3

Google Earth Engine distributes Landsat 7 image composites of indeces such as NDWI and NDVI that, when averaged on a shapefile, give back a coherent value.

But how is SLC failure treated by it https://landsat.usgs.gov/slc-products-background?

3

The missing data is masked prior to creating the composite. Since the SLC-off line errors are not in the same place for multiple satellite passes you still get a full coverage in the multi-image composite.

Landsat 7 Collection 1 Tier 1 composites are made from Tier 1 orthorectified scenes, using the computed top-of-atmosphere (TOA) reflectance

See for instance the 8-day EVI composite documentation.

The Landsat-7 Tier 1 data is calibrated to TOA:

Landsat 7 Collection 1 Tier 1 calibrated top-of-atmosphere (TOA) reflectance. Calibration coefficients are extracted from the image metadata. See Chander et al. (2009) for details on the TOA computation.

You can see the masked stripes of missing data being masked when you display a single image in GEE (i.e. by using the "inspector" tool on masked pixels):

https://code.earthengine.google.com/d72e2444bb77c9d7edc674292fcdc1f2

var dataset = ee.ImageCollection('LANDSAT/LE07/C01/T1_TOA')
                  .filterDate('2010-01-01', '2012-12-31')
                  .filterBounds(geometry);

var geometry = /* color: #d63000 */ee.Geometry.Point([10.540775120310172, 47.90923665980289]);

var trueColor321 = dataset.select(['B3', 'B2', 'B1']);
var trueColor321Vis = {
  min: 0.0,
  max: 0.4,
  gamma: 1.2,
};
Map.setCenter(6.746, 46.529, 6);
Map.addLayer(trueColor321.first(), trueColor321Vis, 'True Color (321)');

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