined

Not
Determined

Yes

No

No

0

Not
Determined

Not
Determined

Not
Determined

Not
Determined

Not
Determined

No

No

No

The QA image can be stretched to emphasize the light ("1"s) and dark ("0") pixels for a quick look at general quality conditions. In the Crater Lake, Oregon image below, the lighter pixels are likely to be affected by a quality condition, in this case snow or clouds.

Quality Band (BQA.TIF) displayed as .jpg for reference only

Quality Band (BQA.TIF) displayed as .jpg for reference only
LDCM sample data Path 45 Row 30 Acquired April 23, 2013.

LandsatLook Quality Image (.png)

The 8-bit LandsatLook Quality Image (.png) is available to download when downloading Landsat 8 data products. This file provides a quick view of the quality of the pixels to determine which scene would work best for each user's application. Only the highest confidence conditions are used to create the LandsatLook Quality image. Similar as stated above, this image may not be useful to all users. (Information on LandsatLook Images = http://landsat.usgs.gov/LandsatLookImages.php)

The table below gives the bits and colors associated with the LandsatLook Quality Image:

8-Bit LandsatLook QA Band
Landsat Look

Landsat Look "Quality" Image (QA.png) displayed as .jpg for reference only
LDCM sample data Path 45 Row 30 Acquired April 23, 2013.





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