Using Statistical Info in EO Browser with a Custom Script

[mfrey (Maya Frey)]

I am attempting to use Ulyssys Water Quality Viewer (UWQV) to create a timeseries using the Statistical Info/Feature Info Service Chart function on EO Browser, and I am having a hard time understanding the output.

I have used UWQV as a custom script (available through the link above) to view chlorophyll and sediment with Sentinel-3 data. The script creates the images I am looking for on the map, and I am able to use the Statistical Info function to create a timeseries. The graph shows two lines labelled “C0” and “C1”, but I cannot tell what these values represent. Looking at this post, it looks like I need to modify the script. Where in the script should I change the values (I want to show rlh and tss)?

Thank you!

[chung.horng (Chung)]

Hi @mfrey ,

The best solution to keep the visualisation and get the statistical info correctly is to refactor the script with setup and evaluatePixel functions to have multiple outputs for visualisation and statistics, e.g., the default NDVI layer has default for visualisation and eobrowserStats and dataMask (both eobrowserStats and dataMask are required) for statistics.

I would suggest focusing on one data source (in your case should be Sentinel-3) and compute the index you need (rlh and tss) with evaluatePixel function. You can find a lot of resource in the Evalscript of our documentation.

If the visualisation is not your focus, the quick fix would be commenting out the backgroundLayer (line 286-288 of this script) and returning the index value (chlIndex or tssIndex) only for the entire area.

For example, to get rlh we can set the PARAMS as below so the sediment layer won’t appear in the output:

const PARAMS = {
  // Indices
  chlIndex: 'default',
  tssIndex: null,
  watermaskIndices: ['ndwi', 'hol'],
  // Limits
  chlMin: -0.005,
  chlMax: 0.05,
  tssMin: 0.075,
  tssMax: 0.185,
  waterMax: 0,
  cloudMax: 0.02,
  // Graphics
  foreground: 'default',
  foregroundOpacity: 1.0,
  background: 'default',
  backgroundOpacity: 1.0
};

Next, we need to comment out the backgroudLayer returned by getValue:

  // if (!isWater(indices.watermask, params.watermaskIndices, params.waterMax, params.cloudMax, isSentinel3)) {
  //   return backgroundLayer;
  // }

Finally, we set the alg as rlh (line 297) and return chlIndex (line 327):

const alg = chl === 'default' ? (isSentinel3 ? 'rlh' : 'mci') : chl;

  // return foregroundOpacity === 1 ? value : blend(value, backgroundLayer, foregroundAlpha, 100 - foregroundAlpha);
  return [chlIndex]

The quick fix allows you to get the statistics of index correctly, but you can barely see anything on the screes as the value of index is too small.

Best Regards