Messages

1

General / Re: Reflectance/Radiometric Calibration on MicaSense Sequoia

« on: June 11, 2018, 06:28:09 PM »

Hello everybody,

I am still having problems with the radiometric correction of the MicaSense Sequoia.

- Firstly, the software never identifies the AirInov grey target;
- The workflow for masking of grey targets and posterior reflectance factor setup is not clear ;
- Sometime I get all black or all white imagery.
- The output range (0 to 1, 8  bits or 10 bits) should be an option on the software.

Is there a tutorial by Agisoft showcasing how can this be done? I reckon a number of users are facing the same problem.

--- SORTED/SOLVED:
Hello everyone,

Quick documentation as these problems seem to apply to a number of users:
- Sequoia creates 3 sets of images (I assume different exposure intervals), allowing for a gradient of DNs.
- Sometimes one of these images is  under/overexposed and Photoscan cannot read the QR Code in all 3 images.
- One should mask (select+right click+add to selection) the area  of the reference panel on all channels. For such, one should  click the chunk + "Set Primary Channel"  + "Green/Red/RedEdge/NIR".
- Adjust band values according to the reflectance of your panels (Micasense are usually around 0.6 and AirInov have the values printed on the panel).
- Reflectance correction should be executed prior to the workflow.
- No problems were found when also checking the "use sun sensor" option.

By doing such, problems such as complete dark or complete white imagery was solved.

Kind regards

2

General / Re: Reflectance/Radiometric Calibration on MicaSense Sequoia

« on: March 08, 2018, 02:36:26 AM »

Hello Selim,

My contribution to the understanding of the mechanics of MicaSense sensor/workflow would be:
- Sequoia TIF files have a bit depth of 16 bits. Thus, 2^16 = 65536. I would assume that the 65000 is the max value found in your imagery data set ( the white reference, possibly?); given that the sequoia optimizes the sensor to use the whole range of bit depth.
- You would not be far off from the meaningful values when normalizing/dividing by the maximum value of each band; after all, the white reference should be 1 or the max DN value.
- You would have a problem if the max DN value would be equal to MicaSense's white reference (thus, setting it to 1), as its reflectance value is actually much lower than 1 (and each panel has a specific value, I reckon that is one of the reasons why there is a QR code).
- Another point is that, without using other exif metadata, one would be not subtract/include dark current aka black level. Thus, the coefficient/slope would be different (possibly why you found different values of reflectance between different software).
- There is also vignetting effects,which are not being accounted for.

I believe that you are familiar with, but personally I found this post to be very helpful: https://support.micasense.com/hc/en-us/articles/115000351194-RedEdge-Camera-Radiometric-Calibration-Model .

To me, is not clear whether PhotoScan has incorporated this calibration as described in the link above. If not, I would argue that it should.

It may be worthwhile to incorporate some of these references in your workflow - if you think is worthwhile. After all, reflectance is a physical property and, therefore, should not vary based on which software one is using.

Cheers,
Gustavo

3

General / Re: Reflectance/Radiometric Calibration on MicaSense Sequoia

« on: March 07, 2018, 03:32:17 AM »

Hi selimerguden,

Sorry, a further extension to my last post:

If the image values are still in DN units (even if calibrated), then then workflow from MicaSense suggests than the values from the reflectance panel haven't been used yet:

https://micasense.github.io/imageprocessing/MicaSense%20Image%20Processing%20Tutorial%201.html

As the suggested workflow is:

• Calibrated digital numbers are converted to radiance units by B1/(2^16) (for 16-bit images)
• The radiance image is converted into reflectance using the radiance values of the panel image of known reflectance to determine a scale factor between radiance and reflectance. With the final values usually in the range between 0 and 1.

If this is the case, then what is Photoscan actually giving us?

I think i'm even more confused!

Cheers
Mark

Hello Mark,

You are right on your considerations. If pixel values are still in DN, no reflectance transformation / radiometric calibration was performed. 
Dividing by the max value of each band (2^16) assumes that the max value is equal to the irradiance; which is most likely not the case.  Also, this would not use any of the EXIF parameter, such as irradiance meter, cosine effect etc. Thus, much of the fine tuning of Mica Sense sensors would be rendered irrelevant.

Mark, you do have the reflectance values of your  AirInov targets?
Has someone actually succeed in going through this process?

Cheers,
Gustavo

4

General / Re: Reflectance/Radiometric Calibration on MicaSense Sequoia

« on: March 06, 2018, 01:02:45 AM »

Hello guys,

I will try tutorial in the coming days and provide an update.

I also had the white or very dark (blueish) resulting orthomosaics (as well as white input imagery)
Something that I did different from the tutorial was that I actually ran the "reflectance calibration" as the final step (tutorial indicates it should be the first) - MDC did the same, I guess.

I will play around and provide additional input in this topic/thread.

Cheers,

5

General / Re: Reflectance/Radiometric Calibration on MicaSense Sequoia

« on: March 05, 2018, 06:48:50 AM »

Also, when I export the orthomosaic, each band has values that are within 0 and 1. How can that be sorted?
Cheers

6

General / Reflectance/Radiometric Calibration on MicaSense Sequoia

« on: March 05, 2018, 06:28:30 AM »

Hello everyone,

Congratulations on adding the "Calibrate Reflectance" feature on the latest version of Agisoft.

A couple of questions about these features:
- When the workflow will be documented?
- After adding the calibration images (as a separate folder) + "Calibrate Reflectance Option" , does Photoscan reads the QR codes/identifies the reflectance values for each channel? Or should user have to insert the values provided by Micasense ?
- As per the calibration images, does the software masks out the non relevant part of the imagery (i.e. everything that is not the panels). Should we do it manually?
- The button "Locate Panels", locate panels+mask within these images?
- If it states that is missing the calibration file (eg. RP02-1603178-SC.csv) where should I source this file or what is the information that should be contained in it?
- At the moment, I am running the workflow, but I cannot notice any changes on pixel/reflectance values.

Kind regards & once again: congratulations.