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I want to use a imagecollection to calculate NDVI, the following code shows how I did. However, I know some values of surface reflectance of Near-infrared band and Red band are less than 0, or larger than 10000, or is Null, so how to calculate the NDVI by avoiding use the Near-infrared band and Red band with those unusual values?

var collection = ee.ImageCollection('LANDSAT/LE07/C01/T1_SR') 
    .filterDate('2019-01-01', '2019-12-31') // the period of interest
    .filterBounds(fc) // the study area
// B4 is near-infrared band and B3 is red band
var NDVI1 = function(image) {
  return image.expression('float(b("B4") - b("B3")) / (b("B4") + b("B3"))');
};
var NDVI = collection.map(NDVI1)
Map.addLayer(NDVI, {}, 'ndvi')
Map.centerObject(fc)

this is the code link: https://code.earthengine.google.com/202776c3d784fed6c7a04947f6b769f6

1 Answer 1

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You can mask those values first, and then use your expression to calculate the NDVI:

var NDVI_maskedVals = function(image) {
  var B3_B4_masked = image.select(['B3', 'B4'])
                          .updateMask(image.select(['B3', 'B4']).gt(0))
                          .updateMask(image.select(['B3', 'B4']).lt(10000)) 
  return image.addBands(
               B3_B4_masked.expression('float(b("B4") - b("B3")) / (b("B4") + b("B3"))')
              .rename('NDVI_masked'));
};

Or use the normalizedDifference function, which automatically forces negatives values to zero:

var NDVI_normDiff = function(image) {
  return image.addBands(image.normalizedDifference(['B4','B3']).rename('NDVI_normDiff'));
};

link code

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