Pixel-based calculation of valid observations in Landsat ImageCollection

Question

I am trying to calculate an ee.Image() object that contains the number of valid (= unmasked) pixels of a given Landsat ImageCollection. Originally I thought a simple ee.ImageColllection.count() would do the job, but as the function treats each image independently of the observation date, the areas where the WRS rows in the Landsat tiles overlap are counted twice. My intention on the other hand is to retrieve a date dependent information of valid pixels.

Given that I have tried the following:

var imgCol_L8_SR = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
          .filterBounds(ROI)
          .filter(ee.Filter.calendarRange(year_start,year_end,'year'))
          .filter(ee.Filter.calendarRange(month_start,month_end,'month'))
          .filter(ee.Filter.lt('CLOUD_COVER', cloud_cover))
          .map(maskL8SR);

// initial Image in list
var ini =ee.List([
  imgCol_L8_SR.select('B1').max()
    .remap([0], [0], 0, "B1")
    .rename('NOBS').int16()
]);

var fun_nobservations = function(date, previous) {
  // Cast
  date = ee.Date(date);
  previous = ee.List(previous);
  var filtered = imgCol_L8_SR.filterDate(date, date.advance(1,'day'));

  // count valid observations per pixel:
  var mask = ee.Image(filtered.count());

  // set valid obs. in overlap back to 1:
  mask = mask.where(mask.gt(1), 1);

  var sum = mask.add(previous.get(-1));
  return ee.Algorithms.If(filtered.size(), previous.add(sum), previous);
};

var start = ee.Date('1985-06-01');
var finish = ee.Date('2017-08-31');
var diff = finish.difference(start, 'day');
var range = ee.List.sequence(0, diff.subtract(1))
                   .map(function(day){return start.advance(day,'day')});

var nobs = ee.Image(ee.List(range.iterate(fun_nobservations, ini)).get(-1));

It counts the number of valid pixels dependent on the date and sums the single image layers iteratively in a list. The problem here is that given the study area size and the time-interval I run into a user memory exceeded error.

Any ideas on how this could be approached differently?

Answer 1

It's not clear what format you expect for the final answer: do you want an aggregate number of valid pixels for a region for each unique date (ie: a single time series) or do you want an image of the total number of valid observations at each pixel, ignoring overlap?

The first is just a mapped reduceRegion. The second you can get by adding a date band to each image (something like days since some epoch) and then reducing with countDistinct.

function maskL8sr(image) {
  // Bits 3 and 5 are cloud shadow and cloud, respectively.
  var cloudShadowBitMask = 1 << 3
  var cloudsBitMask = 1 << 5

  // Get the pixel QA band.
  var qa = image.select('pixel_qa')

  // Both flags should be set to zero, indicating clear conditions.
  var mask = qa.bitwiseAnd(cloudShadowBitMask).eq(0)
      .and(qa.bitwiseAnd(cloudsBitMask).eq(0))

  // Return the masked image, scaled to TOA reflectance, without the QA bands.
  return image.updateMask(mask).divide(10000)
      .select("B[0-9]*")
      .copyProperties(image, ["system:time_start"])
}

var EPOCH = ee.Date('1979-01-01')
var addDate = function(image) {
  var mask = image.mask().reduce(ee.Reducer.min())
  var days = image.date().difference(EPOCH, 'day')
  return ee.Image.constant(days).int()
      .clip(image.geometry())
      .updateMask(mask)
      .copyProperties(image, ["system:time_start"])
}

var year_start = 2015
var year_end = 2018
var month_start = 4
var month_end = 6

var imgCol = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
          .filterBounds(ROI)
          .filter(ee.Filter.calendarRange(year_start,year_end,'year'))
          .filter(ee.Filter.calendarRange(month_start,month_end,'month'))
          .map(maskL8sr)
          .map(addDate)

Map.addLayer(imgCol.reduce(ee.Reducer.countDistinct()), {min:0, max: 60})