Vegetation Condition Index

//VERSION=3
function setup() {
    return {
      input: ["B04", "B08", "CLM", "dataMask"],
      output: { bands: 1 },
      mosaicking: "ORBIT"
    };
  }
  
  const NODATA = -32768;
  
  // tolerance in either direction, so i.e. +- 5 days
  const toleranceDays = 5;
  const msInDay = 24 * 60 * 60 * 1000;
  const msInYear = 365.25 * msInDay;
  const msInHalfYear = msInYear / 2
  const toleranceMs = toleranceDays * msInDay;
  
  var metadata = undefined;
  
  function relDiff(a, b) {
    const diff = Math.abs(a - b);
    return diff > msInHalfYear ? msInYear - diff : diff;
  }
  
  function datetimeToYearEpoch(date) {
    return date - new Date(Date.UTC(date.getUTCFullYear(), 0, 1))
  }
  
  function sortDatesDescending(d1, d2) {
    const date1 = new Date(d1.dateFrom);
    const date2 = new Date(d2.dateFrom);
    return date2 - date1
  }
  
  function preProcessScenes(collections) {
    // sort
    let scenes = collections.scenes.orbits;
    scenes = scenes.sort(sortDatesDescending);
    let newScenes = [];
    // convert first scene to day of year
    const observed = new Date(scenes[0].dateFrom);
    const obsMs = datetimeToYearEpoch(observed)
    for (let i = 0; i < scenes.length; i++) {
      let currentDate = new Date(scenes[i].dateFrom);
      let sceneMs = datetimeToYearEpoch(currentDate)
      let dt = relDiff(obsMs, sceneMs)
      if (dt <= toleranceMs) {
        newScenes.push(scenes[i]);
      }
    }
  
    metadata = {
      observed: observed.toISOString(),
      historical: newScenes.slice(1).map(scene => scene.dateFrom)
    }
  
    collections.scenes.orbits = newScenes;
    return collections;
  }
  
  
  function updateOutputMetadata(scenes, inputMetadata, outputMetadata) {
    outputMetadata.userData = metadata;
  }
  
  function calcNDVI(sample) {
    return index(sample.B08, sample.B04);
  }
  
  function calcMaxMin(samples) {
    let ndvi = calcNDVI(samples[0])
    let max = ndvi;
    let min = ndvi;
    for (let i = 1; i < samples.length; ++i) {
      ndvi = calcNDVI(samples[i])
      if (ndvi > max) {
        max = ndvi;
      } else if (ndvi < min) {
        min = ndvi;
      }
    }
    return [max, min]
  }
  
  function isClear(sample) {
    return sample.CLM == 0 && sample.dataMask == 1;
  }
  
  function evaluatePixel(samples) {
    // if the first value isn't clear, stop
    if (!isClear(samples[0])) {
      return [NODATA]
    }
    const clearTs = samples.filter(isClear)
    const observed = index(clearTs[0].B08, clearTs[0].B04);
    let max = NODATA, min = NODATA, vci = NODATA;
    if (clearTs.length > 0) {
      [max, min] = calcMaxMin(clearTs);
      vci = (observed - min) / (max - min)
    }
  
    return [vci];
  }
  

Evaluate and Visualize

• EO Browser

General description of the script

The vegetation condition index [1] compares the NDVI at current (observed) time to historical values, i.e. to NDVI at similar dates in previous years:

VCI = (NDVI_observed - NDVI_hist_min) / (NDVI_hist_max - NDVI_hist_min)

Please note that in case of Sentinel 2, only a few years of history are available.

The script takes the newest (latest) available scene as the observed one. Then, for each previous year the script finds all values within toleranceDays of the most recent date.

The actual scenes (dates) used can be returned as meta-data with an API requests by replacing the responses part of the request with:

  "responses":  [{
    "identifier": "userdata",
    "format": { "type": "application/json" }
  }]

Example response:

{
  "historical": [
    "2018-09-08T00:00:00.000Z",
    "2016-09-13T00:00:00.000Z",
    "2015-09-09T00:00:00.000Z"
  ],
  "observed": "2019-09-13T00:00:00.000Z"
}

Description of representative images

Vegetation condition index near Dodge City, Kansas, USA. Acquired on 30.05.2020.

References

[1] https://www.indexdatabase.de/db/i-single.php?id=249