Disaster mapping

Mapping Tsunami impacts

Step 1: Setup the visualization and specify the cloud threshold the visualization parameters.


//Script name: Aceh tsunami
//PURPOSE: disaster mapping
//SENSORS: landsat 5

// User specified parameters

// setup Visualization
var vizrgb = {bands: ['B3', 'B2', 'B1'], max: 0.3};
var vizndbi = {palette: ['red']};
var vizdem = {min: 0.0, max: 20.0, palette: ['FF0000','FFFF00','2EFEF7',"013ADF","FF00FF"]};

// set the cloud threshold
var cloud_thresh = 40;

Step 2: Import the administrative boundaries of Sumatra. The shapefile is converted into a linestring and buffered for 10km. This buffer is used to clip only the area of interest close to the coastline.


// Data

// shapefile of sumatra
var sumatra = ee.FeatureCollection("ft:1U1tGgUkzkvnDpJiV0BFsvL_09ozvEssixG0dlBj4");

// convert polygon into linestring
var lineString = ee.Geometry.LineString(sumatra.geometry().coordinates().get(0));

// simplify to reduce calculation time
var lineStringSimple = lineString.simplify(10000);

// buffer 10 to both sides
var coastline = lineString.buffer(10000);

Step 3: Get a landsat 5 image before the Tsunami and one for after the Tsunami.


// get landsat 5 images
var beforeImage = ee.Image("LANDSAT/LT5_L1T_TOA/LT51310562004356BKT00");
print("Before image:" ,beforeImage.get("DATE_ACQUIRED"));

var afterImage = ee.Image("LANDSAT/LT5_L1T_TOA/LT51310562005006BKT00");
print("After image:" ,afterImage.get("DATE_ACQUIRED"));

Step 4: import the srtm digital elevation model


// import digital elevation model
var dem = ee.Image("CGIAR/SRTM90_V4");

Step 5: Mask clouds in both images.


// Functions

// cloud function to remove clouds from the image
var cloudfunction = function(image){
//use add the cloud likelihood band to the image
var CloudScore = ee.Algorithms.Landsat.simpleCloudScore(image);
//isolate the cloud likelihood band
var quality = CloudScore.select('cloud');
//get pixels above the threshold
var cloud01 = quality.gt(cloud_thresh);
//create a mask from high likelihood pixels
var cloudmask = image.mask().and(cloud01.not());
//mask those pixels from the image
return image.updateMask(cloudmask);
};

var beforeImage = cloudfunction(beforeImage);
var afterImage = cloudfunction(afterImage);

Step 6: Calculate the normalized difference bare index.


// calculate the normalized difference bare index
var ndbiAfter = afterImage.normalizedDifference(['B7', 'B4']);
var ndbiBefore = beforeImage.normalizedDifference(['B7', 'B4']);

Step 7: mask the areas that are not bare.


// mask the ares that are not bare
var ndbiAfterth = ndbiAfter.gt(-0.35);
ndbiAfterth = ndbiAfterth.mask(ndbiAfterth);

Step 8: determine the areas that are identified as bare after the tsunami but not before.


// mask the areas that are not bare
var ndbiBeforeth = ndbiBefore.gt(-0.35).not();

// select areas that are affected
var tsunami = ndbiAfterth.updateMask(ndbiBeforeth);

Step 9: select the are lower than 20 meter mean sea level.


// area below +20 meter msl
var waveheight = dem.lt(20);
waveheight = dem.updateMask(waveheight);

Step 10: visualize the results


Map.centerObject(beforeImage,10);
Map.addLayer(beforeImage.clip(sumatra),vizrgb,"before");
Map.addLayer(afterImage.clip(sumatra),vizrgb," after");

Map.addLayer(tsunami.clip(sumatra).clip(coastline),vizndbi,"Affected areas");
Map.addLayer(waveheight,vizdem,"area below 20m MSL");

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