How can I find all the lakes in a region (bounded by polgon) in earth engine? - google-earth

Question

The problem statement is that a region of interest is given.
I need to find all the lakes in a polygon bounded region using the NDWI index for water bodies, which are at a height of more than 1500m. Then display the changes in the area of lake's surface water starting from the year 1984 till 2018 on a 2-year interval in a table like structure in Google Earth Engine. I have used Landsat 5 and 7 data.
I have created the following code:
Earth Engine Code
Now I need to display the results in the polygon marked region in a table sort of structure in the following format:-
Rows - (Lake 1, Lake 2, Lake 3... Lake n)
Columns - (Surface Area in 1984, Surface Area in 1986, ....2018)
How should I go about doing it?

Answer 1

I answer this question in regard of the code posted in the comments, hopefully the question is updated with the code posted in the comments.
Filtering: ok.
Just a comment, I wouldn't name an image collection variable with name img, it's just confusing to me, but variables names are up to you.
var mf = ee.Filter.calendarRange(10, 12, 'month');
var img1 = ee.ImageCollection(l5
.filterDate('1984-01-01','1999-12-31')
.filterBounds(roi)
.filter(mf));
var img2 = ee.ImageCollection(l7
.filterDate('2000-01-01','2018-12-31')
.filterBounds(roi)
.filter(mf));
add NDWI: This is your code:
var addNDWI = function(image){
var ndwi = image.normalizedDifference(['B2', 'B4']).rename('NDWI');
var ndwiMask = ndwi.gte(0.3);
return image.addBands(ndwi);
};
var image1 = img1.map(addNDWI);
var image2 = img2.map(addNDWI);
you are not saving ndwiMask, so you won't be able to use it outside of this function. Again, I wouldn't name them image as they are not images but image collections.
elevation mask: you have to select the elevation band:
var elevMask = elevation.select('elevation').gt(1500)
This mask image will have ones where elevation is greater than 1500 and zeros where not.
applying masks: in this part you have to remember that Earth Engine uses functional programming, so objects are not mutable, this means that you cannot update the state of an object using a method, you have to catch the output of the method you are calling. Here you need ndwi mask, so you have to compute it with NDWI band.
var mask = function(image){
var ndwiMask = image.select('NDWI').gt(0.3)
var ndwi_masked = image.updateMask(ndwiMask);
return ndwi_masked.updateMask(elevMask);
};
var maskedImg = image1.map(mask); // ImageCollection!
var maskedImg2 = image2.map(mask); // ImageCollection!
Visualizing: As the results are ImageCollection, when you add it to the map EE makes a mosaic and that is what you would see. Keep that in mind for further processing.
var ndwiViz = {bands: ['NDWI'], min: 0.5, max: 1, palette: ['00FFFF', '0000FF']};
Map.addLayer(maskedImg, ndwiViz, 'Landsat 5 masked collection');