Google Earth Engine -Iterated image of NDWI

Question

I computed the NDWI of the whole image collection and I expect to output an image that shows water body whenever it has been flooded , even once , throughout the filtered date.

Somehow the output image only show the result of the last image in the image collection.

This is what I currently have :

// Function to clip image collection
var clipimgcol = function(image){ return image.clip(Border)} ;
// ImageCollection
var l8collection = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
.filterBounds(Border)
.filterDate('2014-02-05', '2018-02-05')
.sort('system:time_start')
.map(clipimgcol);

// Mask pixels with clouds and cloud shadows
// Surface reflectance products come with a 'pixel_qa' band based on CFMask
// Create function leaving Water and Clear
var maskCloudWater = function(image){
var pixel_qa = image.select('pixel_qa');
return image.updateMask(pixel_qa.eq(322).or(pixel_qa.eq(386)).or(pixel_qa.eq(834))
.or(pixel_qa.eq(898)).or(pixel_qa.eq(1346)) //pixel_qa = Clear
.or(pixel_qa.eq(324)).or(pixel_qa.eq(388)).or(pixel_qa.eq(836))
.or(pixel_qa.eq(900)).or(pixel_qa.eq(1348))); //pixel_qa = Water
};

// use "map" to apply the function to each image in the collection
var l8masked = l8collection.map(maskCloudWater);

// Compute the Normalized Difference Water Index (NDWI).
var getNDWI = function(img){
  return img.addBands(img.normalizedDifference(['B3','B5']).rename('NDWI'));
};

// map over image collection
var l8ndwi = l8masked.map(getNDWI);
var ndwi = l8ndwi.select('NDWI');
print(ndwi);

I use a threshold of 0.3 for NDWI and tried to iterate the image collection so once the pixel's NDWI is gte 0.3 it returns 1 and the output will be the way I expected, but resulted differently :

var ini = ee.Image(ndwi.first());
function it (current , previousresult){
  var step1 = current.gte(0.3);
  var step2 = ee.Image(previousresult).select('NDWI');
  var laststep = ee.Image.cat([step2 , step1]);

  return laststep;
}
var ndwiit = ee.Image(ndwi.iterate(it , ini)).select('NDWI');
var ndwiViz = {min: -1, max: 1, palette: ['green', 'white','blue']};
Map.addLayer(ndwiit , ndwiViz, 'NDWIIT', false);

I also came up with the idea of remap function, to update the pixel's NDWI value once it is gte 0.3 but i didn't know how to extract band value of every pixel.

Answer 1

I think there can be a simpler approach to this. From my understanding you just want to see if the NDWI value ever exceeds 0.3 in ANY scene of the image collection. To do this you can first find out pixels in each scene that are greater than 0.3 NDWI.

var gt3 = ndwi.map(function(image){
  return image.gte(0.3);
});

This will give you a mask layer having pixels with value 0 and 1, 0 for pixels less than 0.3 NDWI and 1 for pixels greater than or equal to 0.3. Now since all the pixels are either 0 or 1 in each scene. You can take max of this collection to get either 0, for pixels whose ndvi were never greater than 0.3 as all the pixels will be 0, or 1, for pixels which had at least one scene with NDWI gte 0.3

var ndwiit = gt3.max();

Then the rest of code should work fine.