Google Earth Engine: Masking land out of Sentinel-3 imagery

Question

I'm trying to isolate ocean cover from OLCI imagery using a bitmask from the quality_flags band. But it's masking all the pixels, and I notice when I mask the ocean and leave the land, the land is very patchy compared to non-masked imagery.

Here's a link: https://code.earthengine.google.com/8b979aedb7f91352ccd792c63af8ca39

Here's the code I'm using:

//Land mask
function maskLand(image) {
  // Bit 31 is land/sea cover
  var LandBitMask = ee.Number(2).pow(31).int();
  // Get the pixel QA band
  var qf = image.select('quality_flags');
  // The flags should be set to one, indicating water cover.
  var mask = qf.bitwiseAnd(LandBitMask).eq(1);
  return image.updateMask(mask)
}

//Create image collection for area and date range
var lis_collection = ee.ImageCollection(olci)
  .filterBounds(lis)
  .filterDate('2018-05-01', '2018-10-01');

//Mask the image collection to water
var lis_mask = lis_collection.map(maskLand);

//Mosaic, clip, and display the area
var lis_area = lis_mask.mosaic();
var lis_area = lis_area.clip(lis);
print(lis_area);
Map.addLayer(lis_area, oa5vis, "Long Island Sound radiance");

Is there a better way to do this?

Perhaps some kind of mask I can run on the clipped mosaic instead of a function to run on the image collection.

Answer 1

I got this working eventually by modifying a different method I found someone using to mask clouds in Landsat.

//Create functions to mask land from mosaic
var getQABits = function(image, start, end, newName) {
    // Compute the bits we need to extract.
    var pattern = 0;
    for (var i = start; i <= end; i++) {
       pattern += Math.pow(2, i);
    }
    // Return a single band image of the extracted QA bits, giving the band a new name.
    return image.select([0], [newName])
                  .bitwiseAnd(pattern)
                  .rightShift(start);
};

// A function to mask out land and cloud pixels.
var land_pixels = function(image) {
  // Select the QA band.
  var qf = image.select(['quality_flags']);
  // Get the bit that flags land.
  return getQABits(qf, 31,31, 'Land_pixels').eq(0);
  // Return an image masking out cloudy areas.
};

var maskLand = function(image) {
  var lp = land_pixels(image);
  image = image.updateMask(lp);
  return image;
};

Then by running maskLand on an OLCI image. Once I found a good image to use this on, I kept the output mask and applied it to my other images.