1

I am currently working on a time-series in Google Earth Engine. Iam still quite inexperienced, especially when it comes to functions. I want to extract the mean values of Sentinel1-images for each week in a period of 7 month and aggregate them into a new Image Collection I can then further process.

So far I tried to do it for a single month:

 var testsubset = ee.FeatureCollection("users/xyz/test_subset");

//set start-dates    

var images = {
  '2018-02-01': getWeeklySentinelComposite('2018-02-01'),
  '2018-02-08': getWeeklySentinelComposite('2018-02-08'),
  '2018-02-15': getWeeklySentinelComposite('2018-02-15'),
  '2018-02-22': getWeeklySentinelComposite('2018-02-22'),
  '2018-03-01': getWeeklySentinelComposite('2018-03-01')
};

//function to get a weekly composite

function getWeeklySentinelComposite(date) {
  var date = ee.Date(date);
  // Only include the VV polarization, for consistent compositing.
  var polarization = 'VV';
  var sentinel1 = ee.ImageCollection('COPERNICUS/S1_GRD')
                      .filterBounds(testsubset)
                      .filterDate(date, date.advance(1, 'week'))
                      .filter(ee.Filter.listContains('transmitterReceiverPolarisation', polarization))
                      .filter(ee.Filter.eq('instrumentMode', 'IW'))
                      .select(polarization)
                      .mean();
  return sentinel1;
}

I currently run into two main issues: I'am not sure how to return the images I picked and how to turn them into a new ImageCollection. Second: I get

"Invalid argument specified for ee.Date(): [object Object]"

and don't really know why (I used a similar function for another script and it worked fine)

Just to clarify, this is what I want to do with the Image Collection in the end:

//Map.addLayer(testsubset, {color:'green'}, 'Testshape', true , 0.5);

var tempTimeSeries = ui.Chart.image.series({
  imageCollection: sentinel1_IC,
  region: testsubset,
  reducer: ee.Reducer.mean(),
  scale: 200,
  xProperty: 'system:time_start'
});

tempTimeSeries.setChartType('ScatterChart');
tempTimeSeries.setOptions({
  title: 'VV-over-time',
  vAxis: {
    title: 'sigma VV'
  },
  lineWidth: 1,
  pointSize: 4,
  series: {
    0: {color: 'blue'},
    1: {color: 'red'},
    2: {color: 'green'}
  }
});

print(tempTimeSeries);
0

You are almost there. The fact you cannot get an imagecollection from your function now, is that you make a clientside dictionary with dates as key. Instead, first make a server side list of dates seperated a week apart:

//set a list of start dates
var startDate = '2018-02-01';
var endDate = '2018-12-31';
var weekDifference = ee.Date(startDate).advance(1, 'week').millis().subtract(ee.Date(startDate).millis());
var listMap = ee.List.sequence(ee.Date(startDate).millis(), ee.Date(endDate).millis(), weekDifference);

Once you have this, you can map over the list and return an image for every start date of a week with your function. Note that you will have to set something like a date property to every image to work with them later on. You can transform this list of image to an image collection using ee.ImageCollection.fromImages():

var sentinel1_IC = ee.ImageCollection.fromImages(listMap.map(function(dateMillis){
  var date = ee.Date(dateMillis);
  return getWeeklySentinelComposite(date);
}));

You can then continue with your analysis. Link script

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.