If you’ve thought about using aerial photography to study crops, equipment traffic patterns, field drainage, etc., now is a good time to get out and take the photos, says Dr. Tracy Blackmer, research director for the Iowa Soybean Association.
“You can see a lot more from the air than you can looking through a pickup windshield or even from the cab of a sprayer in the field,” Blackmer says.
At this time of year, you can see variations in crop color that are related to nutrition, moisture, insect or disease problems, as well as stage of growth.
In a cornfield, for example, a deep shade of green can suggest nitrogen adequate amounts of nitrogen available. Lighter shades can suggest nutrition, drainage, disease, or insect problems. Color patterns within the field that may be visible only from the air are clues to some of these problems.
Matt Nelson, ISA research coordinator, has shot hundreds of aerial photos of research and side-by-side plots used in crop production economic studies conducted by the association with funding from the Iowa Soybean Promotion Board and other public and private organizations. He has several tips for growers who would like to shoot their own photos.
1) Selecting an airplane. It's very important to fly in a plane with wings above the cabin. When the wings are under the fuselage, it’s very difficult to see fields below or get clear photos of them. Planes with holes cut in the cabin floor are ideal, though not very common. These holes allow for nice shots from directly above without having to tip the airplane in order to shoot out the side window. Photos shot looking straight down, if done correctly, can often be geo-referenced with existing aerial soils maps or GPS/GIS information.
2) Cameras. A simple 35mm camera will suffice. A medium to high quality digital can also be used. A camera with an adjustable lens is very helpful in focusing each shot on the desired field. It’s a good idea to fit lenses with a UV filter to help reduce atmospheric distortions (These are available at most photo shops). If you use a zoom lens, try not to zoom out too much. The greater the focal length of the lens, the more it magnifies the bumps and vibrations of the airplane, often leaving you with blurry photos.
3) Altitude. This is a matter of preference. At lower altitude, resolution is higher and atmospheric haze is lower. However, the lower the altitude, the greater the dimensional distortion. You can have square fields that appear trapezoidal, making your photos more difficult geo-reference. At higher altitudes, distortion is lower but so is resolution and your images may be hazy. “I prefer shooting at around 3,000 ft. At this altitude, I can capture an entire 80 acre field in one picture,” Nelson says. “Also, at lower altitude, you can focus on a specific field without using too much zoom, keeping your photos sharply focused.”
4) Film. “Expensive professional film isn’t necessary,” Nelson says. “I use Kodak Gold ASA 200. It’s easy to find and is often available at the checkout line in grocery stores or convenience shops. It works well in most weather conditions, but if it’s very overcast, you might want to use 400 speed film instead.”
5) Shutter speed and f-stop. The most common mistake made when shooting aerials photos is using incorrect camera settings. Because of the speed of the airplane and other factors, you’ll need both a fast shutter speed (at least 250th of a second) and as high an f-stop setting as possible, (not less than f-11). If your camera has automatic settings, don’t trust them automatically. On your first attempt, before you get into the airplane, point the camera at the ground set shutter speed and f-stop according to the light readings there, and use those settings once you’re aloft. You can also try your automatic settings, but you’ll need to make notes of which photos are which so you’ll know what works best for the next trip up.
6) Weather. A cool, clear day, is ideal for shooting aerial photos. Trouble is, there are too few of them. Try flying early in the morning before clouds pop up and heat, humidity and haze become an issue. If there are a few clouds in the sky, time your shots so that no cloud shadows cover you’re the field you’re photographing. You may need to make several passes over the field while waiting for clouds to move off. Cloud shadows will appear as dark blobs on your images, making them harder to interpret. Don’t rule out flying on days that are completely overcast. If the clouds are high enough to fly under and not too dark, you can use a faster film (ASA 400) and still get good photos. Nelson says another reason to fly on cooler days is they tend to be smoother days. “If it's hot and humid, you may be in for a bumpy ride in a stuffy airplane with little fresh air, so take your Dramamine and a sick sack. Your pilot will appreciate it,” he adds.
7) Food. “This is a tricky one and also a matter of preference. You probably won't want to fly on an empty stomach, on the other hand, you don't want to be stuffed either. I make it a point to eat something about an hour before I go up. I also try to take a bottle of water with me on most flights,” Nelson says.
Both Blackmer and Nelson will be on hand at the Farm Progress Show near Alleman, August 31 through September 2, to help growers interpret what aerial crop photos can tell. If you have your own photos and would like help in learning what to look for, bring them with you to the show. For more information, contact Blackmer at 800-383-1423 or email@example.com.