Hot to get VH in decibels from -23 to -14 db from Sentinel S1 GRD source?


#1

Hi! I added a new layer which Source is Sentinel S1 GRD and we need to use VH in decibels from -23 to -14 db but now it returns values from -20 to 0 I think (//displays VH in decibels from -20 to 0). This is the code we are using:

var val = [Math.max(0, Math.log(VH) * 0.21714724095 + 1)];
var valaux = 13.146*(val) + 317.45;
return colorBlend(valaux,[0,40],[[1,1,0],[0,0,1]]);

Is there a way to get VH in decibels from -23 a -14?

Thanks!


#2

Hi,
The VH linear to decibel conversion formula of which you are probably aware is below:

// displays VH in decibels from -20 to 0
// the following is simplified below
// var log = 10 * Math.log(VH) / Math.LN10;
// var val = Math.max(0, (log + 20) / 20);
var val = [Math.max(0, Math.log(VH) * 0.21714724095 + 1)];

This will visualize values from -20 to 0 decibels, values less than -20 that will be black and more than 0 white. (The script doesn’t actually return values between -20 and 0).

To change the displayed range this needs a bit of modification.
// var log = 10 * Math.log(VH) / Math.LN10;
This line is the same as it is the actual conversion.

Next you need to fit these values between 0 and 1 so it will get visualized correctly.
The general formula is:
// var min
// var max
// var val = Math.max(0, log - min) / (max - min));
This visualizes values between min and max.

In your case min = -23, max = -14, so:
// var val = Math.max(0, (log - -23) / (-14 - -23);
this simplifies to (you don’t actually need this step but it makes it a little faster):
var val = [Math.max(0, Math.log(VH) * 0.11111111111 + 2.55555555556)];


Ratio VH/VV from Sentinel1
#3

Thanks Marko! In this case:
var val = [Math.max(0, Math.log(VH) * 0.11111111111 + 2.55555555556)];
The variable val returns values from -23 to -14? Or what values does the variable take?
Thanks!


#4

Hi Leonardo,
in this case the variable returns values between 0 and 1 so it gets visualized correctly. (0 goes to black, 1 to white). But if you need the actual decibel values for further processing for example, then this is not necessary. You can then use the log variable instead which takes exact values:
var log = 10 * Math.log(VH) / Math.LN10;

If you want values only between -23 and -14 (so anything less than -23 goes to -23, anything more than -14 goes to -14) you can use:
var val = Math.max(Math.min(log, -14), -23);


#5

Hi Marko, the script we are using is:
var val = Math.max(Math.min(log, -14), -23);
var valaux = 13.146*(val) + 317.45;
return colorBlend(valaux,[0,100],[[1,1,0],[0,0,1]]);

How do I colored the map from yellow to blue:
valaux returns 15,092 if val is -23 and 133,406 if val -14.
We tried the colorBlend function in different ways but we couldn’t make it work.
Thanks!


#6

Try:

var log = 10 * Math.log(VH) / Math.LN10;
return colorBlend(log, [-23,-14], [[1,1,0],[0,0,1]]);


#7

Hi Marko, is there a way to use “transparent” color when we use colorBlend, I mean, to see some transparent pixels on the layer instead of a solid color from some value 1 to a value 2 and then use solid colors.
Thanks