FAA Regulation

They can maintain line of sight and without loss of control.  They briefed each other before the flight. They are in radio contact the entire time.

Quadcopter capturing aerial imagery for crop monitoring. UAS used for motion picture filming. Model aircraft for hobby use. 

Finish the mission by paying extra attention to the surrounding. Land as soon as practical and determine if the mission can be restarted without a Visual Observer.  Continue the mission without ...

make sure the UAS is within 400 feet of the control station. maintain line of sight throughout the entire process.   have a visual observer.

The Remote Pilot in Command (RPIC). The owner of the aircraft. The FAA.

Civil and public aircraft operations. Public and military operations. Civil operations. 

Only when visual observers participate in the operation. Alone, if operating during daylight hours. Under the direct supervision of a Remote PIC.  

Operating your sUAS for an imagery company.   Flying for enjoyment with family and friends. Conducting public operations during a search mission.