Landsat7 - converting dns to at sensor radiance

Question

I am trying to convert DNs of Landsat 7 ETM+ bands to at sensor radiance values. I know the formula that I can use in the raster calculator, but I am confused about the values in the Metadata. I am looking for Lmax and Lmin values for all bands.

Are these the same as radiance maximum and minimum values?

Qcal min and max values are there in the metadata but what about the Qcal (not min or max) value from which we need to subtact Qcal min in the algorithm.

Answer 1

Radiance minimum and radiance maximum are Lmin and Lmax, respectively, for each band in the MTL file. The MTL file that comes with each Landsat data will show the Lmin and Lmax like this:

GROUP = MIN_MAX_RADIANCE
    RADIANCE_MAXIMUM_BAND_1 = 193.000 -------> Lmax for Band 1
    RADIANCE_MINIMUM_BAND_1 = -1.520 --------> Lmin for Band 1
    .
    .
    .
    .
  END_GROUP = MIN_MAX_RADIANCE 

Qmin typically equal 1 and Qmax typically equal 255, you can refer to the details of the formula from USGS and also from Yale University. Qcal is the DN value that you want to change to radiance, which is each raster band that you already downloaded and you want to convert to radiance.

So I will quote from the above links the explanation of the formula:

The formula used in this process is as follows:

Lλ = ((LMAXλ - LMINλ)/(QCALMAX-QCALMIN)) * (QCAL-QCALMIN) + LMINλ

Where:

• Lλ is the cell value as radiance
• QCAL = digital number
• LMINλ= spectral radiance scales to QCALMIN
• LMAXλ = spectral radiance scales to QCALMAX
• QCALMIN = the minimum quantized calibrated pixel value (typically = 1)
• QCALMAX = the maximum quantized calibrated pixel value (typically = 255)