Cloudless Mosaic and Image Limit in Sentinel Processing API

[miqedi (Miqedi Moon)]

am currently working on a project that involves creating cloudless mosaics from Sentinel-2 imagery. I am exploring the capabilities of the Sentinel Processing API and have a couple of specific questions regarding its functionality:

1. Is it possible to create a cloudless mosaic using the Sentinel Processing API similar to Google Earth Engine (Creating a composite and then finding the median or n’th quantile of it)? I have tried some publicly available evalscript examples, but they did not work for my case. It is especially a cloudy area, in Google Engine I have used almost 2 years of composite and now want to automate it.
2. For the preProcessScenes function in Evalscript, is there a limit on the number of images it can return?

[william.ray]

Hi,

Yes it is possible to do this using Processing API. With two years of data you may experience some timing out of the API over larger areas though. Below I have an example of a script to calculate a composite of the median value:

//VERSION=3

function setup() {
    return {
      input: [{
        bands: ["B08", "B04", "B03", "B02", "CLM", "SCL"],  //Requests required bands, s2cloudless mask and scene classification layer
        units: "DN"
      }],
      output: {
        bands: 4,
        sampleType: SampleType.UINT16
      },
      mosaicking: "ORBIT"
    };
  }
  
  function filterScenes (scenes, inputMetadata) {
    return scenes.filter(function (scene) {
      return scene.date.getTime()>=(inputMetadata.to.getTime()-12*30*24*3600*1000); //Defines the time range, e.g. from 1st June until 31st October counts 5 months with 30 days, 24 hours...
    });
  }
  
  function getValue(values) {
    values.sort( function(a,b) {return a - b;} );
    return getMedian(values);
  }
  
  function getMedian(sortedValues) {
    var index = Math.floor(sortedValues.length / 2);
    return sortedValues[index];
  }
  function getDarkestPixel(sortedValues) {
    return sortedValues[0]; // darkest pixel
  }
  
  function validate (samples) {
    var scl = samples.SCL;
    var clm = samples.CLM;
  
    if (clm === 1 || clm === 255) {
          return false;
    } else if (scl === 1) { // SC_SATURATED_DEFECTIVE
          return false;
    } else if (scl === 3) { // SC_CLOUD_SHADOW
          return false;
    } else if (scl === 7) { // SC_CLOUD_LOW_PROBA
          return false;    
    } else if (scl === 8) { // SC_CLOUD_MEDIUM_PROBA
          return false;
    } else if (scl === 9) { // SC_CLOUD_HIGH_PROBA
          return false;
    } else if (scl === 10) { // SC_THIN_CIRRUS
          return false;
    } else if (scl === 11) { // SC_SNOW_ICE
      return false;
    }  else {
    return true;
    }
  }
  
  function evaluatePixel(samples, scenes) {
    var clo_b02 = []; var clo_b03 = []; var clo_b04 = []; var clo_b08 = [];
    var clo_b02_invalid = []; var clo_b03_invalid = []; var clo_b04_invalid = []; var clo_b08_invalid = [];
    var a = 0; var a_invalid = 0;
  
    for (var i = 0; i < samples.length; i++) {
      var sample = samples[i];
      if (sample.B02 > 0 && sample.B03 > 0 && sample.B04 > 0 && sample.B08 > 0) {
        var isValid = validate(sample);
  
        if (isValid) {
          clo_b02[a] = sample.B02;
          clo_b03[a] = sample.B03;
          clo_b04[a] = sample.B04;
          clo_b08[a] = sample.B08;
          a = a + 1;
        } else {
          clo_b02_invalid[a_invalid] = sample.B02;
          clo_b03_invalid[a_invalid] = sample.B03;
          clo_b04_invalid[a_invalid] = sample.B04;
          clo_b08_invalid[a_invalid] = sample.B08;
          a_invalid = a_invalid + 1;
        }
      }
    }
  
    var rValue;
    var gValue;
    var bValue;
    var nirValue;
  
    if (a > 0) {
      nirValue = getValue(clo_b08);
      rValue = getValue(clo_b04);
      gValue = getValue(clo_b03);
      bValue = getValue(clo_b02);
    } else if (a_invalid > 0) {
      nirValue = getValue(clo_b08_invalid);    
      rValue = getValue(clo_b04_invalid);
      gValue = getValue(clo_b03_invalid);
      bValue = getValue(clo_b02_invalid);
    } else {
      nirValue = 0;
      rValue = 0;
      gValue = 0;
      bValue = 0;
    }
    return [nirValue, rValue, gValue, bValue]
  }

If your area of interest is larger I would recommend one of the following:

• for areas that are a bit too large, you can look at the large area utility: " the sentinelhub package implements utilities for splitting areas into smaller bounding boxes."
• a second option is to use our eolearn package, which is designed to “to seamlessly access and process spatio-temporal image sequences”. Although it is geared toward ML applications, it is quite good at splitting large areas into tiles and dealing with large time-series.
• The third option, is the most adapted for large ares/time-series: batch processing API, which you can also leverage with the Python SDK. “Sentinel Hub Batch Processing takes the geometry of a large area and divides it according to a specified tile grid. Next, it executes processing requests for each tile in the grid and stores results to a given location at AWS S3 storage. All this is efficiently executed on the server-side. Because of the optimized performance, it is significantly faster than running the same process locally.”