C values when using the moisture index in line graphs

1

Hi All,

I have been trying to figure out the c values and how they influence the line graph when looking at the moisture index without any luck.

Some background.
We configure our own WMS in the sentinel configurator for use in our own portal. Currently, in our WMS we use the default moisture index as provided in the list of layers. I then use the Sentinel EO browser to test.

So in the EO browser I choose the default theme. I choose my area and timeframe in question and then display the default moisture index provided. I then chose the statistical info for the area in question. Note this is the default moisture index use in the EO browser under the default theme.

image
image493×573 68 KB

The statistical info then looks as follows (Figure 1):
image
image801×537 37.6 KB

Next to compare I copy and paste the script of the moisture index in the sentinel configurator (This is the layer we see in our custom portal) to the creat custom rendering tool.

let index = (B8A - B11)/(B8A + B11);

return colorBlend(index, [-0.8, -0.24, -0.032, 0.032, 0.24, 0.8], [[0.5,0,0], [1,0,0], [1,1,0], [0,1,1], [0,0,1], [0,0,0.5]]);

I then choose exactly the same area and timeframe and then choose the statistical info and get the following graph (Figure 2).

image
image796×563 60.9 KB

My question is why is there no correlation whatsoever between figure 1 and 2. How do they calculate the values in the graph of figure 1. How do I use the RGB values in figure 2 to have the same graph as in figure 1.

Please help
Thank you

2

You should not really try to map RGB values back into the actual value. There is no point in that and it will cause errors.
You should simply configure a new layer, which returns actual index values, rather than visualization.
See this FAQ:
https://www.sentinel-hub.com/faq/how-do-i-get-actual-ndvi-values

3

Thank you for the response. This was helpfull.
How would you for instance change the following script to only show the index value.
let viz = ColorMapVisualizer.createDefaultColorMap();

function evaluatePixel(samples) {
let val = index(samples[0].B08, samples[0].B04);
return viz.process(val);
}

function setup(ds) {
setInputComponents([ds.B04, ds.B08]);
setOutputComponentCount(3);
}

Thanks

4

No, as there you have a “color map” component.

The easiest way is written in the FAQ I proposed above:

var ndvi = (B08-B04)/(B08+B04); 
return [ndvi];

With the pattern you propose, it should be something along the lines:

function evaluatePixel(samples) {
let val = index(samples[0].B08, samples[0].B04);
return [val];
}

function setup(ds) {
setInputComponents([ds.B04, ds.B08]);
setOutputComponentCount(1);
}