We want to provide good actionable information to your greens-keepers. We believe that improved data will decrease the amount of time spent finding problems via walking surveys with eyes that can only be calibrated by years of experience. Our “eyes in the sky” will produce good data that will allow your crews to focus more intently on trouble areas and solving existing problems. Likewise, course configuration changes can also be monitored and evaluated as excavation and construction work progresses.
To accomplish this, we will fly aerial drones using standard (RGB) and/or near infrared (NIR) cameras on a periodic basis. Within two days following each flight, the imaging will be completed and made available for your consideration. There will be a clear separation of responsibilities; we are responsible to gather good data and your team is responsible for interpreting and using it.
Essentially, we would like to help you in your efforts to provide your golfers with the best course that you can provide; one that is a supremely healthy and a pleasure to play.
So here is the science
Healthy plants (with homogeneously active chlorophyll) will reflect more near-infrared light and absorb more visible red light than unhealthy plants. Analyzing a plant’s “reflectance” spectrum for both absorption and reflection through the visible and near infrared wavelengths will generally provide good information about the plants health. If you look at the graph on the right, the separation between the green, light green and red lines is telling and is helpful in accurately determining turf health. This methodology is not infallible but it is good.
Here is an example image that shows the edge of a green (upper left) with clear delineation of the second and third cut. In this case, there is significant stress that increases as you move away toward the lower right of the image. Please note that this stress might not be visible to the naked eye since the grass in this location could appear to be generally lush and very green. However, closer visual inspection, “ground-truthing”, is needed to understand the issues which might be related to irrigation, nutrition or diseases such as mold, mildew, rust, etc.
Two ways to do it
There are two helpful indexes to use to measure the amount of red (low frequency) light absorption. One uses a normal (RGB) camera and the other uses a special camera that is sensitive in the red and near-infrared (NIR) spectrum of light. The benefit of one is that most drones have RGB cameras and therefore the index that uses the RGB camera is available. The benefit of the other, which is sensitive to NIR is that it is less susceptible to ambient lighting than the other so, over time, will be a more normalized indicator of plant health.
Below you will find a side-by-side comparison of the three ways of looking at a section of turf planted with a variety of grass.
Normalized Difference Vegetative Index (NDVI) attempts to identify plant stress using a near infrared sensor. The NDVI formula looks like this:
(Rnir – Rred) / (Rnir + Rred)
where R is the measure of reflectance and “nir” indicates near infrared. This index magnifies the amplitude of the near infrared reflectance and nullifies the effect of the red light. The rule is that stressed plants reflect the near infrared light significantly more than healthy plants.
This image shows a standard visible light (RGB) image of the area depicted in the NDVI and VARI images to the left and right, respectively.
This swatch of turf is planted with multiple species in a semi-organized patchwork, hence the variegation. This provides a good example of how coloration test can fool the VARI but not the NDVI.
Visible Atmospheric Resistant Index (VARI) seeks to identify plant stress using an RGB sensor (generally a standard camera). It employs the following formula:
(Rgreen – Rred) / (Rgreen + Rred – Rblue)
where R is the measure of reflectance. This index seeks to measure the absorption of red light by nullifying the effect of the green and blue light. The rule here being that red light is absorbed by healthy plants and is reflected significantly more by stressed plants.
Though the above images above might look the similar, the NDVI image on the left relies on a narrower band of red/NIR light and the VARI image relies heavily on the visible light spectrum. The VARI image appears to highlight much more “stress” where it really is just showing the areas where the red band of light is more prominent which, in this case, is probably due to the color variation between the grasses. If you review the chart at the top of this page, you will realize that the separation might not be as clear cut as it is when the NIR light is in play. So a second advantage to using NDVI over VARI is that the separation is wider and therefore potentially much more accurate. Another big reason that folks don’t embrace NDVI, apart from the lack of RGB images, is that the equipment is a bit more expensive. For instance, both the NIR camera and the drone needed to carry it are rather pricey; all told, you are likely to have $8k in the air while doing an NDVI survey.
The bottom line to all this is:
We believe in using NDVI images to identify stress areas out on your course. Each mission that we fly will produce both NDVI and standard photographic (RGB) images. If desired, the images can be “Photoshopped” together to produce composite course images that make the stress areas even more clear. This imaging will quickly become the go-to reference for discussing and managing course configuration and maintenance efforts.
By now, you’re probably interested in how much this will cost. We will fly a mission for $0.05 / yard. If your course plays 7,000 yards from the blacks, then a mission will be $350 / mission. Because the configuration of your course changes over time and turf-stress is addressed and corrected over time, it would be good to have imaging that tracks both of these over time. Our recommended routine would be to fly 20 missions each year; twice a month during the warmer months (March through October) and once a month during the colder months (November through February).
So, if you are interested and will need to promote this idea in your organization, the business case is:
Penn’s Wall will:
- take care of the required FAA licensing, the aerial mapping equipment and software
- provide your grounds keepers with two important tools
- NDVI images that identify stress areas on or around your course
- RGB images that can be used for configuration change management and NDVI overlay
Your organization will:
- use and interpret the images”ground truth” the issues that the images identify
- blending the images if desired to enhance communication of existing issues
- track/trend issues and configuration changes over time
This could be just the beginning since aerial imagery can be used for a host of other maps which include:
- contour/topographical models
- thermal imaging
- course maps, etc.