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Altum/RedEdge MX processing options in pix4d mapper 4.4.12

I have an Altum camera + DLS2 (same as RedEgde MX + one thermal) and used it 3 separate flights about 1 h apart of the same field between about 10am -1pm. I used the supplied calibration panel in each flight. Day was cloudless with very little humidity so ideal conditions. Sun irradiance was constant during flight as measured by a field spectrometer. Thus the main variable was the sun angle. Now I need to select a variant of the Pix4D mapper workflow to process the spectral data with and without DLS then compare each reflectance dataset. I can’t find info which processing option is preferred for Altum/RedEgde MX imagery aside from this:

Radiometric Processing and Calibration

Allows the users to calibrate and correct the image reflectance, taking the illumination and sensor influence into consideration. It is possible to choose the type of radiometric correction to be done:

  • No correction : no radiometric correction will be done
  • Camera only : corrections will be applied to the parameters that are written in the EXIF metadata and relate to the camera (vignetting, dark current, ISO, etc…): Camera radiometric correction specifications
  • Camera and Sun Irradiance : corrections will be applied to the camera parameters (same as using the option Camera only) as well as for the sun irradiance information written in the XMP.Camera.Irradiance EXIF tag.
  • Camera, Sun Irradiance and Sun angle : corrections will be applied to the camera parameters and the sun irradiance information (same as using the option Camera and Sun Irradiance) as well as to take into account the sun position. This option should only be chosen for flights that were done in clear sky conditions. 

Which I found here:

https://support.pix4d.com/hc/en-us/articles/203891879-Menu-Process-Processing-Options-3-DSM-Orthomosaic-and-Index-Index-Calculator

Latest Pix4D Mapper has another option:

Camera, Sun Irradiance and Sun angle using DLS IMU

Which option is preferred for calculating scene reflectance? How can I determine if Pix4D has used the DLS data?

Thank you. 

Hi Bogdan, Sun angle correction cannot be used for micasense cameras, it is generally used in a fixed-wing drone like E-bee. You can use sun angle using DLS IMU if the tags are available. Otherwise, use the camera and sun irradiance.

It is always recommended to fly perpendicular and not too early or too late, to avoid the sun from a low angle (around noon). Otherwise, your map might have stripes. Using sun angle can correct that, but again, sun angle correction is not recommended for overcast conditions. When capturing images in cloudy conditions, the camera may be over an area that is shadowed, but the sunshine sensor is registering as clear sky conditions. From this, you can see why we cannot compensate cloudy conditions, and we always recommend to fly under the clear sky or uniformly overcast. 

You can see which tags are present by the tool exiv2. Our software uses this tool to read the tags and use them for the radiometric calibration.

How to determine if Pix4D has used DLS data: If you do not have the tags, Pix4D will not do the radiometric calibration. The correction type will be red and disabled. Camera correction does not need DLS data. For sun irradiance correction and sun angle correction, you will need the data. Also, the radiometric correction table in the quality report will say which type of correction has been done.

P.S. The calibration target enables to have an absolute reference, which would allow you to get absolute reflectance values and make it possible to compare data coming from several cameras. However, it is not necessary if you would like to compare data from the same camera (e.g. several different missions acquired with this camera), as there is the sunshine sensor. The latter serves as a reference for the camera. In this case, the reflectance is given on a relative scale, and it is necessary to process with the irradiance data from the sunshine sensor.

Hi, Thank your for the clarification. I used this option: Camera, Sun Irradiance and Sun angle using DLS IMU because this option was available and the “tags” were not red. Pix4D calculated what I think is reflectance for each band and put it in this folder:

project_name/4_index_reflectance/tiles

I picked a tile and read the reflectance in a pixel that was covered with a uniform grey reference tarp, or a tarp that has a reflectance value of 0.32 (used R script to open _noalpha_reflectance_blue_1_1.tif and read pixel values). Then repeated the same for each band but not LWIR band. I finally plotted tarp reflectance for bands 1-5. The result is disappointing: in some bands, for ex. 1, 2, and 5, reflectance values are in good agreement with those of the reference tarp (0.32). Bands 3 but especially 4 (red edge) show very poor agreement. Thus, it appears the reflectance calculation in pix4D is flawed. Do you have any feedback to what can be done to correct it?

Bogdan, What happens when you use camera and sun irradiance? Are the values in accordance to the tarp? Altum has some issues with the red band radiometric correction which will be fixed in the next release of mapper. But the red-edge should not behave weirdly. 

I have not processed the imagery using “camera and sun irradiance” option but based on this initial data and data from the other 2 reference tarp surfaces (not shown) the Pix4D calculated band reflectances are inaccurate for BOTH red (band 3) and red edge (band 4). I then expect most vegetation index calculations, including NDVI, NDRE, and other indices calculated using these 2 bands to be inaccurate. In addition, ALTUM’ DLS camera is designed to calculate sun angle so I would expect any user to use the " Camera, Sun Irradiance and Sun angle using DLS IMU" option. I also expect Pix4D to disable this last option for Altum camera processing if there is a known issue. Thus, I am not planing to continue the evaluation Pix4D in the near future. 

Hi Bogdan,

You can write us a support ticket with your images so that we can examine further. Yes you are right the option with DLS IMU should work. However, I wanted to know if the issue is there with the other option to rule out or troubleshoot more.