For more than 30 years Hank and Marty Detering were contented residents of Londonderry Township. Marty was even chair of the township supervisors.

There's just one problem:

Their house may not be in Londonderry Township, but neighboring Penn Township.

And that little "oops" has already cost Marty Detering her supervisor's job.

The Deterings' placid sense of place came crashing to an end in March while she worked with the Brandywine Conservancy to place a conservation easement on their 38-acre southern Chester County farm. Looking at an aerial map that overlaid their property, Hank noticed that their house appeared to be in Penn Township.

It turns out that while two barns, a shed, the septic system, the driveway, and 22 acres are in Londonderry Township, their house may not be.

"We thought we knew where the lines were," said Marty Detering. "This was a big surprise."

They asked the county to check its records and received a call back on March 15:

Their house is in Penn Township.

Instead of being part of the Octorara School District, they were now in the Avon Grove School District. (Luckily their kids are grown and the reclassification may lower their taxes.) Three days after the call, the Deterings received new voter registration forms.

Since only township residents can serve as supervisors, Marty resigned from the board on April 10.

But the county employee may have acted in haste.

Jeff Laudenslager, director of the county tax-assessment office, said that as soon as the matter was brought to his attention, he asked the staff to put a hold on it until further research was done.

"We want to make sure we get this right," he said.

A team from the county's geographic mapping department is set to visit the area soon, said manager Pierce Eichelberger. Deeds from other properties along the same line are also being checked, he said.

"We have not reviewed that particular boundary before, and now we will," he said.

Eichelberger said that when it comes to township lines, the county only surveys them when a new subdivision comes in along a border. Even then, finding survey markers is a hit-and-miss proposition, since many of them date back to William Penn's day.

And while it will be no consolation to the Deterings, Eichelberger says the problem of questionable township lines stretches across what were the original 13 colonies.

"Even our county boundaries are called into question," he said, "but we haven't had the money to do a quality survey of county and township boundaries."

Marty Detering, who regrets being forced out of office, believes that a ground survey will move them back to Londonderry. Her husband isn't so sure, since all the points on the physical survey match those on the photo map.

"It isn't all bad. I've got my wife back," he said, referring to the many hours she worked as a township supervisor.

Plus, he remains chair of Londonderry's open space committee. It has no residency requirement.

Contact staff writer Nancy Petersen at 610-701-7602 or npetersen@phillynews.com.