Q: Drone imagery is useful, but not scalable. How much can you really expect to cover with a drone?
A: SAIRS has refined service delivery to near military precision. Covering acres means hard work and dedication under a very tight window of opportunity. Your harvest operation is a good analogy. When the weather is right and harvest conditions cooperate, your combine doesn't sit. On a good day, one combine might take off 200 or more acres. Likewise, on a good day, one UAV might cover 2000 acres. When crops are ready, flights often start by 5:30 am and don't stop until Mother Nature says so. Its not uncommon to keep flying all day on overcast days where wind speeds don't pick up throughout the day and lighting conditions remain constant.
Q: How much wind can the drone fly in?
A: The senseFly eBee used by SAIRS is rated to fly in 45 kph winds. SAIRS policy is that where winds on the ground are measured in excess of 20 kph, we won't fly. We've learned from experience that wind speeds at 400 feet can be significantly higher than readings on the ground.
Q: I've heard that variable lighting conditions can dramatically impact NDVI values, making comparisons across space and time difficult.
A: This is true. Much confusion exists around this comment. Whether to trust NDVI values largely depends on how aggressively we intend to follow the outputs and what the field variability is before introducing lighting variability. For instance, in the regions where SAIRS provides service, the land, and therefore the crop is generally highly variable, showing massive differences in the NDVI. Flight operations for conservative on/off fungicide prescriptions need to do little more than differentiate areas of crop vs areas of no crop. There is such a large spread in NDVI values between these dramatically different areas, that variances in lighting values don't have a remarkable impact. In other words, the cloudy day NDVI still delineates the same burned out, flooded out, and otherwise heavily damaged areas as the sunny day NDVI.
Where the difference arises is when we try to draw conclusions from very even and consistent crops. Where a crop has a uniform and even stand that shows little or no difference to our eye, variances in NDVI value may be attributed to minor, almost imperceptible variances in crop health OR differences in lighting values. Here it is critical to understand whether it is crop health or lighting conditions that are impacting NDVI values. Where we cannot positively exclude differences in lighting as the source of the different NDVI value, no conclusion should be drawn.
Q: Do we really need the high spatial resolution that drone imagery provides? Why not just set up our prescription from 10 meter satellite resolution? That's the best resolution my sprayer can deliver anyway.
A: Yes! When higher resolution is available at the same price, go with the resolution every time. In SAIRS opinion, the value of high spatial resolution is often overlooked or downplayed.
Consider the challenge of positively identifying a red area in an NDVI. You're not sure what you are seeing but need to be to be certain with your prescription. Firstly, consider that the drone NDVI you are looking at is actually derived from an image that has captured visible spectrum and near infrared information at the same time. Even though the near infrared influences the color values of the visible spectrum image, it does not influence spatial resolution. The fortunate bi-product of every drone NDVI is the high resolution output in the visible spectrum. In practical application, high spatial resolution contributes like this:
I am studying an NDVI to decide which areas aren't in need of fungicide. I see a red area, possibly meaning poor crop health but then again, it might be shadow cast by a cloud, or maybe even a portion of the field that isn't cropped. I flip over to the high resolution visible layer and zoom in a little. I immediately identify the area as standing water. I zoom in a little more and identify two Canada geese swimming in the water. I can also see that there are very few canola plants in the area immediately surrounding the water but confirm that crop rows are present. I have just used high resolution visible spectrum imagery to corroborate NDVI values. I've identified what I am looking at and strengthened confidence in my prescription.
Q: I can see the value in the higher resolution imagery. How much extra?
A: No extra cost. In post processing we can scale back resolution to mimic satellite values and make the data lighter and easier to work with. Yet native (original) resolution is always there and available if you need it. As always, we strive to stay competitive with satellite imagery.
Q: Can elevation maps be obtained from fungicide flights?
A: Yes. Again at no extra cost. Elevation data is obtained equally as well from an NIR photogrammetry as from RGB photogrammetry. It should be noted however that SAIRS flights are not RTK meaning absolute horizontal/vertical accuracy is only at 1-5 meters. Relative accuracy at 400 foot flight altitude is at consistently better than 5 inches. As a general rule, the elevation maps produced coincidental to a fungicide application map should only be used only for agronomy and basic drainage planning.
More questions for SAIRS? We'd be happy to answer. Direct your questions to firstname.lastname@example.org or the Contact SAIRS link in this website.