// ======================================================================
// URBAN HEAT ISLAND CHANGE ANALYSIS OF CHENNAI
// Land-only Chennai Focus Region
// Before Year: 2014
// After Year : 2024
// Using Landsat 8/9 Collection 2 Level 2 Data in Google Earth Engine
// ======================================================================


// ======================================================================
// SECTION 1: USER CONFIGURATION
// ======================================================================

var CONFIG = {
  // Chennai focus bounding box
  // Sea will be removed using district land boundary
  aoiBox: ee.Geometry.Rectangle([80.05, 12.75, 80.35, 13.25]),

  // Before year summer season
  beforeStart: '2014-03-01',
  beforeEnd:   '2014-05-31',

  // After year summer season
  afterStart:  '2024-03-01',
  afterEnd:    '2024-05-31',

  mapZoom: 11
};


// ======================================================================
// SECTION 2: CREATE LAND-ONLY AOI TO REMOVE SEA AREA
// ======================================================================

var districts = ee.FeatureCollection("FAO/GAUL/2015/level2");

var chennaiLand = districts.filter(
  ee.Filter.and(
    ee.Filter.eq('ADM1_NAME', 'Tamil Nadu'),
    ee.Filter.inList('ADM2_NAME', [
      'Chennai',
      'Thiruvallur',
      'Kancheepuram'
    ])
  )
);

// Final land-only AOI
CONFIG.aoi = chennaiLand.geometry().intersection(CONFIG.aoiBox, ee.ErrorMargin(1));


// ======================================================================
// SECTION 3: MAP INITIALIZATION
// ======================================================================

Map.centerObject(CONFIG.aoi, CONFIG.mapZoom);
Map.setOptions('HYBRID');

print('====================================================');
print('URBAN HEAT ISLAND CHANGE ANALYSIS - CHENNAI');
print('Before Period:', CONFIG.beforeStart, 'to', CONFIG.beforeEnd);
print('After Period:', CONFIG.afterStart, 'to', CONFIG.afterEnd);
print('Dataset: Landsat 8/9 Collection 2 Level 2');
print('AOI: Land-only Chennai Focus Region');
print('====================================================');


// ======================================================================
// SECTION 4: STUDY AREA
// ======================================================================

var studyArea = ee.FeatureCollection([
  ee.Feature(CONFIG.aoi, {name: 'Land-only Chennai Focus Region'})
]);

var outline = studyArea.style({
  color: 'yellow',
  fillColor: '00000000',
  width: 2
});

Map.addLayer(outline, {}, 'Study Area Boundary', true);


// ======================================================================
// SECTION 5: CLOUD MASK FUNCTION
// ======================================================================

function maskLandsatC2L2(image) {
  var qa = image.select('QA_PIXEL');

  var cloudShadowBitMask = 1 << 4;
  var cloudsBitMask = 1 << 3;
  var snowBitMask = 1 << 5;

  var mask = qa.bitwiseAnd(cloudShadowBitMask).eq(0)
    .and(qa.bitwiseAnd(cloudsBitMask).eq(0))
    .and(qa.bitwiseAnd(snowBitMask).eq(0));

  return image.updateMask(mask);
}


// ======================================================================
// SECTION 6: FUNCTION TO CREATE LANDSAT COMPOSITE
// ======================================================================

function getLandsatComposite(startDate, endDate) {
  var landsat8 = ee.ImageCollection('LANDSAT/LC08/C02/T1_L2')
    .filterBounds(CONFIG.aoi)
    .filterDate(startDate, endDate)
    .map(maskLandsatC2L2);

  var landsat9 = ee.ImageCollection('LANDSAT/LC09/C02/T1_L2')
    .filterBounds(CONFIG.aoi)
    .filterDate(startDate, endDate)
    .map(maskLandsatC2L2);

  var collection = landsat8.merge(landsat9);

  return collection.median().clip(CONFIG.aoi);
}

var beforeImage = getLandsatComposite(CONFIG.beforeStart, CONFIG.beforeEnd);
var afterImage  = getLandsatComposite(CONFIG.afterStart, CONFIG.afterEnd);

print('Before Landsat 8 Image Count:',
  ee.ImageCollection('LANDSAT/LC08/C02/T1_L2')
    .filterBounds(CONFIG.aoi)
    .filterDate(CONFIG.beforeStart, CONFIG.beforeEnd)
    .size()
);

print('After Landsat 8 Image Count:',
  ee.ImageCollection('LANDSAT/LC08/C02/T1_L2')
    .filterBounds(CONFIG.aoi)
    .filterDate(CONFIG.afterStart, CONFIG.afterEnd)
    .size()
);

print('After Landsat 9 Image Count:',
  ee.ImageCollection('LANDSAT/LC09/C02/T1_L2')
    .filterBounds(CONFIG.aoi)
    .filterDate(CONFIG.afterStart, CONFIG.afterEnd)
    .size()
);


// ======================================================================
// SECTION 7: FUNCTION TO CALCULATE NDVI, NDBI, NDWI AND LST
// ======================================================================

function calculateUHIProducts(image) {

  // Surface Reflectance scale factor
  var red = image.select('SR_B4').multiply(0.0000275).add(-0.2);
  var green = image.select('SR_B3').multiply(0.0000275).add(-0.2);
  var nir = image.select('SR_B5').multiply(0.0000275).add(-0.2);
  var swir = image.select('SR_B6').multiply(0.0000275).add(-0.2);

  // NDVI = vegetation
  var ndvi = nir.subtract(red)
    .divide(nir.add(red))
    .rename('NDVI');

  // NDBI = built-up
  var ndbi = swir.subtract(nir)
    .divide(swir.add(nir))
    .rename('NDBI');

  // NDWI = water
  var ndwi = green.subtract(nir)
    .divide(green.add(nir))
    .rename('NDWI');

  // Land Surface Temperature
  // ST_B10 scale factor:
  // Kelvin = DN * 0.00341802 + 149
  // Celsius = Kelvin - 273.15
  var lst = image.select('ST_B10')
    .multiply(0.00341802)
    .add(149.0)
    .subtract(273.15)
    .rename('LST');

  return {
    ndvi: ndvi,
    ndbi: ndbi,
    ndwi: ndwi,
    lst: lst
  };
}

var before = calculateUHIProducts(beforeImage);
var after = calculateUHIProducts(afterImage);


// ======================================================================
// SECTION 8: VISUALIZATION PARAMETERS
// ======================================================================

var trueColorVis = {
  bands: ['SR_B4', 'SR_B3', 'SR_B2'],
  min: 7000,
  max: 18000
};

var lstVis = {
  min: 25,
  max: 45,
  palette: [
    '0000ff',
    '00ffff',
    '00ff00',
    'ffff00',
    'ff9900',
    'ff0000',
    '800000'
  ]
};

var ndviVis = {
  min: -0.2,
  max: 0.8,
  palette: [
    'brown',
    'yellow',
    'lightgreen',
    'green',
    'darkgreen'
  ]
};

var ndbiVis = {
  min: -0.4,
  max: 0.5,
  palette: [
    'blue',
    'white',
    'orange',
    'red'
  ]
};

var ndwiVis = {
  min: -0.5,
  max: 0.5,
  palette: [
    'brown',
    'white',
    'cyan',
    'blue'
  ]
};


// ======================================================================
// SECTION 9: LAND SURFACE TEMPERATURE MAPS
// ======================================================================

Map.addLayer(beforeImage, trueColorVis, 'True Color 2014', false);
Map.addLayer(afterImage, trueColorVis, 'True Color 2024', false);

Map.addLayer(before.lst, lstVis, 'LST Map 2014', false);
Map.addLayer(after.lst, lstVis, 'LST Map 2024', true);


// ======================================================================
// SECTION 10: NDVI, NDBI, NDWI MAPS
// ======================================================================

Map.addLayer(before.ndvi, ndviVis, 'NDVI Vegetation Map 2014', false);
Map.addLayer(after.ndvi, ndviVis, 'NDVI Vegetation Map 2024', false);

Map.addLayer(before.ndbi, ndbiVis, 'NDBI Built-up Map 2014', false);
Map.addLayer(after.ndbi, ndbiVis, 'NDBI Built-up Map 2024', false);

Map.addLayer(before.ndwi, ndwiVis, 'NDWI Water Body Map 2014', false);
Map.addLayer(after.ndwi, ndwiVis, 'NDWI Water Body Map 2024', false);


// ======================================================================
// SECTION 11: LST CHANGE MAP
// ======================================================================
// Positive value = temperature increased from 2014 to 2024

var lstChange = after.lst.subtract(before.lst).rename('LST_Change');

var lstChangeVis = {
  min: -5,
  max: 5,
  palette: [
    '0000ff',
    '00ffff',
    'ffffff',
    'ffff00',
    'ff9900',
    'ff0000'
  ]
};

Map.addLayer(lstChange, lstChangeVis, 'LST Change Map 2024 - 2014', false);


// ======================================================================
// SECTION 12: NDVI CHANGE AND NDBI CHANGE
// ======================================================================

var ndviChange = after.ndvi.subtract(before.ndvi).rename('NDVI_Change');
var ndbiChange = after.ndbi.subtract(before.ndbi).rename('NDBI_Change');

var ndviChangeVis = {
  min: -0.3,
  max: 0.3,
  palette: ['red', 'white', 'green']
};

var ndbiChangeVis = {
  min: -0.3,
  max: 0.3,
  palette: ['blue', 'white', 'red']
};

Map.addLayer(ndviChange, ndviChangeVis, 'NDVI Change Map 2024 - 2014', false);
Map.addLayer(ndbiChange, ndbiChangeVis, 'NDBI Change Map 2024 - 2014', false);


// ======================================================================
// SECTION 13: UHI ZONE CLASSIFICATION FUNCTION
// ======================================================================

function createHeatZones(lstImage) {

  var percentiles = lstImage.reduceRegion({
    reducer: ee.Reducer.percentile([25, 50, 75]),
    geometry: CONFIG.aoi,
    scale: 30,
    maxPixels: 1e13,
    bestEffort: true
  });

  var p25 = ee.Number(percentiles.get('LST_p25'));
  var p50 = ee.Number(percentiles.get('LST_p50'));
  var p75 = ee.Number(percentiles.get('LST_p75'));

  var zones = ee.Image(0)
    .where(lstImage.gte(p25), 1)
    .where(lstImage.gte(p50), 2)
    .where(lstImage.gte(p75), 3)
    .rename('Heat_Zone')
    .clip(CONFIG.aoi);

  return zones;
}

var heatZones2014 = createHeatZones(before.lst);
var heatZones2024 = createHeatZones(after.lst);

var heatZoneVis = {
  min: 0,
  max: 3,
  palette: [
    '00ffff',
    'ffff00',
    'ff9900',
    'ff0000'
  ]
};

Map.addLayer(heatZones2014, heatZoneVis, 'UHI Zone Map 2014', false);
Map.addLayer(heatZones2024, heatZoneVis, 'UHI Zone Map 2024', false);


// ======================================================================
// SECTION 14: HOTSPOT MAPS
// ======================================================================

var hotspot2014 = heatZones2014.eq(3).selfMask().rename('Hotspot_2014');
var hotspot2024 = heatZones2024.eq(3).selfMask().rename('Hotspot_2024');

Map.addLayer(hotspot2014, {palette: ['orange']}, 'Very High Heat Hotspots 2014', false);
Map.addLayer(hotspot2024, {palette: ['red']}, 'Very High Heat Hotspots 2024', false);


// ======================================================================
// SECTION 15: EXTRACT BUILT-UP, VEGETATION AND WATER
// ======================================================================

var builtUp2014 = before.ndbi.gt(0).selfMask().rename('Built_up_2014');
var builtUp2024 = after.ndbi.gt(0).selfMask().rename('Built_up_2024');

var vegetation2014 = before.ndvi.gt(0.3).selfMask().rename('Vegetation_2014');
var vegetation2024 = after.ndvi.gt(0.3).selfMask().rename('Vegetation_2024');

var water2014 = before.ndwi.gt(0).selfMask().rename('Water_2014');
var water2024 = after.ndwi.gt(0).selfMask().rename('Water_2024');

Map.addLayer(builtUp2014, {palette: ['orange']}, 'Extracted Built-up Area 2014', false);
Map.addLayer(builtUp2024, {palette: ['red']}, 'Extracted Built-up Area 2024', false);

Map.addLayer(vegetation2014, {palette: ['lightgreen']}, 'Extracted Vegetation Area 2014', false);
Map.addLayer(vegetation2024, {palette: ['green']}, 'Extracted Vegetation Area 2024', false);

Map.addLayer(water2014, {palette: ['cyan']}, 'Extracted Water Area 2014', false);
Map.addLayer(water2024, {palette: ['blue']}, 'Extracted Water Area 2024', false);


// ======================================================================
// SECTION 16: AREA CALCULATION FUNCTION
// ======================================================================

var pixelAreaKm2 = ee.Image.pixelArea().divide(1000000);

function areaSqKm(mask, bandName) {
  return mask.unmask(0)
    .multiply(pixelAreaKm2)
    .reduceRegion({
      reducer: ee.Reducer.sum(),
      geometry: CONFIG.aoi,
      scale: 30,
      maxPixels: 1e13,
      bestEffort: true
    })
    .get(bandName);
}

function printArea(mask, bandName, label) {
  print(label + ' Area sq.km:', areaSqKm(mask.rename(bandName), bandName));
}


// ======================================================================
// SECTION 17: HEAT ZONE AREA CALCULATION
// ======================================================================

print('');
print('================ HEAT ZONE AREA 2014 ================');
printArea(heatZones2014.eq(0), 'area', 'Low Heat Zone 2014');
printArea(heatZones2014.eq(1), 'area', 'Moderate Heat Zone 2014');
printArea(heatZones2014.eq(2), 'area', 'High Heat Zone 2014');
printArea(heatZones2014.eq(3), 'area', 'Very High Heat Zone 2014');

print('');
print('================ HEAT ZONE AREA 2024 ================');
printArea(heatZones2024.eq(0), 'area', 'Low Heat Zone 2024');
printArea(heatZones2024.eq(1), 'area', 'Moderate Heat Zone 2024');
printArea(heatZones2024.eq(2), 'area', 'High Heat Zone 2024');
printArea(heatZones2024.eq(3), 'area', 'Very High Heat Zone 2024');

print('');
print('================ LAND COVER AREA COMPARISON ================');
printArea(builtUp2014, 'area', 'Built-up Area 2014');
printArea(builtUp2024, 'area', 'Built-up Area 2024');
printArea(vegetation2014, 'area', 'Vegetation Area 2014');
printArea(vegetation2024, 'area', 'Vegetation Area 2024');
printArea(water2014, 'area', 'Water Area 2014');
printArea(water2024, 'area', 'Water Area 2024');


// ======================================================================
// SECTION 18: LST STATISTICS
// ======================================================================

function lstStatistics(lstImage, yearLabel) {
  var stats = lstImage.reduceRegion({
    reducer: ee.Reducer.minMax()
      .combine({
        reducer2: ee.Reducer.mean(),
        sharedInputs: true
      }),
    geometry: CONFIG.aoi,
    scale: 30,
    maxPixels: 1e13,
    bestEffort: true
  });

  print('LST Statistics ' + yearLabel + ':', stats);
}

lstStatistics(before.lst, '2014');
lstStatistics(after.lst, '2024');

var meanLSTChange = lstChange.reduceRegion({
  reducer: ee.Reducer.mean(),
  geometry: CONFIG.aoi,
  scale: 30,
  maxPixels: 1e13,
  bestEffort: true
});

print('Mean LST Change 2024 - 2014:', meanLSTChange);


// ======================================================================
// SECTION 19: MEAN LST BY LAND COVER TYPE
// ======================================================================

function meanLSTOverMask(lstImage, mask, label) {
  var mean = lstImage.updateMask(mask).reduceRegion({
    reducer: ee.Reducer.mean(),
    geometry: CONFIG.aoi,
    scale: 30,
    maxPixels: 1e13,
    bestEffort: true
  });

  print(label, mean);
}

print('');
print('================ MEAN TEMPERATURE BY LAND COVER ================');
meanLSTOverMask(before.lst, builtUp2014, 'Mean LST over Built-up Area 2014');
meanLSTOverMask(after.lst, builtUp2024, 'Mean LST over Built-up Area 2024');

meanLSTOverMask(before.lst, vegetation2014, 'Mean LST over Vegetation Area 2014');
meanLSTOverMask(after.lst, vegetation2024, 'Mean LST over Vegetation Area 2024');

meanLSTOverMask(before.lst, water2014, 'Mean LST over Water Area 2014');
meanLSTOverMask(after.lst, water2024, 'Mean LST over Water Area 2024');


// ======================================================================
// SECTION 20: LEGEND
// ======================================================================

var legend = ui.Panel({
  style: {
    position: 'bottom-right',
    padding: '10px',
    backgroundColor: 'rgba(255,255,255,0.9)'
  }
});

legend.add(ui.Label({
  value: 'UHI Legend',
  style: {
    fontWeight: 'bold',
    fontSize: '15px',
    margin: '0 0 6px 0'
  }
}));

function legendRow(color, name) {
  var colorBox = ui.Label({
    style: {
      backgroundColor: color,
      padding: '9px',
      margin: '0 6px 4px 0'
    }
  });

  var label = ui.Label({
    value: name,
    style: {
      margin: '0 0 4px 0',
      fontSize: '12px'
    }
  });

  return ui.Panel({
    widgets: [colorBox, label],
    layout: ui.Panel.Layout.Flow('horizontal')
  });
}

legend.add(legendRow('0000ff', 'Low Temperature'));
legend.add(legendRow('00ffff', 'Low Heat Zone'));
legend.add(legendRow('ffff00', 'Moderate Heat Zone'));
legend.add(legendRow('ff9900', 'High Heat Zone'));
legend.add(legendRow('ff0000', 'Very High Heat / Increase'));
legend.add(legendRow('green', 'Vegetation'));
legend.add(legendRow('blue', 'Water'));
legend.add(legendRow('yellow', 'Study Area Boundary'));

Map.add(legend);


// ======================================================================
// SECTION 21: TITLE
// ======================================================================

var title = ui.Label({
  value: 'Urban Heat Island Change Analysis of Chennai from 2014 to 2024',
  style: {
    position: 'top-center',
    fontWeight: 'bold',
    fontSize: '20px',
    padding: '8px',
    backgroundColor: 'rgba(255,255,255,0.85)'
  }
});

Map.add(title);


// ======================================================================
// SECTION 22: FINAL OUTPUT NOTE
// ======================================================================

print('');
print('================ PROJECT OUTPUTS =================');
print('1. LST Map 2014');
print('2. LST Map 2024');
print('3. UHI Zone Map 2014');
print('4. UHI Zone Map 2024');
print('5. LST Change Map 2024 - 2014');
print('6. NDVI Change Map');
print('7. NDBI Change Map');
print('8. Heat Zone Area Comparison');
print('9. Mean LST over Built-up, Vegetation and Water');
print('==================================================');


// ======================================================================
// SECTION 23: EXPORT OPTIONS
// ======================================================================

/*
Export.image.toDrive({
  image: before.lst,
  description: 'Chennai_LST_2014_LandOnly',
  folder: 'GEE_UHI_Project',
  fileNamePrefix: 'Chennai_LST_2014_LandOnly',
  region: CONFIG.aoi,
  scale: 30,
  crs: 'EPSG:4326',
  maxPixels: 1e13
});

Export.image.toDrive({
  image: after.lst,
  description: 'Chennai_LST_2024_LandOnly',
  folder: 'GEE_UHI_Project',
  fileNamePrefix: 'Chennai_LST_2024_LandOnly',
  region: CONFIG.aoi,
  scale: 30,
  crs: 'EPSG:4326',
  maxPixels: 1e13
});

Export.image.toDrive({
  image: lstChange,
  description: 'Chennai_LST_Change_2024_2014_LandOnly',
  folder: 'GEE_UHI_Project',
  fileNamePrefix: 'Chennai_LST_Change_2024_2014_LandOnly',
  region: CONFIG.aoi,
  scale: 30,
  crs: 'EPSG:4326',
  maxPixels: 1e13
});
*/

// ======================================================================
// END OF SCRIPT
// ======================================================================