The radiometric corrections are used to improve the radiometric quality of the data. They allow correcting the image 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.
IN THIS ARTICLE
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: Menu Process > Processing Options... > 3. DSM, Orthomosaic and Index > Index Calculator.
When producing a reflectance map, the image 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:
- Sensor settings: shutter speed, ISO, aperture
- Sensor properties: light transmission in the optics, sensing, and digitization in the chip
- Scene conditions: incoming sunlight, camera location, and orientation
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 Exif/Xmp tags for radiometric correction.
Note:
Radiometric corrections
are not applicable
f
or images that 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
type
Description
Requirements
Processing option
Camera only
Such corrections take into account the camera properties and settings (vignetting, dark current, ISO, etc...) and these parameters are obtained from the Exif metadata. For more information: Exif/Xmp tags for radiometric correction.
* Exif tags: EXIF and XMP tag information for project creation.
Camera only
Camera and Sun Irradiance
Camera, Sun Irradiance and Sun angle
For more information: Menu Process > Processing Options... > 3. DSM, Orthomosaic and Index > Index Calculator.
Sun Irradiance
Such corrections take into account the information provided by the sun irradiance sensors (light sensors). 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 camera integrated with Downwelling Light Sensor.
* XMP tags: EXIF and XMP tag information for project creation.
Camera and Sun Irradiance
Camera, Sun Irradiance and Sun angle
For more information: Menu Process > Processing Options... > 3. DSM, Orthomosaic and Index > Index Calculator.
Sun angle
This takes into account the direction of the incoming sun ray, and its projection onto the scene and onto the sun sensor.
This option should only be selected for flights that were performed in clear sky conditions.
This is more precise than the one with DLS IMU.
* 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: EXIF and XMP tag information for project creation.
Camera, Sun Irradiance and Sun angle
For more information: Menu Process > Processing Options... > 3. DSM, Orthomosaic and Index > Index Calculator.
Sun angle using DLS IMU
Same function as sun angle correction.
This option is available where there is no rig (fixed geometry) between the sun sensor and the camera. The IMU pose of the sun sensor should be tagged in the EXIF.
This option should only be selected for flights that were performed in clear sky conditions (similar to sun angle correction)
If there is an IMU embedded in the sun sensor, the orientation should be tagged in Xmp.Camera.SunSensorYaw, Xmp.Camera.SunSensorPitch, and Xmp.Camera.SunSensorRoll (in degree).
Camera, Sun Irradiance and Sun angle using DLS IMU
For more information: Menu Process > Processing Options... > 3. DSM, Orthomosaic and Index > Index Calculator.
Reflectance targets
Reflectance targets can be used to do an (additional) radiometric calibration in field conditions. The calibration target enables us to have reflectance values such that it is possible to compare data coming from several cameras in case the cameras are not calibrated. However, if you have a Parrot sequoia+ camera, you will not need targets as the cameras are factory calibrated. For more information: Radiometric calibration target and Parrot sequoia+ targetless workflow
Troubleshooting
- Error message "Unable to estimate direct sunlight fraction: The sun is behind the horizon."
- 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:
- Don't apply the sun angle correction and choose Camera and Sun Irradiance in the radiometric corrections options. For more information: Menu Process > Processing Options... > 3. DSM, Orthomosaic and Index > Index Calculator.
- Edit the GPSTimeStamp EXIF tag of the images, changing the time from local to UTC.
- 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:
- Empty pixels/holes in the orthomosaic:
- If certain pixels cannot be radiometrically corrected, they are marked internally, and not displayed (i.e. they are transparent). Such pixels appear as holes in the orthomosaic or index images. Pixels cannot be radiometrically corrected if they are over or underexposed. This happens most often for highly reflective objects, such as cars or roofs.
Book Reference
"Introduction to Radiometry and Photometry" by William Ross McCluney