There was once a lighthouse on a headland where the water changed its mind often, and a keeper named Saif who had lived in it for eleven years.
Saif was proud of one possession above all others: a map of the coast, drawn by a famous surveyor, framed and hung at the foot of the stairs. It showed every rock and every channel. Sailors who climbed up for shelter were shown the map before they were shown the tea.
"Everything is on it," Saif would say. "There is no surprise left in this water."
In the tenth year, a storm moved a sandbar. Nobody saw it move. Sandbars do not announce themselves. But afterwards the safe channel on the eastern side was two metres shallower than the map promised, and the map, being framed, said nothing about it.
A fishing boat went aground there in the spring. Saif rowed out and brought the crew back and gave them tea, and when the captain said, "There is something wrong with the eastern channel," Saif went and stood in front of the frame and read it again.
"The channel is four metres," he said. "It has always been four metres."
"It was not four metres tonight," said the captain.
A second boat went aground in the summer. A third in the autumn, and that one lost its nets.
An old woman came up the stairs after the third. She had sold fish on that shore since before Saif was born, and she did not want tea.
"How deep is the eastern channel?" she asked.
"Four metres," said Saif.
"When did you last put a rope in it?"
Saif did not answer, because the honest answer was eleven years, and he had begun to hear how it would sound.
The next morning he took a rope with a weight on the end, and a boat, and he went out to the eastern channel and dropped the weight in eleven places, and he wrote down eleven numbers in a notebook. Not one of them was four.
He rowed back. He took the map out of its frame. He did not tear it up, because it was still right about a great many things. He drew a line through the eastern channel with a pencil, and beside it he wrote 1.9 m — measured, and the date.
Then he hung it up again with the pencil tied to the corner on a piece of string.
Sailors who came after that were shown the map, the pencil, and the notebook, and then the tea. Some of them added numbers of their own.
The lighthouse still stands. The map in it is ugly now, crossed out and written over in six different hands, and there has not been a grounding on that coast in thirty years.