Processing NDVI in Google Earth Engine

Question

I would like to analyze NDVI in the region of Bangkok.

Here is the code :

{    
    //Define time range
    var startyear = 2006;
    var endyear = 2016;

    //Set date in ee date format
    var startdate = ee.Date.fromYMD(startyear, 1, 1);
    var enddate = ee.Date.fromYMD(endyear, 12, 31);

    ///Import a fusion table of your study area
    // The Ca basin
    var Ca = ee.FeatureCollection(geometry)

    //filter on date and area (bounds)
    var l5images = l5.filterDate(startdate,enddate).filterBounds( Ca);
    var l7images = l7.filterDate(startdate,enddate).filterBounds( Ca);
    var l8images = l8.filterDate(startdate,enddate).filterBounds( Ca);

    ///Include a funtion to remove the clouds 
    //set cloud threshold
    var cloud_threshold = 40;

    // Select the red and NIR bands
    l5images = l5images.select(["B4","B3"]);
    l7images = l7images.select(["B4","B3"]);
    l8images = l8images.select(["B5","B4"]);

    // calculate ndvi from landsat 8
    function l8ndvi(img) {
      var ndvi = img.normalizedDifference(['B5', 'B4']).rename('NDVI');
      return img.addBands(ndvi);
    }

    // calculate ndvi from landsat 5 & 7
    function l57ndvi(img) {
      var ndvi = img.normalizedDifference(['B4', 'B3']).rename('NDVI');
      return img.addBands(ndvi);
    }


    // calculate ndwi for each image in imagecollection
    var l5ndvi = l5images.map(l57ndvi);
    var l7ndvi = l7images.map(l57ndvi);
    var l8ndvi = l8images.map(l8ndvi);

    // combine all data in single image collection
    var allcollection =  ee.ImageCollection((l5ndvi.merge(l7ndvi)).merge(l8ndvi));

    // add map to canvas
    Map.addLayer(allcollection);
    Map.centerObject(allcollection,8);
    print()
}

Google Earth Engine Link that I used

But then there is trouble, but I don't really understand what is it about.

So Is there anything I can do about this. I am still very new to programming.

Answer 1

NormalizedDifference() returns floating point values. Your band B3/B4/B5 are probably of some integer value. GEE cannot make composites with difference band types. Easiest solution would be casting the input B3-4-5 bands to floating point numbers. Change the following function of your code, and I think it should work fine (note that your link doesn't work for me).

// calculate ndvi from landsat 8
function l8ndvi(img) {
  var ndvi = img.normalizedDifference(['B5', 'B4']).rename('NDVI');
  return img.toFloat().addBands(ndvi);
}

// calculate ndvi from landsat 5 & 7
function l57ndvi(img) {
  var ndvi = img.normalizedDifference(['B4', 'B3']).rename('NDVI');
  return img.toFloat().addBands(ndvi);
}

Answer 2

I'd suggest that you comment out these lines and let us know if it works.

// Select the red and NIR bands
    l5images = l5images.select(["B4","B3"]);
    l7images = l7images.select(["B4","B3"]);
    l8images = l8images.select(["B5","B4"]);

If it works, I'll explain what I have in mind. I'm also a newbie, so maybe my hypothesis is wrong.