Index Calculator - Reflectance Map Radiometric Correction

When using the index calculator, the interest is in the reflectance of the terrain. However the pixel value depends on many other factors, which need to be corrected in order to get a measure of the reflectance.

These factors include:

  • Incoming light
  • ISO
  • Aperture 
  • Shutter speed
  • Sensor response and optical system
  • Vignetting

When generating the Reflectance Map, Pix4Dmapper uses the values of some parameters present in the EXIF (205732309) of the images in order to correct (at least partially) for the factors mentioned above.

The type of radiometric corrections can be changed in the Processing options: 203891879.

 
Important: When applying the radiometric corrections:
  • The Camera Only option will apply the correction for the parameters that are written in the EXIF metadata and relate to the camera (vignetting, dark current, ISO, etc...).
  • The Camera and Sun Irradiance option will take into account the camera information from the point above as well as the information written in the XMP.Camera.Irradiance EXIF tag for radiometric correction.
  • The Camera, Sun Irradiance and Sun angle will take the sun position into account, as well as the camera information and the irradiance data. This option should only be chosen for flights that were done in clear sky conditions. 
 
Important:
  • For images not modified by the camera firmware, the original pixel value of the image is the value obtained in the sensor, thus it is better to do the radiometric correction, even if not all the factors are present, since then the values will be a better estimation of the real reflectance of the object that appears in the image.
  • For images which were processed by the camera firmware (or another software) by using filters and formulas to obtain a good image for visualization, the pixel value might not represent the reflectance values of the material itself. A radiometric correction is not possible since the factors mentioned at the top of this article were modified in an unknown way and doing the whole radiometric correction cannot work.
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