Hello,
I’ve been struggling with my evalscript to return the fetched data in the correct chronological order. Below are the main parts of my initial script:
function setup() {
return {
input: [{bands: ["B03", "B04", "B08", "SCL", "CLD", "dataMask"], units: "DN"}],
output: [
{id: "dailyNDVI", bands: 1, sampleType: "UINT16"},
{id: "weeklyNDVI", bands: 1, sampleType: "UINT16"},
{id: "B03", bands: 1, sampleType: "UINT16"},
{id: "B04", bands: 1, sampleType: "UINT16"},
{id: "B08", bands: 1, sampleType: "UINT16"}
],
mosaicking: "ORBIT"
};
}
function calcNDVI(sample) {
// Invalid data.
if (!isValid(sample)) {
return MIN_INT16
}
// Cloud detection.
if (isCloud(sample)) {
return -2
}
// Water detection.
if (isWater(sample)) {
return 3
}
if (isWaterCandidate(sample)) {
return 2
}
return index(sample.B08, sample.B04)
}
function isValid(sample) {
return sample.dataMask === 1
}
function calcDailyNDVI(samples, scenes) {
return samples.reduce(function (dailyNdvis, sample, index) {
const date = JSON.stringify(getDate(scenes[index].date));
const dailyNdvi = calcNDVI(sample);
if (!dailyNdvis[date]) {
dailyNdvis[date] = MIN_INT16
}
dailyNdvis[date] = dailyNdvi
return dailyNdvis
}, {})
}
function preProcessScenes (collections) {
// Sort scenes by dateFrom in ascending order
collections.scenes.orbits.sort(function (s1, s2) {
const date1 = new Date(s1.dateFrom)
const date2 = new Date(s2.dateFrom)
return date1 - date2
})
return collections
}
function updateOutput(outputs, collections) {
let uniqueDailyDates = new Set();
let uniqueWeeklyDates = new Set();
collections.scenes.forEach(scene => {
uniqueDailyDates.add(JSON.stringify(getDate(scene.date)));
uniqueWeeklyDates.add(JSON.stringify(getSunday(scene.date)));
});
sampleDates = Array.from(uniqueDailyDates).sort();
sampleWeeklyDates = Array.from(uniqueWeeklyDates).sort();
outputs.weeklyNDVI.bands = Math.max(1, sampleWeeklyDates.length);
outputs.dailyNDVI.bands = Math.max(1, sampleDates.length);
outputs.B03.bands = Math.max(1, sampleDates.length);
outputs.B04.bands = Math.max(1, sampleDates.length);
outputs.B08.bands = Math.max(1, sampleDates.length);
}
function evaluatePixel(samples, scenes) {
let weeklyNDVI = calcScenesNDVI(samples, scenes);
let dailyNDVI = calcDailyNDVI(samples, scenes);
let dailyBands = calcDailyBands(samples, scenes);
return {
dailyNDVI: sampleDates.map(x => dailyNDVI[x]).map(y => y !== MIN_INT16 ? y * FACTOR + OFFSET : 10000),
weeklyNDVI: sampleWeeklyDates.map(x => weeklyNDVI[x]).map(y => y !== MIN_INT16 ? y * FACTOR + OFFSET : 10000),
B03: sampleDates.map(date => dailyBands[date] ? dailyBands[date].B03 : MIN_INT16),
B04: sampleDates.map(date => dailyBands[date] ? dailyBands[date].B04 : MIN_INT16),
B08: sampleDates.map(date => dailyBands[date] ? dailyBands[date].B08 : MIN_INT16)
};
}
I noticed that for a given field and search interval, there was one date where all daily NDVI pixels were 0, but the corresponding bands (which were also 0) were returned as the last date—resulting in a shift of the dates (each date being placed one day before its correct position).
So I thought that this was the case when all the values in the bands were invalid and that SH API was returning this lastly. A behaviour that I don’t fully understand, since I thought preProcessScenes() and updateOutput() ensured a consistent order of dates in both metadata and tiles.
I updated my script to use:
function calcDailyBands(samples, scenes) {
const res = {};
for (let i = 0; i < samples.length; i++) {
const sample = samples[i];
const scene = scenes[i];
const date = JSON.stringify(new Date(scene.date));
res[date] = {
B03: isValid(sample) ? sample.B03 : 0,
B04: isValid(sample) ? sample.B04 : 0,
B08: isValid(sample) ? sample.B08 : 0
};
}
return res;
function evaluatePixel(samples, scenes) {
let weeklyNDVI = calcScenesNDVI(samples, scenes);
let dailyNDVI = calcDailyNDVI(samples, scenes);
let dailyBands = calcDailyBands(samples, scenes);
return {
dailyNDVI: sampleDates.map(x => dailyNDVI[x]).map(y => y !== MIN_INT16 ? y * FACTOR + OFFSET : 0),
weeklyNDVI: sampleWeeklyDates.map(x => weeklyNDVI[x]).map(y => y !== MIN_INT16 ? y * FACTOR + OFFSET : 0),
B03: sampleDates.map(date => dailyBands[date] ? dailyBands[date].B03 : 0),
B04: sampleDates.map(date => dailyBands[date] ? dailyBands[date].B04 : 0),
B08: sampleDates.map(date => dailyBands[date] ? dailyBands[date].B08 : 0)
};
}
and by doing so, I finally got the chronological order that seemed correct.
However, I thought that the underlying issue to this bug was that the pixels were in fact invalid, so I changed the default returning value to 10000, for the bands and the daily NDVI… and when refetching the data, I got all 0 again for the bands and NDVI. Is it possible to have dataMask==1 when all bands==0? That seems to be a bit incomprehensible. If so, what is supposed to return the calcNDVI function as division by 0 is not determined?
I’m quite at a loss here to understand what is going on and what might be wrong with my script. Any idea would be welcome, thanks!