There have been recent posts on the “wall” about scientific and “toy” cameras for mapping. The focus is on NDVI which is simply an index that provides information about the difference between reflected red and near-infrared radiation from a target. It's an index because it is unitless and it is normalized so values always fall between -1 and +1. It tends to be a good indication of plant vigor and has been correlated to different aspects of plant productivity.
In any digital camera that I'm familiar with a pixel starts its life as a voltage. The next step is where the the scientific and point-and-shoot cameras diverge. In a scientific camera voltage is simply calibrated to output radiance and in a point-and-shoot camera it follows a more complex processing path to output something pleasing to the human eye. Scientific cameras are trying to measure physical variables as accurately as possible and point-and-shoot cameras are trying to make a good looking photograph – science vs art. Point-and-shoot cameras are more complex than scientific imagers but they use lower quality and mass produced parts to keep the costs down whereas the scientific cameras use precision everything which are produced in low volumes. That's a brief summery but the bottom line is that the two different cameras are designed for different uses. To imagine that a camera designed for making pretty pictures can be used for scientific studies seems a bit ludicrous – or does it? It' depends on what you want to do.
There is a good bit of work going on to try and convert point-and-shoot camera from an art tool to a scientific tool. This is an area that fascinates me. I realize there are serious limitations when working with low quality sensors and imaging systems but some (perhaps many) of those radiometric and geometric imperfections can be modeled and adjusted using calibration techniques and software. For example, there are a few articles in the peer-reviewed literature about people calibrating commercial digital cameras (usually DSLRs) to record radiance and the results are pretty encouraging. I have been developing my own work flow to calibrate point-and-shoot cameras although I'm using simple DIY approaches since I no longer have access to precision lab equipment that would allow me to more accurately characterize my cameras. If anyone is interested I post my calibration experiments on the Public Labs web site (http://publiclab.org/). I'm always looking for feedback to advance this work so comments are welcome. My intent is to convert simple cameras to the best scientific tools that is possible.
When deciding which instrument to use you need to consider the goals of the project and available financial resources. For the financial resources you need to consider purchase cost, maintenance and replacement costs if it gets damaged. There is no comparison from a cost perspective. On the bargain side of scientific imagers you should expect to pay a few thousand dollars and if you want a large format mapping camera it's in the ball-park of $1 million. The precision/scientific-grade cameras are very expensive, require careful maintenance and recalibrating (can also be costly), and if you have one in a UAV that crashed you will likely lose a lot. You can get a used digital camera and convert it to an NDVI capable imager for well under $100 or purchase one designed for mapping like the Mapir for about $300.
What about accuracy, precision and stability? Clearly instruments designed with these qualities in mind will be better than something made to make pretty pictures. A more appropriate question is what is good enough for our purposes? I'll focus on NDVI mapping and it's important to realize different applications (e.g., creating 3D point clouds, ortho-mapping, land cover classification) will have other qualities to consider. One important factor to consider is radiometric accuracy. Although I'm trying to improve what we can get from point-and-shoot cameras I realize I will never attain the accuracy or precision possible with scientific imagers. How important are radiometric qualities for NDVI mapping? In most of the applications I see on this and similar forums people are mostly interested in relative changes in NDVI throughout an image and not absolute NDVI values. Some folks want to monitor NDVI over time and in that case it's important to be able to standardize or normalize NDVI but that is possible with calibration work flows. For these applications a well designed and calibrated point-and-shoot cameras can perform good enough to provide the information required such as to spot problem areas in an agricultural field. One point that is often overlooked is that close-range imaging and NDVI typically do not go well together. The problem is that we are imaging scenes with leaves, stems and soil and at the fine resolution provided by most point-and-shoot cameras we are trying to get the NDVI values from very small areas on the ground. For example, we can see different parts of a leaf and each part of the leaf is angled somewhat differently which will effect the NDVI value. Our scenes tend to be very complex and you can have the most accurate and precise instrument available and you might still be disappointed because of the physical issues (bi-direction reflectance, mixed pixels, small area shadows...) that create noise in the images. It is certainly nice to reduce as many sources of noise as possible but with a scientific camera I'm not convinced (at least not yet) that the improved radiometric performance is significant enough to overcome all of the noise coming from the scene to justify their use.
As far as the Mapir camera I received one of these last week and am trying to set time aside to calibrate it and see how well it performs. My initial reaction is that it is a nice compact camera well suited to small UAV mapping. I would prefer a red dual-pass filter but I expect that and other enhancements will become available in future versions. I like the fact that someone is focused on developing practical low-cost mapping cameras.
I welcome any comparisons between cameras and hope we can work together to improve the output we get from simple inexpensive cameras.
Hello! Has anyone done real tests of Vegetation Indices with Mapir 3 and its radiometric calibration panel?
I'm trying to stay out of this because I would consider myself biased. I'm a big fan of Micasense and a big supporter of John Sulik (I've read many of his white papers).
So, take this as you will I guess.
I agree with you LW, and I believe that Micasense does as well. This is why they're dedicating so much of their resources to R&D while other companies are dumping all of it into marketing and trying to catch the gold rush of the drone boom.
Micasense isn't the only company putting a lot of money and effort into researching and testing capabilities (like identifying infestations), but they're definitely one of the leaders. This is a work in progress, and i'll support any company that is continuing to develop the capabilities.
NDVI tells me I have biomass and to a degree how much over the area of my field. That's good and I can get some information from that. But more is needed so instead of the Right Tool For The Job maybe I should ask what is the Right Script For The Job? I flew cotton yesterday with a good infestation of spider mites and aphids. We've known this was there for two weeks and the producer finally said its time to spray.
Spider mites cause small rust like growth on leaves. Aphid cause a sticky shine on the leaves. Both easy to scout.
Issue is NDVI and other indices can't tease that damage out from the crop canopy. I said canopy not individual leaves. Doesn't matter if its a Rededge or a camera out of a Cracker Jack box it can't detect canopy damage unless its really bad then it maybe too late. If Airwolf or Sulik or anyone else could write a script that manipulated vegetation indices to tease out important information other than the state of biomass that would be truly beneficial.
Thanks Airwolf. Do you plan on performing some more in depth comparisons as well? I like this idea and would be interested in seeing some more.
Did you happen to make a comparison between stitched results? Did you have access to a spectrometer?
What are the alternatives when it comes to purchasing red filter for S100 ?
I cannot find anything more than just publiclabs DIY kit. Can Anyone post some links to where can I buy one?
MidOpt (http://midopt.com/) sells a dual pass 660/850 (passes red and NIR) filter that from my experience is superior to other filters I have tested. I have a colleague who put one in an S100 and it took a good bit of tweaking to get the focus set properly but he is very happy with it now.
Thanks Ned , I will give it a try. I wonder what are the drawbacks of setting a filter on the camera lens instead of inserting it inside? Of course I willhave to first take out the IR filter. But the ability to so easily interchange filters for example Blue vs Red on the same camera, and experimenting with it , is very teasing.
here is one suggestion.