This magazine isn't online, but you can get it on your iPad, Kindle or Android tablet via NextIssue. The article is a pretty basic overview, with examples of using a Rite Wing Zephyr and a hexacopter along with a Canon s100 modified with the IR filter removed. But it's notable that drones are getting this kind of attention in agriculture. 

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Comment by Deon van der Merwe on October 31, 2013 at 1:19pm

I expect you will be happy with the results from a MaxMax camera. They use filters that preserve the ability of the camera to auto focus correctly, and you will not have hot spots or other artifacts.

Many people use the SX260 with good results. I like the S100 (you have to call them to order one). It also has GPS, but it has a bigger sensor, exceptional dynamic range for a point-and-shoot, and it has  "DSLR-like" manual controls. Some people have experienced lens vibration wit the S100, but it only seems to be an issue on some aircraft. Probably due to the prop vibration frequency resonating with the floating lens.

Comment by Deon van der Merwe on October 31, 2013 at 1:35pm

This image shows sensor characteristics from a MaxMax-converted S100. It was done using a diffraction grating and halogen light. The spectral calibration was done using a mercury lamp. It should be fairly accurate in terms of wavelength, but the peak magnitudes are not calibrated for the response in sunlight.

Comment by Kevin Price on October 31, 2013 at 3:55pm

Mark , yeah, I am not surprised at the image quality of the TetraCam.  I have not talked to a single person who has purchased one who is impressed with the image quality.  The term I often hear used to describe the image quality is "adequate."  The comparison between the Tetracam and the MaxMax images is night and day.

Comment by James Dunthorne on October 31, 2013 at 4:45pm

Thanks for the info guys.

One problem i foresee with getting the maxmax conversion is that a true NDVI requires NIR light, and this starts at 760nm. The spectral intensity graphs shows that the conversion is not actually capturing any part of this spectrum. It is in fact a part of the spectrum inbetween red and NIR. Is this part of the spectrum actually good for looking at plant health?

If a cheap camera existed which would measure R G B and NIR (760-1100nm), it would be perfect. The tetracam does 760-900nm in the NIR spectrum with R G NIR1 but is expensive and crap resolution. The maxmax conversion maxes out at 760nm, so im unsure how useful this will be for plant health detection.

Does anyone know of any information regarding using these bands for crop health monitoring?

Comment by James Dunthorne on October 31, 2013 at 5:22pm

Comparing grayscales of a field at different wavelengths you might see what i mean (see below)

Is the conversion going to be good for NDVI, because there isn't a massive amount of reflection in this scale. Not as much as there is at 800nm anyway.

Cheers

Comment by James Dunthorne on October 31, 2013 at 5:30pm

Anyone had a play with this? Looks like it might be a slightly better solution than the maxmax conversion...

http://www.event38.com/ProductDetails.asp?ProductCode=CAM-NGB260

Comment by Kevin Price on October 31, 2013 at 5:45pm

James, as they say, "the proof is in the pudding."  We are getting r-square values using the MaxMax NDVI cameras of 0.91 for corn yields and 0.94 for tallgrass biomass estimates.  The NIR band is located along the red edge, which is where a lot of the action is taking place for many plant components.  We are getting amazing correlations with our MaxMax NDVI cameras. I have a 2,151 band $100,000 spectroradiometer that we used to collect spectral readings on 10,000 soybean plots and when we ran statistical analyses on the data, the NIR bands on the red edge were selected among the 2,151 bands as the best predictors of soybean yield.  I think the NIR band on the MaxMax camera is placed right in the right spot.

Comment by James Dunthorne on October 31, 2013 at 8:09pm

I hadn't realised you had done so much testing, i just wanted to ask some questions to ensure i am buying the right camera before i shell out a tonne of money! Thanks for the help though, i am relatively new to these cameras, i am used to operating UAVs so it may take me a little bit up time to get up to speed. Only a few weeks ago i was looking at tetracam and now i wouldn't touch it from what some people have said. This maxmax conversion is definitely the most promising camera i have seen so far, especially when combined with the Agpixel software. 

Does anyone know how much Agpixel costs? I couldnt find a price anywhere :/

Comment by Deon van der Merwe on November 1, 2013 at 5:04am
James, thanks for sharing the Event38 link. I was not aware of them.
The graph they provide can be misleading. The graph shows % transmission, and not the sensor response. The MaxMax conversion has a very similar transmission graph, but the important parameter is the sensor response. I'm not sure why the colors in their example do not appear "correct", but it may just be related to how the image was processed.
I'd like to see their camera compared to the MaxMax. If they are willing to let me do a head to head comparison, using the same methods and equipment on both, I'd be happy to share the results with the community here.
Comment by Deon van der Merwe on November 1, 2013 at 5:21am
On the series of wavelength images: if you look closely at the detail, as opposed to the overall brightness, you will notice that where you have the densest chlorophyll (the trees), the brightness is actually best in the 720nm image. Some of the dark contrasting areas in the field at higher nm values, that appear bright in the lower nm range, are due to the strong NIR absorption by moisture at the higher NIR wavelengths. This effect may result in an apparent low plant biomass in those areas that is not related to the actual biomass. When interpreting an NDVI derived from the higher wavelengths for areas like the example where there are strong differences in soil moisture, there is a danger of misinterpretation as far as biomass is concerned.

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