Using scaling factor for collection 2 level 2 with Roy et al constants

Question

I am using the Landsat ETM+ to OLI harmonization tutorial but I need it for Landsat collection 2 L2 data, this means the scaling factors have changed. I am not sure how to apply the scaling factors in this case. The tutorial multiplies the y-intercept constants by the scaling factor (10,000, see below) but I'm not sure how would that apply to the scaling factor and offset for collection 2 (0.0000275 + -0.2).

var coefficients = {itcps: ee.Image.constant([0.0003, 0.0088, 0.0061, 0.0412, 0.0254, 0.0172])
         .multiply(10000), slopes: ee.Image.constant([0.8474, 0.8483, 0.9047, 0.8462, 0.8937, 0.9071])};

I noticed some one already asked this question here, but I am wondering if any one could expand with code on how to actually use the new scaling factors for collection 2 in this case?

Answer 1

To compute reflectance value using scale factor for both collections (1 and 2) use the following functions:

var col1_DN = ee.ImageCollection("LANDSAT/LE07/C01/T2_SR");
var col2_DN = ee.ImageCollection("LANDSAT/LE07/C02/T1_L2");


var col1_SR = col1_DN.map(function(image){return image.multiply(0.0001)})
var col2_SR = col2_DN.map(function(image){return image.multiply(0.0000275).add(-0.2)})

Given the example you mentioned, you can avoid the use .multiply(10000) and harmonize over reflectance values instead of DN values.

You can add an extra step for this purpose:

var coefficients = {
  itcps: ee.Image.constant([0.0003, 0.0088, 0.0061, 0.0412, 0.0254, 0.0172])
  slopes: ee.Image.constant([0.8474, 0.8483, 0.9047, 0.8462, 0.8937, 0.9071])
};

// Here goes the other code skipped in this snipet

function etmToOli(img) {
  return img.select(['Blue', 'Green', 'Red', 'NIR', 'SWIR1', 'SWIR2'])
      .multiply(0.0000275)
      .add(-0.2) 
      .multiply(coefficients.slopes)
      .add(coefficients.itcps)
      .addBands(img.select('pixel_qa'));
}

Since this works with reflectance data, .round() and .toShort() are out because output is float. You can apply also a 10000 factor for matching Collection 1 scheme after computing reflectance as well, but it all depends on your needs (if integer values made this code faster for instance).