bandArithmetic.js

///// 
// Band Arithmetic 
///// 
// Calculate NDVI using Sentinel 2 
// Import and filter imagery by location and date. 
var sfoPoint = ee.Geometry.Point(-122.3774, 37.6194);
var sfoImage = ee.ImageCollection('COPERNICUS/S2')
    .filterBounds(sfoPoint)
    .filterDate('2020-02-01', '2020-04-01')
    .first();
// Display the image as a false color composite. 
Map.centerObject(sfoImage, 11);
Map.addLayer(sfoImage, {
    bands: ['B8', 'B4', 'B3'],
    min: 0,
    max: 2000
}, 'False color');
// Extract the near infrared and red bands. 
var nir = sfoImage.select('B8');
var red = sfoImage.select('B4');
// Calculate the numerator and the denominator using subtraction and addition respectively. 
var numerator = nir.subtract(red);
var denominator = nir.add(red);
// Now calculate NDVI. 
var ndvi = numerator.divide(denominator);
// Add the layer to our map with a palette. 
var vegPalette = ['red', 'white', 'green'];
Map.addLayer(ndvi, {
    min: -1,
    max: 1,
    palette: vegPalette
}, 'NDVI Manual');
// Single-Operation Computation of Normalized Difference for NDVI

// Now use the built-in normalizedDifference function to achieve the same outcome. 
var ndviND = sfoImage.normalizedDifference(['B8', 'B4']);
Map.addLayer(ndviND, {
    min: -1,
    max: 1,
    palette: vegPalette
}, 'NDVI normalizedDiff');


// Use normalizedDifference to calculate NDWI 
var ndwi = sfoImage.normalizedDifference(['B8', 'B11']);
var waterPalette = ['white', 'blue'];
Map.addLayer(ndwi, {
    min: -0.5,
    max: 1,
    palette: waterPalette
}, 'NDWI');

// Create an NDVI image using Sentinel 2. 
var seaPoint = ee.Geometry.Point(-122.2040, 47.6221);
var seaImage = ee.ImageCollection('COPERNICUS/S2')
    .filterBounds(seaPoint)
    .filterDate('2020-08-15', '2020-10-01')
    .first();
var seaNDVI = seaImage.normalizedDifference(['B8', 'B4']);
// And map it. 
Map.centerObject(seaPoint, 10);
var vegPalette = ['red', 'white', 'green'];
Map.addLayer(seaNDVI,
    {
        min: -1,
        max: 1,
        palette: vegPalette
    },
    'NDVI Seattle');

// Implement a threshold. 
var seaVeg = seaNDVI.gt(0.5);
// Map the threshold. 
Map.addLayer(seaVeg,
    {
        min: 0,
        max: 1,
        palette: ['white', 'green']
    },
    'Non-forest vs. Forest');

// Implement .where. 
// Create a starting image with all values = 1. 
var seaWhere = ee.Image(1)
    // Use clip to constrain the size of the new image. 
    .clip(seaNDVI.geometry());
// Make all NDVI values less than -0.1 equal 0. 
seaWhere = seaWhere.where(seaNDVI.lte(-0.1), 0);
// Make all NDVI values greater than 0.5 equal 2. 
seaWhere = seaWhere.where(seaNDVI.gte(0.5), 2);
// Map our layer that has been divided into three classes. 
Map.addLayer(seaWhere,
    {
        min: 0,
        max: 2,
        palette: ['blue', 'white', 'green']
    },
    'Water, Non-forest, Forest');

// Implement masking. 
// View the seaVeg layer's current mask. 
Map.centerObject(seaPoint, 9);
Map.addLayer(seaVeg.mask(), {}, 'seaVeg Mask');
// Create a binary mask of non-forest. 
var vegMask = seaVeg.eq(1);
// Update the seaVeg mask with the non-forest mask. 
var maskedVeg = seaVeg.updateMask(vegMask);
// Map the updated Veg layer 
Map.addLayer(maskedVeg,
    {
        min: 0,
        max: 1,
        palette: ['green']
    },
    'Masked Forest Layer');
// Map the updated mask 
Map.addLayer(maskedVeg.mask(), {}, 'maskedVeg Mask');

// Implement remapping. 
// Remap the values from the seaWhere layer. 
var seaRemap = seaWhere.remap([0, 1, 2], // Existing values. 
    [9, 11, 10]); // Remapped values. 

Map.addLayer(seaRemap,
    {
        min: 9,
        max: 11,
        palette: ['blue', 'green', 'white']
    },
    'Remapped Values');