Chapter 13: Data Quality Bitmasks¶

This chapter provides a workflow to explore quality flag 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¶

/**
 * 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);
}

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 Confidence
- 0: None
- 1: Low
- 2: Medium
- 3: High
Bits 8-9: Cirrus Confidence
- 0: None
- 1: Low
- 2: Medium
- 3: High
Bit 10: Terrain Occlusion

Bitmask value of 1 for single bits means flag is set (i.e. water == 1 –> pixel is water). Bitmask value for multiple bits indicates condition (cirrus confidence of 3).

// Define RMNP Boundary
var rmnp_boundary = ee.FeatureCollection("users/calekochenour/Rocky_Mountain_National_Park__Boundary_Polygon");

// Define Landsat 8 collection
var landsat8_t1_sr = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR');

// Get single image for QA evaluation
var rmnp = landsat8_t1_sr
  .filter(ee.Filter.calendarRange(2013, 2020, 'year'))
  .filter(ee.Filter.calendarRange(6, 9, 'month'))
  .filterBounds(rmnp_boundary)
  .first()
  .clip(rmnp_boundary);

print('Baseline Image', rmnp);

// Get the pixel QA band
var rmnp_qa = rmnp.select('pixel_qa');
print('QA Band:', rmnp_qa);

// EXTRACT QA BITS
// Fill bitmask (bit 0)
var fill_quality = extract_qa_bits(rmnp_qa, 0, 0, 'fill_bitmask');
print('Fill Bitmask (Bit 0):', fill_quality);

// Clear bitmask (bit 1)
var clear_quality = extract_qa_bits(rmnp_qa, 1, 1, 'clear_bitmask');
print('Clear Bitmask (Bit 1):', clear_quality);

// Water bitmask (bit 2)
var water_quality = extract_qa_bits(rmnp_qa, 2, 2, 'water_bitmask');
print('Water Bitmask (Bit 2):', water_quality);

// Cloud shadow bitmask (bit 3)
var cloud_shadow_quality = extract_qa_bits(rmnp_qa, 3, 3, 'cloud_shadow_bitmask');
print('Cloud Shadow Bitmask: (Bit 3)', cloud_shadow_quality);

// Snow bitmask (bit 4)
var snow_quality = extract_qa_bits(rmnp_qa, 4, 4, 'snow_bitmask');
print('Snow Bitmask (Bit 4):', snow_quality);

// Cloud bitmask (bit 5)
var cloud_quality = extract_qa_bits(rmnp_qa, 5, 5, 'cloud_bitmask');
print('Cloud Bitmask (Bit 5):', cloud_quality);

// Cloud confidence bitmask (bits 6-7)
var cloud_confidence_quality = extract_qa_bits(rmnp_qa, 6, 7, 'cloud_confidence_bitmask');
print('Cloud Confidence Bitmask (Bits 6-7):', cloud_confidence_quality);

// Cirrus confidence bitmask (bits 8-9)
var cirrus_confidence_quality = extract_qa_bits(rmnp_qa, 8, 9, 'cirrus_confidence_bitmask');
print('Cirrus Confidence Bitmask (Bits 8-9):', cirrus_confidence_quality);

// Terrain occlusion bitmask (bit 10)
var terrain_occlusion_quality = extract_qa_bits(rmnp_qa, 10, 10, 'terrain_occlusion_bitmask');
print('Terrain Occlusion Bitmask (Bit 10):', terrain_occlusion_quality);

// MASK PIXELS BASED ON VARIOUS BITMASK FLAGS (CLOUD & SHADOW, CLEAR, ETC.)
// Clear pixels
var rmnp_masked_clear = rmnp.updateMask(clear_quality.eq(1)); // 1 means clear pixel

// Shadows and clouds (indicates shadow free AND cloud free)
var rmnp_masked_shadows_clouds = rmnp.updateMask(
  cloud_shadow_quality.eq(0).and(cloud_quality.eq(0)));

// Shadows only
var rmnp_masked_shadows = rmnp.updateMask(cloud_shadow_quality.eq(0));

// Snow
var rmnp_masked_snow = rmnp.updateMask(snow_quality.eq(0));

// Clouds only
var rmnp_masked_clouds = rmnp.updateMask(cloud_quality.eq(0));

Data Processing¶

// No data processing in this lab.

Data Postprocessing¶

// No data post-processing in this lab.

Data Visualization¶

// Define Landsat 8 RGB visualization parameters
var l8_vis_params_rgb = {
  'bands': ['B4', 'B3', 'B2'],
  'min': 0,
  'max': 3000
};

// Define Landsat 8 CIR visualization parameters
var l8_vis_params_cir = {
  'bands': ['B5', 'B4', 'B3'],
  'min': 0,
  'max': 3000
};

// Center map to Rocky Mountain National Park, Colorado
Map.setCenter(-105.6836, 40.3428, 10);

// Baseline image
Map.addLayer(rmnp, l8_vis_params_rgb, 'RMNP Baseline Image');

// Pixel QA band
Map.addLayer(rmnp_qa, {}, 'QA Band', false);

// Bitmask layers
Map.addLayer(fill_quality, {min: 0, max: 1}, 'Fill Bitmask');
Map.addLayer(clear_quality, {min: 0, max: 1}, 'Clear Bitmask');
Map.addLayer(water_quality, {min: 0, max: 1}, 'Water Bitmask');
Map.addLayer(cloud_shadow_quality, {min: 0, max: 1}, 'Cloud Shadow Bitmask');
Map.addLayer(snow_quality, {min: 0, max: 1}, 'Snow Bitmask');
Map.addLayer(cloud_quality, {min: 0, max: 1}, 'Cloud Bitmask');
Map.addLayer(cloud_confidence_quality, {min: 0, max: 3}, 'Cloud Confidence Bitmask');
Map.addLayer(cirrus_confidence_quality, {min: 0, max: 3}, 'Cirrus Confidence Bitmask');
Map.addLayer(terrain_occlusion_quality, {min: 0, max: 1}, 'Terrain Occlusion Bitmask');

// Masked layers
Map.addLayer(rmnp_masked_clear, l8_vis_params_rgb, 'Masked for Clear Pixels');
Map.addLayer(rmnp_masked_shadows_clouds, l8_vis_params_rgb, 'Masked for Shadow and Cloud Pixels');
Map.addLayer(rmnp_masked_shadows, l8_vis_params_rgb, 'Masked for Shadow Pixels');
Map.addLayer(rmnp_masked_snow, l8_vis_params_rgb, 'Masked for Snow Pixels');
Map.addLayer(rmnp_masked_clouds, l8_vis_params_rgb, 'Masked for Cloud Pixels');

Data Export¶

// No data export in this lab.

Chapter 12: Cloud Masking Chapter 14: Image to Asset