Here's a primer on custom geofencing with Solo, which has never been easier. It has just been released in the new 3DR Solo app for iOS and will be coming to Android soon:
On your map view, you can enable or disable geofencing. When it’s on, you will have four points (like dropped pins) that you use to make a virtual quadrilateral flight cage around your drone. Solo uses GPS to set and obey this boundary. You can move your four points to change the shape, size and location of your quadrilateral at any time in flight, blocking off any objects or areas you choose. This also means that if you want to fly in another area, simply move your pins on your screen and you’ll create a new safe zone — or you can turn the geofence off altogether.
Why this is important:
Safe: Because it’s a hard fence, it really does keep your drone away from objects, even power lines.
Customizable: If you don’t want that hard fence there, you can move it to where you want it, or easily disable or enable it.
Engaging: Perhaps the most important advantage. Instead of stopping you from thinking about the environment around you, like hardware sensors would do, geofencing keeps pilots engaged and aware. This level of interaction with your drone and with your environment is critical for any truly holistic approach to drone safety, as opposed to blindly trusting an imperfect technology.