NDVI index based on QA bands for MCD43A4 , in GEE

Question

I want to calculate the NDVI index based on the MODIS/006/MCD43A4 collection and based on quality bands, but as a new GEE user I have no idea If I am on the rght way to do this. So far I have the following :

  var imageCollection = ee.ImageCollection("MODIS/006/MCD43A4")
        //.select(['Nadir_Reflectance_Band2', 'Nadir_Reflectance_Band1'], ['NIR', 'RED']).multiply(0.0001)
        .filterBounds(geometry)
        .map(function(image){return image.clip(geometry)}) 
        .filter(ee.Filter.calendarRange(10,11,'month')),
    geometry = /* color: #d63000 */ee.Geometry.Polygon(
          [[[74.31655930070895, 56.84897198026647],
            [71.72378586320895, 53.54030739149663],
            [81.04019211320895, 51.563412328672136],
            [87.80777023820895, 55.429013452403986]]]);;


var modNDVI = imageCollection.select("Nadir_Reflectance_Band2","Nadir_Reflectance_Band1","BRDF_Albedo_Band_Mandatory_Quality_Band1","BRDF_Albedo_Band_Mandatory_Quality_Band2");


       var filterqa = function(feature){
     var image = ee.Image(feature);
     var QAImage = image.select(['BRDF_Albedo_Band_Mandatory_Quality_Band1']);
    var QAImage2 = image.select(['BRDF_Albedo_Band_Mandatory_Quality_Band2']);

     var maskedImage = ee.Image(0)
                         .where((QAImage.eq(0),1)&&(QAImage2.eq(0),1));

     return image.mask(maskedImage);  
};
var md_masked= modNDVI.map(filterqa);




        var ndvi = (function(image) {
          return image.expression('(NIR - RED) / (NIR + RED)', {
            'NIR': image.select('Nadir_Reflectance_Band2').multiply(0.0001).float(),
            'RED': image.select('Nadir_Reflectance_Band1').multiply(0.0001).float(),
          }).clamp(-1, 1);
        }); 

    print(imageCollection)


        //var ndviParams = {min: -1, max: 1, palette: ['blue', 'white', 'green']};
        //Map.addLayer(, ndviParams, 'NDVI');

From the imageCollection, I see that each image has 14 bands from which, 7 (7-13) have quality information. How can I calculate the NDVI index based on the good quality observations? I am really confused as there is no a SummaryQA band simillar to the one from the MOD13Q1.

Answer 1

If you're interested in just using QA information from Bands 1 and 2, this should get you started. Note that most of the code below is the same as yours, just re-organized a bit. You're off to a great start! Also note that the final image collection contains 1159 images, and only the first of those is being added to the map in the final step.

var geometry = ee.Geometry.Polygon(
          [[74.31655930070895, 56.84897198026647],
            [71.72378586320895, 53.54030739149663],
            [81.04019211320895, 51.563412328672136],
            [87.80777023820895, 55.429013452403986]]);
Map.addLayer(geometry,{},"Geometry");

// Create NDVI function
var addNDVI = function(image) {
  var ndvi = image.normalizedDifference(["Nadir_Reflectance_Band2","Nadir_Reflectance_Band1"]);
  return image.addBands(ndvi.rename("NDVI"));
};

// Create a QA mask function
var filterqa = function(image){ 
  var mask1 = image.select("BRDF_Albedo_Band_Mandatory_Quality_Band1").eq(0);
  var mask2 = image.select("BRDF_Albedo_Band_Mandatory_Quality_Band2").eq(0);
  return image.updateMask(mask1).updateMask(mask2);
};

// Compile image collection and map relevant functions
var imageCollection = ee.ImageCollection("MODIS/006/MCD43A4")
        .filterBounds(geometry)
        .filter(ee.Filter.calendarRange(10,11,'month'))
        .map(filterqa)
        .map(addNDVI)
        .map(function(image){return(image.clip(geometry))});

print(imageCollection);
Map.addLayer(imageCollection.select("NDVI"),{},"Image");