Computing other vegetation indices

[rit]

I would like to compute vegetation indices like GCVI or EVI and wanted to see how I can adapt the script below (taken from Sentinel Hub Batch Statistical — Sentinel Hub 3.9.1 documentation):

ndvi_evalscript = """
//VERSION=3

function setup() {
  return {
    input: [
      {
        bands: [
          "B04",
          "B08",
          "dataMask"
        ]
      }
    ],
    output: [
      {
        id: "ndvi",
        bands: 1
      },
      {
        id: "dataMask",
        bands: 1
      }
    ]
  }
}

function evaluatePixel(samples) {
    return {
      ndvi: [index(samples.B08, samples.B04)],
      dataMask: [samples.dataMask]
    };
}
"""

What I am unclear about here is the ndvi: [index(samples.B08, samples.B04)], line, how is NDVI being calculated there? I thought there would be an explicit formula.

The formula for GCVI is:

(band[‘b08’] / band[‘b03’]) - 1.

thanks!

[paolo.filippucci]

Hi @rit ,
“Index” is actually a function that allow you to obtain the difference divided by the sum:

https://docs.sentinel-hub.com/api/latest/evalscript/functions/

To obtain what you want, you just need to substitute that line with the formula you want (and also fix input and output. Like this:

indices_evalscript = """
                        //VERSION=3
                    
                        function setup() {
                            return {
                                input: [{
                                    bands:[“B03”,“B08”,“dataMask”]}],
                                output:[{
                                    id: "GCVI",
                                    bands: 1
                                },
                                    {
                                id: “dataMask”,
                                bands: 1
                                }]
                            }
                        }
                    
                        function evaluatePixel(sample) {
                            return {
                                GCVI: [(samples.B08/samples.B03)-1],
                                dataMask: [samples.dataMask]
                            };
                        }
                    """

I guess that this should work

[rit]

@paolo.filippucci

indices_evalscript = """
                        //VERSION=3
                    
                        function setup() {
                            return {
                                input: [{
                                    bands:[“B03”,“B08”,“dataMask”]}],
                                output:[{
                                    id: "GCVI",
                                    bands: 1
                                },
                                    {
                                id: “dataMask”,
                                bands: 1
                                }]
                            }
                        }
                    
                        function evaluatePixel(sample) {
                            return {
                                GCVI: [(samples.B08/samples.B03)-1],
                                dataMask: [samples.dataMask]
                            };
                        }
                    """

Thanks! I tried it but getting the following error:
sentinelhub.exceptions.DownloadFailedException: Failed to download from: https://services.sentinel-hub.com/api/v1/statistics with HTTPError: 400 Client Error: Bad Request for url: https://services.sentinel-hub.com/api/v1/statistics Server response: "{"status": 400, "reason": "Bad Request", "message": "Failed to evaluate script!\nevalscript.js:7: SyntaxError: Invalid or unexpected token\n

Any idea about what might be going wrong?

[paolo.filippucci]

There is a missing “s” in “function evaluatePixel(sample)”

try to substitute with
“function evaluatePixel(samples)”

[rit]

That worked. Also, I had to change the quotes around dataMask to match those around GCVI. Thank you!