On a quiet evening late last week, Angela Cywinski carefully slid the brown paper wrapping off a framed map at the Aquinnah old town hall and set it aside.

The black-and-white map — a certified copy dated 1971 and bearing the official seal of the commonwealth — shows the town line between Gay Head and Chilmark as drawn by Jeremiah Pease in 1855. The line cuts straight through Menemsha Pond, then meanders down the middle of the channel to Vineyard Sound.

A couple of people stood quietly, eating Key lime pie in the evening light.

Map shows old town line, which was changed this year. — Alex Elvin

Ms. Cywinski, the town assessor, pointed to the part of the map where Gay Head and Chilmark face each other across the channel. “Every time there was a storm, this whole area would change,” she said. “It was never consistent.” As Menemsha continued to change, the town line stayed the same, eventually running directly through three fishing shacks on Boathouse Road — causing headaches for assessors and property owners alike.

The evening ceremony on Thursday was timely. Ninety days earlier, Gov. Charlie Baker had signed a piece of state legislature rendering the old line obsolete.

“It takes 90 days for it to go into effect,” Ms. Cywinski said of Chapter 452, an act changing the boundary line between the towns of Aquinnah and Chilmark. “So today it’s officially in effect: The line has changed.”

Voters in Chilmark and Aquinnah had set the change in motion at their annual town meetings three years ago, in effect filing a home-rule petition with the state. The new line places each shack squarely in one town or another: three in Chilmark and two in Aquinnah. Governor Baker signed the bill into law on Jan. 13.

Ms. Cywinski had discovered the map rolled up in the vault in town hall and had it framed under non-glare glass. She plans to hang it somewhere on the town hall campus.

It’s her job to archive all of the town’s historic maps, she said, and there were more where this one came from. “I’m slowly finding them all,” she said. “And when I find good ones like this, I will have them preserved for archival purposes.”

With the sun hanging low in the sky last Thursday, the once-in-a-lifetime ceremony dispersed as quietly as it had begun.