Since crossing the Cornmill Stream, in the previous square, I have been following the high fence which separates the woods belonging to the people of Waltham Abbey from those belonging to the Royal Gunpowder Mills. The Mills' portion is apparently awaiting restoration and as a result is unsuitable for casual visitors, although I am informed that the owners run a popular landtrain tour. On this side of the fence, seemingly identical in aspect, no restoration is considered necessary. I press on, sniffing for signs of untreated gunpowder.
The path passes a carved granite monolith close to the edge of the wood, at the end of a dead straight track. I am crossing the Greenwich Meridian, the base-line of the global grid of longitude and latitude, the mother grid to which all the world's maps defer. But this grid does not consist of squares: the lines of longitude converge at the poles, the lines of latitude are not always straight. To make a map like mine, with its regular four-centimetre cadence, compromises must be made. If I were to visit the four corners of any square on the map and carefully measure each side, I would discover none of them were exactly the same length, and none exactly a kilometre long either. The Ordnance Survey's National Grid is an updated fiction, discarding the mucky earth for an airy ellipsoid, a convenient planet where squares are squarer and angles righter.