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I'm trying to mask each modis image using the QA layer. As far as I understand, the part .select("QA").eq(0) should remove all pixels with no ideal quality for all bands. However, when I make a plot with Map.addLayer(ls7.first()) it seems like there is no pixel in any part of the world with the best quality.

It seems strange, is something wrong with my code? When I use .select("StateQA").eq(0) it seems that only some sparse pixels (most of them in the sea) are ok. Any suggestions?

This is my code:

//select collection

var ls7SR = ee.ImageCollection('MODIS/006/MOD09A1');

// calculate normalized burn ration

var ls4_7_Indices = function(lsImage){
  var nbr = lsImage.normalizedDifference(['sur_refl_b02', 'sur_refl_b07']).toFloat();
  nbr = nbr.updateMask(lsImage.select("QA").eq(0));
  return nbr.copyProperties(lsImage, ['system:time_start']);
  };

// Map functions across modis Collection

var ls7 = ls7SR.map(ls4_7_Indices)


Map.addLayer(ls7.first())

1 Answer 1

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You have to do some bitwise operations when using these types of bands. If you look at the image collection in the catalog, you'll find this under Bitmask for QA:

Bits 0-1: MODLAND QA bits

0: Corrected product produced at ideal quality - all bands
1: Corrected product produced at less than ideal quality - some or all bands
2: Corrected product not produced due to cloud effects - all bands
3: Corrected product not produced for other reasons - some or all bands, may be fill value (11) [Note that a value of (11) overrides a value of (01)]

This means that you have to look at the value of bit 0 to 1 from the QA band. When this value is 0, all bands have ideal quality. I have a little utility function I use to help with this. See bitwiseExtract() here below.

var bitwiseExtract = function(value, fromBit, toBit) {
  if (toBit === undefined)
    toBit = fromBit
  var maskSize = ee.Number(1).add(toBit).subtract(fromBit)
  var mask = ee.Number(1).leftShift(maskSize).subtract(1)
  return value.rightShift(fromBit).bitwiseAnd(mask)
}

var ls7SR = ee.ImageCollection('MODIS/006/MOD09A1')

var ls4_7_Indices = function(lsImage) {
  var nbr = lsImage.normalizedDifference(['sur_refl_b02', 'sur_refl_b07']).toFloat()
  var modlandQa = bitwiseExtract(lsImage.select("QA"), 0, 1)
  nbr = nbr.updateMask(modlandQa.eq(0))
  return nbr.copyProperties(lsImage, ['system:time_start'])
}

var ls7 = ls7SR.map(ls4_7_Indices)
Map.addLayer(ls7.first())

https://code.earthengine.google.com/5d9083fbcdb93330077c791a97473d6d

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  • Thank DAniel. It seems to work, but the truth is that I didn't undertand how the function works. Can you explain it a little bite?
    – jose maria
    Mar 8, 2021 at 17:05
  • 1
    The function takes three arguments: A value (like the QA band here, or an ee.Number), the first bit to use and the last bit to use (e.g. 0 and 1 for MODLAND QA bits, or 2 and 5 for band 1 data quality). The function look at the value as a binary number. As an example, 2470774279 is 10010011010001010000011000000111 in binary. In your case, you look at bits 0 and 1, that's the first two bits. These bits can be 00, 01, 10, or 11. That's the same as 0, 1, 2, 3 in decimal. In the example number here, you have 11 (start reading from the right), which is 3 (Corrected product not produced for other). Mar 8, 2021 at 19:29
  • Thanks Daniel. Now I understand how it works. So, for instance, if I was intererested in best quality for band n1 (bits 2-5), i need to put: bitwiseExtract(lsImage.select("QA"), 2, 5) , and then inside the function "ls4_7_Indices" nbr = nbr.updateMask(modlandQa.eq(4)) right? just to know how to manipulate the function in the future.
    – jose maria
    Mar 9, 2021 at 9:02
  • or eq(0) becasue it is looking only bits 2 to 5?
    – jose maria
    Mar 9, 2021 at 9:08

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