NDWI(Normalized Difference Water Index,归一化水指数)，用遥感影像的特定波段进行归一化差值处理，以凸显影像中的水体信息。目前对于NDWI有S. K. McFEETERS和Bo-cai Gao的两种不同做法，且都于1996年发表在权威国际遥感学术杂志上。对于NDWI的命名尚有争议。
NDWI = (绿波段 - 近红外波段) / (绿波段 + 近红外波段)
landsat8: NDWI = (band3 - band5) / (band3 + band5)
landsat5/7: NDWI = (band2 - band4) / (band2 + band4)
sentinel2: NDWI = (band3 - band8) / (band3 + band8)
接下来我们使用GEE来处理landsat8的NDWI，这段代码和前两篇的NDVI计算代码是一致的。这里的代码只是讲解相关原理如何实现，其中某些参数需要在实践中灵活调整。
//landsat 8 NDWI Demo
function NDWI_V1(img) {
var nir = img.select("B5");
var green = img.select("B3");
var ndwi = green.subtract(nir).divide(green.add(nir));
return ndwi;
}
function NDWI_V2(img) {
var nir = img.select("B5");
var green = img.select("B3");
var ndwi = img.expression(
"(B3 - B5)/(B3 + B5)",
{
"B5": nir,
"B3": green
}
);
return ndwi;
}
function NDWI_V3(img) {
var ndwi = img.normalizedDifference(["B3","B5"]);
return ndwi;
}
//landsat8 and roi
var l8_col = ee.ImageCollection("LANDSAT/LC8_L1T_TOA");
var roi = ee.Geometry.Point([124.1455078125,45.644768217751924]);
var img = ee.Image(l8_col.filterBounds(roi)
.filterDate("2017-02-01", "2017-09-23")
.first());
var ndwi1 = NDWI_V1(img);
var ndwi2 = NDWI_V2(img);
var ndwi3 = NDWI_V3(img);
var visParam = {
min: -0.5,
max: 0.5,
palette: ['00FFFF', '0000FF']
};
Map.addLayer(img, {bands:["B4", "B3", "B2"], max:0.3}, "raw_img");
Map.addLayer(ndwi1, visParam, "ndwi_1");
Map.addLayer(ndwi2, visParam, "ndwi_2");
Map.addLayer(ndwi3, visParam, "ndwi_3");
Map.centerObject(roi, 9);
//show charts
var ndwi_list = l8_col.filterDate("2017-01-01", "2017-09-23")
.map(function(image) {
var cloud = ee.Algorithms.Landsat.simpleCloudScore(image).select("cloud");
var mask = cloud.lte(20);
var ndwi = image.normalizedDifference(['B3', 'B5']).rename('NDWI');
return image.addBands(ndwi).updateMask(mask);
});
var chart1 = ui.Chart.image.series({
imageCollection: ndwi_list.select('NDWI'),
region: roi,
reducer: ee.Reducer.mean(),
scale: 30
}).setOptions({title: 'NDWI IMAGE SERIES'});
print(chart1);
var chart2 = ui.Chart.image.doySeries({
imageCollection: ndwi_list.select('NDWI'),
region:roi,
regionReducer: ee.Reducer.mean(),
scale:30
}).setOptions({title: "ROI NDWI EACH DAY SERIES"})
print(chart2)