VASSALBORO – Setting his tackle box aside, Tim Thurston steers his 18-foot power boat on a straight-as-an-arrow course across China Lake as he prepares to pull something other than brown trout or bass from the greenish waters.

Thurston’s objective on this sunny, midsummer day is numbers: he is measuring the water’s depth at some of the 1,200 target points that will enable him to produce a detailed and accurate map depicting how the bottom of the central Maine lake is shaped.

The technology that makes it possible is a combination global positioning system and depth recorder. It pinpoints his position at target points 400 feet apart and records the depth at each one.

“Basically, you hit a couple of buttons at key times and it records new latitude and longitude, along with the depth,” said Thurston, who uses a laptop computer in the boat to save the data.

Maine Lake Charts Inc. is a sideline business for Thurston, 41, of Pittston, whose regular job also puts him out on the water. As navigational aids supervisor for the state Department of Conservation, he and an assistant are responsible for maintaining 1,300 buoys on 25 Maine lakes.

The primary market for his maps consists of fishermen and recreational boaters. Fishermen tend to steer toward waters with significant changes in bottom structure on the theory that fish are often found there; maps spelling out the contours of the lake bottom can signal the most productive fishing spots.

His most ambitious project, completed last winter, was Sebago Lake. It was the first time that Maine’s second-largest lake, at 28,771 acres, was mapped using modern technology, he said. The Sebago map is by far his biggest seller.

The president of Sebago Lake Anglers, Don Sicotte, said his members were impressed by the new map, which is a lot more detailed than older versions.

“We see a lot more (contour) lines,” Sicotte said, noting that the information should prove especially helpful to fishermen unfamiliar with Sebago. “It tells the newcomers mainly, the guys from away, where they should be fishing.”

When Thurston began his lake mapping business in 2001, the only lake depth data available dated to the 1930s and 40s when it was collected as part of a Depression-era program designed to put people back to work.

“They would send three or four guys out with a lead line – a weight on a hand-held line that’s marked off with a knot at graduated intervals,” he said.

Those old-timers often relied on estimates and guesswork, he said. “They were guessing how far off the shore they were and they were guessing that they were running in a straight line.”

Consequently, his numbers are often far different from what he finds in the state’s data. The maximum depth on the old China Lake map is 85 feet, but Thurston found a deep hole at 93 feet. At other points, his depth finder read 69 feet where the old map showed 44 feet; at another, he found 49 feet at a point shown as 70.

He found that Sebago, the state’s deepest lake, isn’t quite as deep as previously thought. What had been generally accepted as a maximum depth of 316 feet is actually 307 1/2, Thurston said. That’s because his depth finder showed 305 feet at a time when the water level was down 2 1/2 feet.

Thurston doesn’t fault the work of his predecessors, saying technology is responsible for today’s more accurate data.

“Think back to the 30s and 40s and what they had for surveying equipment,” he said. “They were still navigating with a sextant.”

Maine Lake Charts has published 42 maps covering the entire state, from Mousam Lake in Shapleigh to Aroostook County’s Portage Lake. Sixteen of the maps have new depth data and lake contour lines that Thurston has created.

The colorful maps come in two versions: a foldable version on waterproof paper and an 11-by-17 inch laminated chart that can be mounted on the wall or used as place mats.

Aside from Sebago, other top sellers include Rangeley Lake, Cobbosseecontee Lake in Monmouth and Winthrop, Great Pond in Belgrade and Long Lake in Naples.

In addition to China Lake, he is currently compiling new data on Long Pond in Belgrade. And Thompson Lake in Oxford is on his to-do list.

“I’m just kind of picking them off one at a time,” he said.

One project he’s in no hurry to take on is Moosehead Lake, the state’s largest, at 74,890 acres. He estimates that it would take a month and a half to collect data on Moosehead, an undertaking he might leave until he retires from his state job.

Thurston begins each project by obtaining an aerial photo of the lake, which he uses to detail the shoreline. He also checks the image for any shallows that might be visible.

The mapping of Sebago took seven consecutive days, followed by three days “to verify and fill in the blanks.” Thurston covered more than 800 miles and charted depth readings at the 5,900 points that appear on the map. He actually ended up with 80,000 data points in all, which enhance the accuracy of the contour lines.

It’s during the winter, when the lakes are covered with ice, that Thurston uses the data to make the maps. His wife, Delani, handles the Internet sales and the packing.

He does most of his depth measurements early in the spring or late in the fall, when lakes are less crowded and he is able to keep to his average 12 mph speed without having to slow down for fishermen, water skiers or other boaters. During the height of summer, he sets out early in the day to avoid the crowds.

Once he has programmed the GPS with his target points, he simply drives the boat to them. “Connecting the dots is what I call it,” he said.

Even on the sunniest day, Thurston is all business. “I bring my tackle box, but I never use it. When I come out here, I’m here to work.”

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On the Net: http://www.mainelakecharts.com