//บทเรียน MODIS data Hand on 
//// Code provided by Dr.Sitthisak Moukomla 

// Define the region of interest: Thailand
var thailand = ee.FeatureCollection('USDOS/LSIB_SIMPLE/2017')
                .filter(ee.Filter.eq('country_na', 'Thailand'));

// Load MODIS Surface Reflectance Image Collection
var modisSR = ee.ImageCollection('MODIS/061/MOD09GA')
               .filter(ee.Filter.date('2024-01-01', '2024-03-30'))
               .filterBounds(thailand);

// Calculate NDWI using the formula (Green - NIR) / (Green + NIR)
// Green band is 'sur_refl_b04' and NIR band is 'sur_refl_b02'
var calculateNDWI = function(image) {
  var ndwi = image.normalizedDifference(['sur_refl_b04', 'sur_refl_b02']).rename('NDWI');
  return image.addBands(ndwi);
};

// Apply the NDWI calculation to the Image Collection
var ndwiCollection = modisSR.map(calculateNDWI);

// Select and visualize the NDWI band
var ndwi = ndwiCollection.select('NDWI').median();
var ndwiVis = {
  min: -1.0,
  max: 1.0,
  palette: ['0000ff', '00ffff', 'ffff00', 'ff0000', 'ffffff'],
};

// Center the map on Thailand and add the NDWI layer
Map.centerObject(thailand, 5);
Map.addLayer(ndwi.clip(thailand), ndwiVis, 'NDWI Colorized');