Radiometric corrections

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.

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 and XMP tag information read by Pix4D Desktop.

 
Note: Radiometric corrections are not applicable for 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
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: Camera radiometric correction specifications.

* Exif tags: EXIF and XMP tag information read by Pix4D Desktop.

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 read by Pix4D Desktop.

 

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 read by Pix4D Desktop.

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

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: Menu Process > Processing Options... > 3. DSM, Orthomosaic and Index > Index Calculator.
2. Edit the GPSTimeStamp EXIF tag of the images, changing the time from local to UTC.

Book Reference

"Introduction to Radiometry and Photometry" by William Ross McCluney

 

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7 comments

  • Josh Abbott

    Hello, in the text above it mentions "Such corrections take into account the information provided by the sun irradiance sensors (from Parrot Sequoia or MicaSense RedEdge)"

    I just want to confirm that Phantom 4 Multispectral is also included?

    Thank you

    Josh.

  • Momtanu (Pix4D)

    Hi Josh, we will correct the article, any camera that has the DLS sensor (and the tags written correctly in the image EXIF) will have sun irradiance correction supported. P4m is also included, however, the sun angle tag values are 0 for this camera, thus sun angle correction will not be performed. 

  • Julien Lieb

    Hello,

    I wonder if Pix4D can deal with dataset coming from several flights with parrot sequoia (not sequoia+)?

    I have a project made from 8 consecutive flights, and it seems that the software take only the first calibration image of the set.

    Is there an option for integrate the 8 calbration images, with the time in the exif for example?

    Thanks,

    julien

  • Momtanu (Pix4D)

    Julien, Pix4D supports only one radiometric target images set. However, it can be any of the 8 sets and not the first one, you can manually import the target images you want. You can also try processing the 8 flights as separate projects and merging in QGIS.

  • Julien Lieb

    Thank you,

    Would it be possible to (radiometric) calibrate each picture before the process, with the closest calibration picture?    I'm still noob about multispec survey, maybe it has no sense but I try my question anyway! ;) 

  • Momtanu (Pix4D)

    The DLS (light sensor) is already correctly for each image. The target images are used to get absolute reflectance values. The radiometric correction happens in step 3 (at the end) so we use only one set. However, if you want to use all the targets, process all the flights (using their respective target) separately (all steps, so 8 projects) and then merge the maps in QGIS.

  • Julien Lieb

    Thank you for advice, but it is impossible, as I used interlaced flightlines on big areas.

    this is clearly the problem of the parrot sequoia, as the manufacturer only says that it is soooo simple and efficient. And when you use it, it's a bit more complicated.

    The overlap that is mentionned on the manual, and the reachable GSD for RGB sensor are totally out of reality. Big disapointment  on this cam. I now know that I can use it for single flight Multispectral only, with important overlap, with the internal memory only (SD card has a lot of bug and freeze the cam), and be very patient for exporting the pictures out of the cam (tons of corrupted files and lost files). 

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