The right tool for the job

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 ( 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.  

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    • I'm also wondering about what's going on in this field. It seems like producing NDVI is more like magic than science. That Pix4D's guide is the first clear document I've seen about the subject.

      There has been a lot of talk about calibration, but very little information about the actual procedure and no mention that JPG compression screws everything up.

    • Martin - I (and other folks) have a few notes posted on the Public Labs web site: In my notes I try to explain image calibration and provide some software tools to do the calibration. It's a work in progress and my explanations are not always clear but it might be a place for you to start. The claim that "it is imperative not to use the JPG" seems a bit extreme to me. I agree RAW images are much better but that doesn't mean JPEG are useless. I'm working on methods to transform JPEG images so they have a linear response to radiance with mixed but encouraging results. The goal is to do the best with what we have.

    • Moderator

      I agree 100% and the Micrasense is one of the top 3 items listed in the grant proposal I'm writing, but the truth is that it's financially unobtainable. I have more interest in building up to that level than I do with loans or investors. 

      I did contact them after you and I were emailing last month and am hoping to test it for a few weeks while working with the local colleges, but I would also like to support continued development of more affordable options. I don't think any of us want to perpetuate false claims about their accuracy, but I'd like to play a part in finding the niche. 

      Also, 100% of my "clients" are getting images/results for free, and the agreement is that they offer me feedback regarding the accuracy and usefulness of the images and allow me to use the images elsewhere. I'm not willing to take payment until my 333 is approved and until I'm confident that they're getting something worth the money - which makes upgrading hard to do at the moment. My plan with the schools is to allow them access to platforms with (eventually) more advanced sensors in a supervised manner, but they've expressed interest in cheaper options that the students will be allowed more freedom with in the robotics class. 

    • Mario, after you factor in the cost of the modified cameras (S100s, etc.) and the time you spend it makes more sense to me just to buy the MicaSense camera, especially if you are going to use it for commercial use. Also, it would help instill confidence in your clients that you are using a scientific grade camera. It has an upward sensor for the reflectance calibration so all images can be stitched and compared over time. It easily integrates with the 3DR GPS and has all kinds of other conveniences. It will save you time, which is quite valuable, and circumvent much of the frustration that I and others on here have had trying to modify cameras. That said, modifying the cameras is a fine idea for the DIYer or hobbyist. Do the drone yourself (DIYdrones) but I suggest buying a camera that integrates easily so you can focus on the other drone issues (regulations, safety, flight planning, etc).

    • Moderator

      I'm trying to encourage more affordable and ready-made options - The process of converting cameras doesn't seem sustainable to me in the long term. Maybe I'm jaded due to a shelf full of S100s. 

    • Ned, I am looking forward to converting a Xiaomi Yi camera. It can output RAW images and has a bigger sensor than Mapir (sjcam 4000).

    • Moderator

      Thanks Ned, that was much better wording for what I was trying to say. 

      "The claim that "it is imperative not to use the JPG" seems a bit extreme..."

      and "The goal is to do the best with what we have".

      As long as the capabilities of "...what we have" is properly conveyed and understood.

      And again, thank you Ned, for all the work you continue to put into this and publicly share. 

    • Moderator
      Right, with the idea being to take the necessary steps to avoid false values in the NDVI output. So to clarify, I've been looking for a clear explanation of the differences between high end multispectral cameras and converted consumer cameras. Now that we have that, I'm wondering what use the converted cameras incapable of RAW outputs have.
  • Moderator

    I wanted to share a couple image sets-

    The S100 is RGB, and only some of the images are georeferenced. Add some GCPs before stitch.

    I generally use Agribotix's field extractor to geotag, but I used mission planner for the MAPIR this time and my offset was not correct. Having learned on the S100, geotagging after the fact is new to me. These images aren't geotagged, but I included the tlog. 

    The .tiff is what I stitched together with Pix4d, but not georeferenced.

    tiff is still uploading

    Dropbox - Error
    Dropbox is a free service that lets you bring your photos, docs, and videos anywhere and share them easily. Never email yourself a file again!
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