NDVI Processing

NDVI Cameras - Plants, Lies and Videotape

NDVI, or Normalized Difference Vegetation Index, can be a great tool for gauging the health of of plants by remote means. Maps Made Easy is designed to process calibrated NDVI imagery only.

https://en.wikipedia.org/wiki/Normalized_Difference_Vegetation_Index

With the proper equipment, NDVI imagery can be collected with a drone and processed to create map overlays that can give a wealth of potentially cost saving information. 

"Proper Equipment" is the key phrase here. There is no shortage of companies who are willing to sell users "Ag" or "NDVI" cameras that are nothing more than hacked cameras with the IR blocking filters removed and a blue filter installed. We will not link to them here because they are all very misleading in the capabilities and accuracy. Accurate NDVI data processing requires the capability to calibrate the camera and the imagery to ensure that the ratios of near infrared light and visible light throughputs are known and can be appropriately scaled. Hacked consumer cameras cannot do this and a scientific-grade camera must be used for accurate results. (We are purposefully leaving spectral content out of this for now to keep this article easy to read.)

The reason Drones Made Easy, a company founded by a team of military multispectral imaging systems experts, does not currently offer any cheap NDVI solutions is because they are all currently not scientifically accurate. We do now offer a Parrot Sequoia based system but it is not exactly cheap. With NDVI, you can pick cheap or you can pick good, as things are right now you can't have both. The Drones Made Easy Agronaut system is the cheapest system available to include a flight control app, unlimited processing and a real NDVI camera: Drones Made Easy Agronaut Agricultural Mapping System

NDVI cameras are expensive. Currently, the most inexpensive option for real NDVI is the Sequoia by Parrot at ~$3500. Also available are the TetraCam's ADC Snap which costs ~$4500 to MicaSense's RedEdge ~$6200 camera, few people are willing to make such an investment for something that they don't fully understand when it seems like you can just buy one of these hacked cameras and get similar results.

Below is a sample of the Sequoia camera being used to make absolute measurements over time. Three visits to the same area on April 1, July 8 and August 12 show how the vegetation has dried out. These layers were all taken under pretty different light conditions and you can see it does a really good job of maintaining constant values for unchanged areas. RGB and NDVI layers for each date are shown. 

The data was take using the Drones Made Easy Agronaut system which is a DJI Matrice 100 with some custom electronics to drive a H4-3D gimbal to stabilize the camera. The custom on board computer also does the data preparation for upload to Maps Made Easy.

For further examples of the data that is taken by the Agronaut and how it is processed, see our article about NDVI Image Stack Processing.

Notice the full range of red to dark green that are representative of values that range from -.3 to 1. This is a good range and you can see that dead/non-plant objects, like road, are red. Certainly not healthy plants. The plant areas of the fields and trees are green with values from .3 to .9 representing active photosynthesis.

The numbers ranging from -1 to +1 at the bottom of the map show the scale of the index values as calculated using the following formula:

NDVI = (NIR - VIS) / (NIR + VIS)

The ND in NDVI is "Normalized Difference" and means that this is a self correcting measurement of the ratios of near infrared (NIR) and visible (VIS) light. The full range of values should take up a good amount of the space of the range from -1 to 1.

In hacked cameras, it is common for the amount of NIR light being accepted by the camera to be far different than that being collected in the visible which results in the range being calculated having a range of only .2 or .3 of the full range. If the whole image is green, does that mean that everything is healthy? Nope. It means the camera (or data wasn't calibrated properly and the ratios that got calculated do not represent what is actually going on on the ground.

Here is an example of a poorly calibrated NDVI camera:

Notice that the range is between .8 and 1. This camera was allowing way too much NIR light in or was not properly calibrated to account for the large discrepancy between the NIR and VIS light levels.

The numbers between -1 and 1 actually mean something. You can't just stretch values that were calculated as ranging from .2 to .5 over the -1 to 1 range and call it accurate NDVI data. It may look nice and have some red and some green in it, but it won't MEAN anything. The information created by doing this is not actionable intelligence. If a bad NDVI mapper gives a farmer such information and he acts on it, it will likely kill his whole crop. Luckily, farmers are smart and know better. This is where Ground Truthing comes in.

The Workaround - Manual Ground Truthing

If you absolutely must use and uncalibrated camera, you can still use Maps Made Easy to stitch and calculate your index imagery.

Our tone mapping (which can be adjusted) assumes that the NDVI calculated values of 0 or less are dead or inanimate. Values of .3 or higher are generally in the healthy range. This is the science behind using NDVI as a indicator of plant health.

Custom Tone Mapping

Tone mapping is the process where we apply a color to a calculate value. A value of .8 is nice and healthy so we give it a dark green. A value of -.3 is dead so we give it a dark red. Through the following steps is is possible to 

• Make sure the camera's white balance is fixed if you can.
• Use a fixed exposure time, if you can.
• Take some close up ground truth images of known healthy and known dead areas (like bare dirt).
• Manually calculate a few values for known good and known bad surfaces using a free tool like ImageJ to get the raw RGB values.
• Determine what your captured range is.
• Adjust the tone mapping table at the time of upload by adjusting the "NDVI Settings" to reflect your custom range.

The color map on the right is drawn from the standard values Tetracam uses that span the full range of -1 to +1. The modified color map on the right will show the compressed value range of .25 to .8 as colors ranging from red to green. This modification needs to be made at the time of upload.

After custom tone mapping, the data will still not mean anything. The values will still be uncalibrated but the imagery will at least show the colors you want to map. Maps Made Easy does not, and will not, stretch the calculated values in order to have the data "look" correct. Data is correct or it is not. 

We have never been shy about sharing our opinions on the mis-service these vendors are doing the industry. If too many people start sharing this bad data commercially, the Agricultural industry will very quickly get turned off to the technology when in fact it could be a great tool as long as people do it properly.