Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.
The i.landsat.qa module generates reclassification rule files which can be used in r.reclass for filtering the QA band according to pixel quality characteristics the user defines as unacceptable. It works with both Collection 1 and Collection 2 data from Landsat 8 OLI/TIRS, 4-7 TM/ETM+. The dataset the QA band belongs to is specified like in i.landsat.download.
Values defined as unacceptable for a given condition will be set to NULL in the output raster map. All other values will be set to 1.
The Quality Assessment (QA) band from Landsat contains 16bit integer
values that represent "bit-packed combinations of surface, atmosphere, and
sensor conditions that can affect the overall usefulness of a given pixel".
The following quality relevant conditions are represented as "single bits" in the Landsat QA band:
Condition | Collection |
Designated Fill | 1,2 |
Dilated Cloud | 2 |
Terrain Occlusion / Dropped Pixels | 1 |
Cloud | 1,2 |
Cirrus | 2 |
Cloud Shadow | 2 |
Snow | 2 |
Clear | 2 |
Water | 2 |
Possible choices for the "single bits" are:
Value | Description | Bit representation |
No | This condition does not exist | 0 |
Yes | This condition exists | 1 |
The following quality relevant conditions are represented as "double bits" in the Landsat QA band:
Condition | Collection |
Radiometric saturation | 2 |
Cloud Confidence | 1,2 |
Cloud Shadow Confidence | 1,2 |
Snow/Ice Confidence | 1,2 |
Cirrus Confidence | 1,2 |
Possible choices for the "double bits" are:
Value | Description | Bit representation |
Not Determined | Algorithm did not determine the status of this condition | 00 |
Low | Algorithm has low to no confidence that this condition exists (0-33 percent confidence) | 01 |
Medium | Algorithm has medium confidence that this condition exists (34-66 percent confidence) | 10 |
High | Algorithm has high confidence that this condition exists (67-100 percent confidence) | 11 |
"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."
# Create a cloud mask: i.landsat.qa --overwrite --verbose cloud_confidence="Medium,High" \ dataset="landsat_ot_c2_l2" \ output=./Cloud_Mask_rules.txt r.reclass input=LC81980182015183LGN00_BQA \ output=LC81980182015183LGN00_Cloud_Mask rules=./Cloud_Mask_rules.txt # Create a water mask (only available for collection 2): i.landsat.qa --overwrite --verbose water="Yes" \ dataset="landsat_ot_c2_l2" \ output=./Water_Mask_rules.txt r.reclass input=LC81980182015183LGN00_BQA \ output=LC81980182015183LGN00_Water_Mask rules=./Water_Mask_rules.txt
Available at: i.landsat.qa source code (history)
Latest change: Monday Jan 30 19:52:26 2023 in commit: cac8d9d848299297977d1315b7e90cc3f7698730
Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.
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