Alfred chuckled at her question. "That's actually not as exciting as you might think. It's nothing fancy at all. They send two blokes out in a rowboat with an anchor and a long line, and they toss it down and see how long the line is when it stops." There was a bit more to it than that, but not much. The line they used was weighted so that it didn't drift aimlessly in the tide and give a false measurement, and it was marked every fathom to make it easy to measure when pulling it back up. One of the men had to have some navigational prowess, because the boat had to be in exactly the right spot when the line was cast or else the sounding would be useless — but really, those were all quite trivial details. The answer, basically, was that they just counted.

"There are some ways of mapping it out magically, but they're not very efficient," he continued with a shrug. "They use them in some highly trafficked ports where the measurements change pretty often — Port Said is like that, in Egypt. But for almost everywhere else in the world, the old fashioned Muggle way works just as well."