I've interpreted your questions as asking for an efficient method to scan and area.
This is not a trivial problem.
For some years I've been looking for a method to make a robotic lawnmower that didn't use a random mow sequence.
The most simple process for your problem is as follows:
1. Determine the sensor width of the robot.
2. Create a tube on stand. The tube has a diameter of 1/3rd the sensor width.
3. Coil a length of rope around the tube. Attach one end to the tube, the other end to the robot.
4. Have the robot drop the "tube on the stand" in the middle of the field.
5. Have the robot rotate around the tube, using the length of the unwinding rope to set its distance from the pole.
This idea came from a next door neighbor who attached his powered mower to a rope tied to a tree. As the mower wrapped the rope around the tree it cut a smooth track, getting closer to the tree. As the tree grew, the mower width had to get bigger.
I'd also like to hear from anyone who has a better idea.