[the Phantom], and I used the standard DJI app to fly it and take photographs. It doesn’t have a way of automating a flight mission, so it was a manual flight. The image on the iPad shows you where you are, and I had to estimate where I needed to take photos. I would move forward 70 feet and take a picture, move forward 70 feet and take another picture, and so on. I was trying to fly in swaths, so I would move over 70 feet and start another swath. I thought I was doing a pretty good job of creating a nice grid of photos.”
Later, with the GPS coordinates for each photo displayed on a map, he could see that instead of a regular grid of photo points, the points appeared to be scattered randomly over the project area, with more in some areas and few in others. In the article on page 7, Keith Steele, managing partner at Sitka Technology Group, explains what this meant: not enough coverage to produce a 3-D model.
Flight planning software lets you specify the amount of overlap needed between photos. Overlap allows image-processing software to identify common points between the photos in order to stitch them together into mosaics and create 2-D and 3-D maps and models.
“Overlap is important, especially when you’re flying over timber,” Gakstatter says. “The normal overlap is 70 percent—that’s what I tried to use at Yankee fork—but you need 90 percent over a heavy tree canopy or ground that is homogeneous, like an ag field, where there is not a lot of variation in the terrain.”
Gakstatter recently used his UAS in a small project in New York, over a heavy forest canopy.
“The results just weren’t good at all,” he says. “There was a nice lodge right in the middle of the project area, but with the tree canopy the software had a devil of a time matching up the images after the fact.”
This image, a sample from Maps Made Easy (www.mapsmadeeasy.com), is a georeferenced, colorized relative elevation and NDVI image created from 124 geo-tagged images taken from an altitude of 330 feet with a UAS-mounted Canon PowerShot S100 fitted with a special near-infrared filter. This is a portion of a larger image of a 50-acre site in Chile, from Latitud Drones (www.latituddrones.cl).
Gakstatter uses Map Pilot, an iPad/iPhone flight-planning app from Maps Made Easy (www.mapsmadeeasy.com).
“This is a simple app—$10—and it has some limitations, but at least you can define a polygon of the flight area and the amount of overlap you want, and then it will automatically compute the location of each photo point that it will take. It also computes battery time, which is very useful,” says Gakstatter.
Maps Made Easy also offers an online map-processing service. You upload imagery taken by your UAS, and Maps Made Easy stitches them together and produces high-resolution maps. The service works on a point system. Small maps with 125 points or less are free, which, according to the company, lets users test the system and run sample areas before running big maps. All new accounts come with 500 points as a starting balance, enough to map 200 acres, assuming you need 60-percent overlap and 2 inches per pixel. You buy additional points as needed. $9.99 gets you 300 points; $669.99 buys 50,000 points, enough to map 30,000 acres.
Another lesson learned: battery power is limited.
“At Yankee Fork I spent two batteries, which is about 40 minutes. Since you can fly for about 23 minutes on one battery charge, every minute counts,” says Gakstatter.
Near the end of the last flight at Yankee Fork, with the Phantom running low of power, Gakstatter took a short-cut to the landing zone. Instead of moving to a point above the landing site and descending slowly, he approached in a straight line. In his haste, the Phantom clipped a tree and crashed—fortunately, from a low altitude. The only damage was to a propeller, an easy repair.
Now, with more experience under his belt—and the Map Pilot app— Gakstatter reckons he could fly the same area in 15 minutes, on less than one battery charge. Since the Yankee Fork project, “I’ve tried to do all of my missions on one battery, so I don’t have to stop and switch,” he says.
However, having spare batteries and a means of charging them from a car battery are must-haves. A UAS fighting a light breeze might drain two or more batteries over an area that would take one battery on a calm day. Gakstatter is planning for a project with a larger area—about 165 acres—that he figures will take more than two hours of flight time and six full battery charges to complete. Depending on the wind.