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.