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Question:

How is at-sensor reflectance calculated for Landsat 4-5 or Landsat 7?

Answer:

Before at-sensor reflectance can be determined for Landsat 4-5 or Landsat 7 data, the radiance needs to be first calculated using the Digital Number (DN) values. Details on the conversion to radiance, and radiance to reflectance can be found in the Landsat 7 Data Users Handbook.