GEE学习——用公式写SAVI/NDVI指数！

// Filter the L7 collection to a single month.筛选一个月的影像
var collection = ee.ImageCollection('LANDSAT/LE07/C01/T1_TOA')
    .filterDate('2002-11-01', '2002-12-01');
// A function to compute NDVI.用表达式计算NDVI
var NDVI = function(image) {
  return image.expression('float(b("B4") - b("B3")) / (b("B4") + b("B3"))');
};
// A function to compute Soil Adjusted Vegetation Index.用表达式计算SAVI
var SAVI = function(image) {
  return image.expression(
      '(1 + L) * float(nir - red)/ (nir + red + L)',
      {
        'nir': image.select('B4'),
        'red': image.select('B3'),
        'L': 0.2
      });
};
// Shared visualization parameters.视觉参数配色
var vis = {
  min: 0,
  max: 1,
  palette: [
      'FFFFFF', 'CE7E45', 'DF923D', 'F1B555', 'FCD163', '99B718',
      '74A901', '66A000', '529400', '3E8601', '207401', '056201',
      '004C00', '023B01', '012E01', '011D01', '011301'
  ]
};
Map.setCenter(-93.7848, 30.3252, 11);
// Map the functions over the collection, reduce to mean and display.
//用map遍历集合中的NDVI和SAVI并且求取平均值和展示
Map.addLayer(collection.map(NDVI).mean(), vis, 'Mean NDVI');
Map.addLayer(collection.map(SAVI).mean(), vis, 'Mean SAVI');