Micasense calibration target for Sequoia

[Allison_Shaw]

Hi there,

We used a Micasense calibration target (not sure which version, but guessing older since Pix4D doesn’t do automatic marking–see photo) with a Parrot Sequoia camera.
Only 1 of 7 eBee Plus flights had the green calibration photo not overexposed according to Pix4D’s automatic cutoff of dn < 63,000, and only 2 had the red calibration photo not overexposed, and they were early morning flights with long shadows which are not recommended.  When using a Phantom 4 Pro, we had some green and red calibration photos overexposed but not nearly as many.  We had no overexposure when using an Airinov calibration target instead of Micasense. Is the above Micasense panel just not a good choice when using a Sequoia camera due to being bright white? 

Is it just a coincidence that flying with the eBee Plus caused more overexposure than with the Phantom 4 Pro?  We don’t have the time to cover 1000 Ha sites with the slow Phantom 4 Pro. 

I guess we are stuck relying on just the sun sensor, no calibration, for the whole site last year since the one flight that didn’t overexpose is clearly not representative of all the other flights’ conditions.  How much might it affect our NDVI results to not calibrate?

Thanks,

Allison

[momtanu.chakraborty]

Hi Allison,

The target above can has only automatic detection (not marking). For automatic marking, the reflectance factors will be present on the panel itself.

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.

Therefore, it is possible to process projects without using a calibration target. These can then be compared with projects captured with the same camera. However, if you have several cameras from which you would like to compare data, or if you require absolute reflectance values, it is necessary to have a radiometric calibration target.

If possible, we usually recommend checking the calibration target shots while on the field to check for under/overexposure. We would recommend taking the image of the calibration target on top of a fairly reflective surface for the auto exposure not to overexpose. For example, we do it on light asphalt and that typically works well.

With datasets having such issue we see two possible solutions

1. Remove all radiometric calibration images: then all reflectance maps would be relative (as I explained before, shouldn’t be an issue if the camera being used is the same for comparison) or

2. Only remove the radiometric calibration image for the overexposed bands. In this case, it should be taken into account in the analysis of the Index Maps that are produced (produce vegetation indices that do not require those bands, for example, if green and red are overexposed, calculate NDRE, NIR-rededge/NIR+rededge)

Let us also see what our other users think.

[Victor_Ludena]

Hello I have the same problem, I have the version 3.1.23 of the pix4d and a camera sequoia, when processing the multispectral images in step 3 I appear with exclamation the calibration part, I do not have the calibration card but I have the sunshine sensor . The question is whether processing is correct and I should not enter any value in the calibrate button. Best regards.

[momtanu.chakraborty]

Hi Victor, The exclmation means the data from DLS sensor will be used, but the calibration target is not there. Otherwise, the camera and sun irradiance would be red and disbaled (if the DLS sensor information was not there). 

We recommend using calibration target as well as DLS sensor for your projects as it is more accurate. If you are using a target, you will need to click calibrate and import the target images, mark them (draw polygon on the surfae on the target) and enter the reflectance factor values. The exclamation will then turn into green checkmarks.

If you do not have the target, and only the DLS sensor, The exclamation mark will be there. The data from the DLS however, is being used by the software.