How to Draw a Low Place · Low Ground 低处

In February, at two hundred and sixty metres, I came to a place in the floor of the valley where the ground simply gave up — a round depression a few hundred paces across, with water standing at the bottom and nothing growing on its sides.

I wanted to put it on the map. I found that I could not.

There is nothing in a hollow to draw. A hill has a summit you can stand on and mark with a cross; a hollow has only its absence, air where the ground should be. What a surveyor does instead is walk the rim. You go round at one height and draw that line, then step down twenty metres and go round again. The hollow arrives on the sheet as a set of rings tightening towards a centre you never actually draw — defined entirely by the ground that holds it.

I sat on the rim for a long time after I understood this.

It is the same with what I came down here carrying. I could never say what it was. Asked directly, I had nothing — the way a hollow has nothing. But I could tell you its edges: the hour of the afternoon when it was worst, the things I had stopped doing, the small objects that had grown too heavy to lift. Go round those, at one height and then the next, and a shape appears. Not the thing itself. The ground around it. That turns out to be enough to put it on a map.

The method has a plain name: marching squares. Lay a grid over the ground and take the height at every point. Choose one height. Mark each point as above it or below. Then march across the grid, square by square; wherever the four corners of a square disagree, the line must pass between them. There are only sixteen ways it can, and the method simply looks each one up.

Here it is. Filled points stand above the height you choose, hollow points below, and the amber line runs exactly between them. Press on the map and drag, and you can dig a low place of your own.

Height 0 m Cells crossed 0 Lowest 0 m

Height Show grid points Shade terrain

Slide to change the height. Press and drag on the map to dig a low place.

I dug one myself, on the sheet, the evening I wrote this. The amber line would not go into it. It went round, patiently, at every height I tried, and when I lowered it far enough the ring tightened, and tightened, and was gone.

Every hollow on every map has a bottom. That is the difference between a hollow and the world.