Calculating water body area in Google earth engine using Landsat images?

Question

I am attempting to calculate the area of water bodies in my classified map using landsat images in Google earth engine. from youtube I found a tutorial on GEE which showed me a method of calculating an area. However I struggled to apply it because I don't understand why my code is showing error.

Can someone guide me through how to calculate area, whether using the same method, or another?

Here's the link to my map:

https://code.earthengine.google.com/ff99d299831a9768fc70cd8fec15807b

the code is:

var l5 = ee.ImageCollection("LANDSAT/LT05/C01/T1_SR"),
    table = 
    /* color: #d63000 */
    /* displayProperties: [
      {
        "type": "rectangle"
      }
    ] */
    ee.Geometry.Polygon(
        [[[90.95591178825589, 23.73559234169589],
          [90.95591178825589, 23.528001478587907],
          [91.1265431969473, 23.528001478587907],
          [91.1265431969473, 23.73559234169589]]], null, false);

var cloudMaskL457 = function(image) {

  var qa = image.select('pixel_qa');


// If the cloud bit (5) is set and the cloud confidence (7) is high

// or the cloud shadow bit is set (3), then it's a bad pixel.

  var cloud = qa.bitwiseAnd(1 << 5)
                  .and(qa.bitwiseAnd(1 << 7))
                  .or(qa.bitwiseAnd(1 << 3));


// Remove edge pixels that don't occur in all bands
  var mask2 = image.mask().reduce(ee.Reducer.min());
  return image.updateMask(cloud.not()).updateMask(mask2);
};  


l5 = l5.filterDate('1989-09-01', '1990-03-30')
      .filterBounds(table)
      .filterMetadata('CLOUD_COVER_LAND', 'less_than', 10)
      .map(cloudMaskL457);


Map.centerObject(table, 12);
print('l1: ', l5);


var visParams = {bands: ['B3','B2', 'B1'], min: 600, max: 2500};


//['B3_median','B2_median', 'B1_median'], min: 600, max: 2500};

var visParamsFalse = {bands: ['B4','B3', 'B2'], min: 600, max: 2500};

var median = l5
//.reduce(ee.Reducer.median())
      .median()
      .clip(table);

print(median)

//Map.addLayer(median, visParams, 'median');
Map.addLayer(median, visParamsFalse, '1990False');




//Map.addLayer(awei1, {min: -1, max: 1, palette: ['FF0000', '0000FF']},'1990l1');

var awei2 = median.expression(
    'blue + 2.5 * green - 1.5 * (NIR + MIR) - 0.25 * SWIR', {
      'blue': median.select('B1'),
      'green': median.select('B2'),
      'NIR': median.select('B4'),
      'MIR': median.select('B5'),
      'SWIR': median.select('B7')
}).rename('awei2');


print(awei2)


Map.addLayer(awei2, {min: 1, max: 2, palette: ['FF0000', '0000FF']},'classified');

var area_pxa = awei2.multiply(ee.Image.pixelArea()) 
                    .reduceRegion(ee.Reducer.sum(),table,30,null,null,false,1e13)
                    .get('constant')

 area_pxa = ee.Number(area_pxa).divide(1e6)                 
 print ('Area using ee.Image.pixelArea (km²)', area_pxa)

Answer 1

I believe you may have copied large portions of your code from the tutorial.

You unfortunately have kept some of the dictionary key calls. Modify the end of your script as such: change constant to awei2

var area_pxa = awei2.multiply(ee.Image.pixelArea()) 
                    .reduceRegion(ee.Reducer.sum(),table,30,null,null,false,1e13)
                    .get('awei2')

Your script should work as expected now.