What is samples in the evalscript

1

For the evalscript posted below, as shown in evaluatePixel(samples) method, i would like to know:

1- what samples is? and what is samples.length?does sample.length is the number of acquisition times?
For example, for the following time-series: TS1: 01.02.2024 and TS: 30.05.2024, if we assumed that withing the aforementioned times-series there were 7 acquisition times, does that mean that sample.length will return 7?!!

2-for the contents of samples, does it values of NDVI with cloude coverage or without it please?

eval-script:

//VERSION=3
   // Script to extract a time series of NDVI values using 
  // Sentinel 2 Level 2A data and  metadata file.
  
  function setup() {
	return {
	  input: [{
		bands: ["B04", "B08"],
		units: "DN"
	  }],
	  output: [
	{
	  id: "default",
	  bands: 1,
	  sampleType: "FLOAT32",
	  nodataValue: NaN,
	},
  ],
	  mosaicking: Mosaicking.ORBIT
	}
  }

  function updateOutput(output, collection) {
	output.default.bands = collection.scenes.length
  }

  function evaluatePixel(samples) {
	// Precompute an array to contain NDVI observations
	var n_observations = samples.length;
	let ndvi = new Array(n_observations).fill(NaN);
  
  //Fill the array with NDVI values
	samples.forEach((sample, index) => {
	  ndvi[index] = (sample.B08 - sample.B04) / (sample.B08 + sample.B04) ;
	});
					  
	return ndvi;
  }
2

Hello Burkhard,

You are right in your assumptions. samples holds the band values with cloud coverage. It’s on you to filter out clouds or other missing values.

If you have 7 acquisitions over your area of interest it holds 7 items, so samples.length would be 7.

But do keep in mind that it will only ever hold the values for a single pixel of the image you are requesting. That is why it is called evaluatePixel. This function will be applied to every single pixel in your output image individually.

4

In the case of mosaicking: ORBIT it will give you an array of all 7 values which are in the time of interest. So samples is correct it will give you all values for that pixel which are available.

You can have a look at how samples is structured by running

  function evaluatePixel(samples) {
	throw new Error(JSON.stringify(samples))
  }

How those 7 values are handled is entirely up to you. samples just returns the raw values. You can do whatever you want with them. If you want a temporal average you need to sum up all the values in the samples array and divide them by the length.

6

If you mean the structure of the samples object which evaluatePixel gets, then you can find the information here: Evalscript V3