Laser and sonic range finders can measure the distance to ground. The biggest problem in ground based robots, vision, is not needed. A robot plane recently crossed the Atlantic Ocean. Every year colleges compete with robot helicopters in an international competition that is similar in difficulty to crop spraying. It would be very difficult for the pilot to kill himself.
Robots are also very good at remembering safety procedures.
They also don't get mad or tired. This increases profitability considerably. By removing the weight of the pilot and all of his safety gear you get a lighter aircraft. In fact, it is no longer necessary to build spray planes to the human scale at all. A spray plane could be one of a swarm of planes small enough to fill the need of the job at hand.