John Massingill carries a map of the Chesapeake Bay around in his head. He can tell you the difference between the two Wicomico rivers and the 19 places called Back Creek, and he knows which lighthouse is the one that leans.

He has to know all this, because he knows you don't.

Massingill is a search-and-rescue controller for the U.S Coast Guard. He works in the Chesapeake's equivalent of a 911 center -- a spot that could be busy with distress calls this holiday weekend as the bay fills with boaters.

The job here can be harder than trying to help people on land, because no address pops up with the distress call. In fact, many pleasure boaters who call have little clue where they are.

So before they can send help, Coast Guard controllers must ask the caller about buoys, landmarks, water depth and other details -- and then match those clues to their mental maps of the Chesapeake.

"We try to get the search out of it" with this detective work, Massingill said, "and just have the rescue."

The controllers work in a windowless command center in the Coast Guard's yard in the Curtis Bay section of south Baltimore. Their office handles distress calls, accidents and oil spills from the bay's northern tip south to the Maryland line.

When a call comes in, either on marine radio or a cellular phone, a Coast Guard radio operator usually talks to the caller while Massingill or another controller stands by to help.

The first thing they ask is: "Where are you?"

The answer is not always clear.

"I'm by buoy Number 2," is a common example.

"There's lots of 'Number 2s' out there," Massingill said -- in fact, almost all channels have numbered buoys, starting with Number 1 at the mouth.

Or, the caller might say, "I'm by Turkey Point."

Chief Boatswain's Mate Joseph Spillman chuckles at this one.

"Which Turkey Point?" asked Spillman, who supervises the Baltimore controllers.

There are, by the controllers' estimation, about 28 places called Turkey Point around the bay, including one south of Annapolis and another miles away at the bay's northern tip.

The same-name problem also goes for the two Wicomico rivers -- one a tributary of the Potomac in Southern Maryland, the other across the bay on the Eastern Shore. And that's not counting the Great Wicomico and Little Wicomico rivers, both in Virginia.

There are streams named Back Creek all the way from Salisbury in the south nearly to Delaware in the north.

To give the controller a better idea of their location, the boaters are asked a series of questions: "Where did you leave from? Where are you going? What do you see?"

Here, it helps that the bay is not very wide: Often, the boaters are close enough to land to pick out nearby landmarks.

The Coast Guard controllers have trained for these situations by going out on the bay in boats and on planes. They have studied the terrain, so they know that if boaters report seeing large red-and-white radio towers, they are near the mouth of the Severn River at Annapolis.

"If they can say, 'I passed a light that was leaning 10 minutes ago' -- well, that's Sharps Island Light" near the mouth of the Choptank River, said Chief Operations Specialist Scott Jacobs, a former controller. "All the controllers know that's the only lighthouse on the bay that leans."

After dark, however, many of the landmarks disappear. That leaves buoys, many of them set up to flash at different intervals. A green light that comes and goes every two seconds can be a valuable clue.

When all else fails, Massingill said, he asks the boater to check the depth of the water -- easily done with a fish finder.

"If they tell me it's 10 feet, then there's some areas I can definitely rule out" -- like the deep channels of the middle bay, he said.

Once controllers have a fix on the position of a distress call, they can start coordinating the rescue. Using computer programs that analyze tides, currents and wind speed, the controller can figure out where the boat is likely to drift, and send out boats or helicopters to search for the boater.

The controllers can deal with heart attacks or cases in which someone has fallen overboard. They address minor problems, such as a boat running out of gas or people who got clobbered when their sailboat booms swing around. "They don't call it a boom for nothing," Massingill said.

They also have to deal with "overdue" cases, such as those in which a woman says her husband has not returned from a fishing trip.

These are difficult, because a wife might not know exactly where her husband had intended to fish.

The Coast Guard often mounts a rescue attempt, only to learn that the missing boater is on land.

"Nine-point-five times out of 10, there's another reason" besides trouble on the water, Jacobs said.

"He could be down at Jack's [bar], pounding them," Spillman said.

Last Memorial Day weekend, search-and-rescue controllers dealt with 13 distress calls, including a collision between a sailboat and a barge, and a 6-year-old on a boat who had a dislocated shoulder.

The controllers said that if the current weather holds, this holiday weekend could be a busy one, too.

"More boats on the water, combined with a late-afternoon thunderstorm -- could turn pretty hectic around here," Spillman said.

Massingill, a civilian Coast Guard employee, said he is reminded of the job's importance every time he sees the boats on his daily commute from the Eastern Shore.

"I drive across the Bay Bridge every day to work," Massingill said.

"And I look out there and say, 'I'll take care of you.' "

U.S. Coast Guard controllers rely on many tools for search-and-rescue missions, but sometimes their own mental maps of the area are the most vital element.Lt. Andrew Ely, left, and search-and-rescue controller John Massingill check maps in Coast Guard command center.