Radiometric corrections

The radiometric corrections are used to improve the radiometric quality of the data. They allow to correct the images reflectance, taking the scene illumination and sensor influence into consideration. Radiometric calibration and corrections of multispectral images can be performed during Step 3. DSM, Orthomosaic and Index of a project.

Radiometric calibration

Radiometric calibration of multispectral images can be performed during Step 3. DSM, Orthomosaic and Index using a calibration target. The calibration target enables to have an absolute reference, which would allow to get absolute reflectance values and make it possible to compare data coming from several flights and cameras. For more information: 206494883.

Radiometric corrections

Radiometric corrections of multispectral images are applied during Step 3. DSM, Orthomosaic and Index and can be set in the Processing Options. For more information: 203891879.

When producing a reflectance map, the images pixel value depends on many factors, which need to be corrected in order to get a radiometrically trustful measure of the terrain reflectance. These factors include:

  • Incoming sunlight irradiance
  • 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 of the images in order to correct (at least partially) for the factors mentioned above. For more information 205732309.

 
Note: Radiometric corrections are not applicable for images which were processed by the camera firmware (or another software) using filters and formulas to obtain a good image for visualization. In such a case, the pixel value might not represent the reflected radiance values originally measured and thus the reflectance cannot be computed.
Correction
type
Description Requirements Processing option
Camera only Such corrections are applied to the parameters that are written in the EXIF metadata and relate to the camera (vignetting, dark current, ISO, etc...). For more information: 115001846106.

* Exif tags: 205732309.

Camera only
Camera and Sun Irradiance
Camera, Sun Irradiance and Sun angle

For more information: 203891879.

Sun irradiance

Such corrections take into account the information provided by the sun irradiance sensors (from Parrot Sequoia or MicaSense RedEdge). Such sensors provide a record of the light conditions during the flight in the same spectral bands as the one captured by the multispectral sensor.

Provided with such information, Pix4Dmapper can normalize the images captured during the flight and thus allows to compare images taken in different illumination conditions.

* Sun irradiance sensor: flying with a Sequoia Sunshine Sensor or MicaSense RedEdge Downwelling Light Sensor.

* XMP tags: 205732309.

 

 

Camera and Sun Irradiance

Camera, Sun Irradiance and Sun angle

For more information: 203891879.

Sun angle

Such corrections take into account the angle between the sun irradiance sensor and the sun. This option compensates for the directionality of the sun reflections. 

This option should only be selected for flights that were performed in clear sky condition.

* Known geometry: The relative angle between the sun irradiance sensor and the camera must be known and tagged in the EXIF for every image (e.g. using Sequoia with Parrot Disco-Pro AG or senseFly eBee drones, Micasense RE(M) or any arbitrary camera-drone setup).

* XMP tags: 205732309.

 

Camera, Sun Irradiance and Sun angle

For more information: 203891879.

 

Troubleshooting

If the error message "Unable to estimate direct sunlight fraction: The sun is behind the horizon." appears, it is most probable that the GPS firmware had some inconsistency in the timestamp writing (e.g. images tagged in local time, images without timestamp, etc.).

In order to apply the sun angle radiometric correction, Pix4Dmapper expects the EXIF tag Exif.GPSInfo.GPSTimeStamp to be in UTC time (the universal norm for GPS). If such an error message appears, there are two workarounds:

1. Don't apply the sun angle correction and choose Camera and Sun Irradiance in the radiometric corrections options. For more information: 203891879.
2. Edit the GPSTimeStamp EXIF tag of the images, changing the time from local to UTC.

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