Computing ENL for BMP image in SNAP

Microwave Toolbox
#1

I am opening bmp image in SNAP and converting it to BEAM-DIMAP format. Then I am finding ENL from SNAP statistics for some region. Mean and Sigma statistics are correct through SNAP but ENL is not equal to mean^2/sigma^2.
As the image is bmp, pixel values range from 0 to 255.

Can anyone tell me why is it happening?

#2

I think the conversion to 8bit (0-255) rescaled the original pixel values so that the statistics are no longer representing the actual distribution of backscatter values.
But some questions remain: What data are you using? Why did you convert it to it in BMP format? Did you calibrate the data? How did you get the ENL values?

#3

I am using level 1.5 data and applying speckle filtering on that data. Actually I am applying filter through program where it is image is used in bmp format. To get ENL values, I selected homogeneous region from the image.

As I mentioned previously, mean and sigma are correct but only ENL is different than it should be according to equation.

#4

Hi,
Remember that the mean and sigma used to calculate the ENL have to be based in the intensity of DN values rather than DN values. Note that for SAR, the mean intensity of a homogeneous region is not equal to the square of the mean DN values (i.e. you need to square all the actual DN values before taking the mean).

To assist further can you give the product type and mode together with the mean and sigma values. Provided there is not DN saturation in the BMP image, this should be sufficient to calculate the ENL.

Note that expected ENL values can be found in Table 5-1 of the Sentinel-1 IPF product definition document (https://sentinel.esa.int/documents/247904/1877131/Sentinel-1-Product-Definition).

#5

Thanks. I was computing ENL directly from DN values and so, I was facing problem.

Thanks again. Your answer helped a lot.

Can we convert 0-255 values of bmp or tif image to actual DN values of intensity image through some process?

#6

Hi, As mentioned above the 0-255 values in bmp values cannot be used to determine the DN values (or backscatter). If you need to determine the DN values, you should use the 16bit measurment files within the S1 product.