Hello everyone, I am posting this because some may find it interesting and I need some assistance. I am working on a project using a modified DJI Phantom 3 Pro to get NDVI images in order to quantify changes in grazing vegetation quality over time and differences between sites. The imaging portion will be backed up with some other methods, including looking at carbon to nitrogen ratios using stable isotope analysis.
Testing to date:
So far I have only managed a few test flights and images. I am using Image J and the Photo Monitoring plugin to process my images. Here is one of my test images, I am not getting the resolution I need. I may have to change the height at which I am flying or camera settings. This will require more testing.
The image is a worn grass field used as a car park in the summer months. The grass is showing up a pretty even value and based on how worn it is I am assuming the grey is a good representation of where bare earth is. Now the evenness of the value may just be due to the characteristics of this particular field, but I am also hoping to see differences within the field itself. I may need to fly lower, currently the autopilot software I am using puts a floor on my flight at 40m. Or something in the camera settings may tease out a bit more detail. The lens is a PaeuProductions lens and the image is using the NDVI_VGYRM Lut.
What I need to be able to do in order to compare images between sites and over time is a way to extract numerical values for the images. What would be ideal is to get a max NDVI value for an image and a mean. This may already be possible, however I cannot locate a plugin, piece of software etc. that would allow me to do this. If someone here has experience with this any assistance would be greatly appreciated.
PeauProductions has this filter here:
This must be a dichroic filter. The narrow bands could produce good results. We don't know how much cross contamination there will be because we don't know the sensitivity of the blue channel to NIR light, but it could be substantial. The red channel is probably not very sensitive to blue light in that narrow band, so the red channel should have a pretty pure NIR signal. The PeauProductions site seems to suggest that there is no "spectral cross-talk" between the bands, but that is false and depends completely on the camera (Bayer filters) being used.
Unfortunately the visible light it captures is blue, so it will do a poor job producing NDVI that distinguishes healthy from stressed (or dead) foliage. That should be added to my list above of things that will allow more discrimination among levels of plant health -- 5) use a red filter, not blue.
Do you know what shutter speed and ISO you were able to use with this filter? Not much light passes through this filter, so ISO might have to be high to allow a fast shutter speed. High ISO will produce noise in the photos, but that might not matter much for NDVI images.
I'm a little confused about something. NDVI for healthy foliage should be around 0.5, and that requires the NIR:VIS ratio to be about 3:1.
Your filter passes equal amounts of NIR and blue light, and sunlight has a ratio of NIR:blue at those wavelengths of about 1:2.
In order for NDVI values to compute to reasonable values, the red channel (which is capturing NIR) has to record values three times higher than the blue channel (which captures blue light). We don't know what the sensitivity of those channels is to the two bands of light they are capturing. Assuming they are equally sensitive, the light captured by the red channel (NIR) would have to be six times brighter than the light captured by the blue channel (blue). I guess healthy foliage reflects an order of magnitude more NIR than it does blue, so that works. A complicating factor is that the blue channel will be contaminated with NIR light and will be brighter than it should be. But it must work out to be close enough to produce NDVI that looks right.
This might be why some blue filters can provide direct computation to NDVI without custom white balance. When a red filter is used, the blue channel captures NIR and that channel is not as sensitive to NIR as the red channel is to red. Maybe I am less confused now.
I think you are right about a DIY approach to calibration. You might not need to know the exact proportion of incoming NIR and blue reflected from your targets. If you are close, and use the same targets and values in all your photos (or missions), you should be able to correct for the day to day (or hour to hour) variation in light quality and sun angle.
Your camera captures a rather pure NIR (800 nm) image in the red channel. You can use that to measure the relative NIR brightness of various target materials. A normal camera could be used to measure the relative blue brightness of the targets. It would help a lot to have some materials with known spectral reflectances, so check this out: http://biodiversityinformatics.amnh.org/interactives/spectralcurves.php.
Is this a question? Click here to post it to the Questions page.
Chris, Thank you again for all the great information. I would assume the filter you found on PeauProductions website is the one in the lens I purchased from them. That would only make sense. When I shot the composite image above, camera settings were ISO 100, 1/320 shutter speed, 0 exposure comp, normal colour saturation and white balance set to cloudy.
As I understand from the information you provided, the camera should be pretty good at providing NDVI images. What I still need to find is materials for calibration. Tar paper, and pine are not necessarily ideal materials. I found this: http://www.mosaicmill.com/products/hardware/ndvi_tools.html while looking around and it appears to be just a white, middle grey and black panel. However they do not provide any real information. I am wondering if I could do something similar. It would be nice to find a material that reflected quite high in the 450nm range and one quite high in the 800nm range. As I understand it that would provide me with the best form of calibration. Although essentially anything would do, so long as I know the ndvi value of the material or not?
Aluminium has a pretty high reflectance of all wavelengths, I wonder if I could use it somehow? Making a panel out of aluminum foil would be quite easy and inexpensive. I may have to test this. I will continue to look further, and unless you can think of a reason I am oblivious to that foil is a silly idea, I will try and take some test shots.
Is this a question? Click here to post it to the Questions page.