SAN DIEGO — Police agencies across the nation are increasingly using drones to improve public safety, but need clear operations policies and limits to win public trust, experts said at a law enforcement conference in San Diego.

To that end, a model policy on use of drones – or "small unmanned aircraft systems" – was rolled out Wednesday by the International Association of Chiefs of Police.

The policy, which could be adopted or revised by any law agency, sets out specific procedures for deploying a drone, lists restrictions on its use, details how data would be retained or deleted and how operators should be trained.

"Do not start a (drone) program without a policy ... Engage the public," advised Alan Frazier, a Grand Forks (N.Dak.) County part-time sheriff's deputy and member of the association's aviation committee.

He spoke to an audience of about 30 law enforcement officials from around the country and other countries attending the three-day conference focusing on technology from body cameras to digital evidence and social media.

Frazier, also an associate professor for the University of North Dakota's Department of Aviation, said his sheriff's agency has used its drones 18 times across the state in the past two years. He said they have helped search flood zones, photograph homicide scenes and track felony suspects.

He said other agencies in North Dakota haven't asked to borrow a drone and its sheriff's operators as often as he expected, but figured the program was still too new for them to think of it.

A major advantage of drones over manned helicopters, Frazier said, is cost and pilot safety. He estimated that a helicopter runs at least $1.8 million before being equipped with law enforcement gear and costs $575 an hour to run it, not counting the crew. Drone systems average about $30,000 and cost $20 an hour to run.

"A UAS can run about one-third of the missions a helicopter can run," he said.