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Chapter 13: Data Quality Bitmasks

This chapter provides a workow to explore quality ag bitmasks for imagery in Rocky Mountain National Park, Colorado, United States. The full

GEE code can be found here.

Code for functions was adapted from this Geographic Information Systems Stack Exchange post and this Open Geo Blog post.

Functions

Data Acquisition & Preprocessing

Landsat 8 T1 SR ‘pixel_qa’ band bitmasks:

Bit 0: Fill

Bit 1: Clear

Bit 2: Water

Bit 3: Cloud Shadow

Bit 4: Snow

Bit 5: Cloud

Bits 6-7: Cloud Condence

0: None

1: Low

2: Medium

3: High

Bits 8-9: Cirrus Condence

0: None

1: Low

2: Medium

3: High

Bit 10: Terrain Occlusion

Bitmask value of 1 for single bits means ag is set (i.e. water == 1 –> pixel is water). Bitmask value for multiple bits indicates condition (cirrus

condence of 3).

/**

* Extracts QA values from an image

* @param {ee.Image} qa_band - Single-band image of the QA layer

* @param {Integer} start_bit - Starting bit

* @param {Integer} end_bit - Ending bit

* @param {String} band_name - param_description

* @return {ee.Image} - Image with extracted QA values

*/

function extract_qa_bits(qa_band, start_bit, end_bit, band_name) {

// Initialize QA bit string/pattern to check QA band against

var qa_bits = 0;

// Add each specified QA bit flag value/string/pattern to the QA bits to check/extract

for (var bit = start_bit; bit <= end_bit; bit++) {

qa_bits += Math.pow(2, bit); // Same as qa_bits += (1 << bit);

}

// Return a single band image of the extracted QA bit values

return qa_band

// Rename output band to specified name

.select([0], [band_name])

// Check QA band against specified QA bits to see what QA flag values are set

.bitwiseAnd(qa_bits)

// Get value that matches bitmask documentation

// (0 or 1 for single bit, 0-3 or 0-N for multiple bits)

.rightShift(start_bit);

}

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2024/11/4 10:17

Chapter 13: Data Quality Bitmasks — Remote Sensing with Google Earth Engine

https://calekochenour.github.io/remote-sensing-textbook/03-beginner/chapter13-data-quality-bitmasks.html

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