Step 2. Calculating Radiance

The purpose of this section is to briefly instruct users who are given a Landsat scene how to convert the raw DN values into radiance.

<<<< Step 1. Image Preparation                                           >>> Step 3. Calculating Reflectance

As outlined in the online Landsat 7 User's Guide there is a standard procedure to convert Landsat DN values into radiance values. In summary, you must first convert the raw DN values into the amount of energy that the sensor received, which is called the radiance. To do this we make use of a simple formula:

L_λ = "gain" * (DN) + "offset"

In this formula, L_λ is the radiance of the pixel and is calculated by multiplying the gain of the sensor's band by the DN value. To cut a long-story short, this gain is calculated to account for the fact that there are different Landsat satellites and different Landsat formats.

To convert your image into radiance you need some information from the vendor. Namely you need to find out whether each band was "high or low gain". This information can commonly be found in the header file. The header file typically has a file type like *.MTL. It is often very small (~10K) and can be read by most txt editors. Here is a snippet from LE7042028000219850.MTL to show you what to look for:

If you then look at Chapter 11 of the online Landsat 7 User's Guide you use this information within an "expanded" version of the above equation to calculate the radiance of each band. You can use most equation editors in GIS or image processing packages to convert each band.