She surveyed it.
That is the thing nobody had done in a hundred and eleven years, and it is the reason for everything that came afterwards. One and thirty surveyors had gone down the Salt Road, and every single one of them had walked it and drawn it, and Tamsin Reed did not walk it.
She shot it.
She set a baseline at the stone at seven in the morning.
Two hundred and eleven paces, chained twice in opposite directions, with the chain she had made herself at nineteen. She set four stations and took bearings on both ridges and on a rock and on a dead tree, and she wrote every one of them in a book with the time against it.
It took her four hours to move nine hundred yards.
Ivar sat down on a rock and watched her do it and did not say one word.
"You are laughing at me," she said, at about the third hour.
"I am not."
"You are sitting on a rock with your face."
"I have taken one and thirty surveyors down this road," said Ivar, "and every one of them walked it in a day and a half and drew a line with some hills on it.
"And you have been in the same field since dawn with a chain, and I have not seen anybody do that in nineteen years, and I am sitting on this rock because I do not want to be in your way."
At dusk she had nine hundred yards of the Salt Road on paper and it was the best work she had ever done.
"Right," she said. "Tomorrow — "
"Tamsin."
"What?"
Ivar was standing at the edge of the firelight with his back to her, looking north, the way they had come.
"Look at the ridge," he said.
There are two ridges above the Salt Road and there is a notch in the eastern one, about a third of the way along.
She had shot it four times that morning from four different stations and it was in her book with a mean bearing of eleven degrees and a closure of a foot and a half, which for that ground is extraordinary.
She turned round.
The notch was in the western ridge.
"That is not — " Tamsin Reed had the book open. "That is not possible. I have got it four times, I have got it on a closed traverse — "
"Aye."
"I have closed the traverse, which means it cannot be wrong, that is the entire — "
"Your book is right," said Ivar.
He came and sat down by the fire.
"That is the thing I have been waiting all day for you to find," he said. "Your book is not wrong. You are not tired and you have not made an error and you have not been given the wrong glass.
"The road has moved."