While there are no rules set by the Federal Aviation Administration process for flying unmanned aerial vehicles for agricultural uses, farmers who are buying them are following rules set for hobby airplane enthusiasts. The FAA limit set for that purpose is 400 feet above the ground.
Just how high is that? At a recent demonstration one company took its UAV up to 380 feet, the preset limit in the controls for the machine. They set it at 380 feet to make sure they're staying within the legal limit.
Once it gets close to that range it disappears from sight with the naked eye. It's time to have blind faith that it will return to you. When it begins to descend it comes back out of the clouds and you can see it again.
Take a quick look at the picture here and you may think it's just blue sky and white, puffy clouds. Look closer and you can see a UAV in the lower corner of one of the white clouds. At this level it is well below 400 feet, or even 380 feet.
Whether or not you want to go to as close to 400 feet in the first place depends on what you're trying to do, officials with Crop Copter say. If you're wanting broad brush pictures to answer certain questions, you might run it fairly high and fairly fast. If you want to have images that zoom in closer, then you may run at a much lower altitude and perhaps a slower rate of speed.
Battery run time on UAVs depends on how much you're willing to pay. Generally as the price goes up so does battery life, increasing the amount of time you can stay in the air and the range you can go out.
The Crop Copter people note that 20 minutes of battery life meaning 20 minutes of flight time doesn't mean you can scout for the full 20 minutes. You must allow time for the UAV to return to home base. That's included when they calculate flight time.