Stand a levelling laser on the ground where you are and let it sweep all the way round. Level draws what it would touch: every place on earth that is exactly as high as you are, seen from above, as one green line. Walk up a dike and the line pulls back; step into a polder and it comes at you from the horizon. Below sea level it stops being a figure of speech — that green line is the waterline of a sea risen to your shoes.

Flood turns it blue and fills it in: take away every dune, dike and storm barrier and the sea level simply carries on over the land. Reach is the point — the water is let in from the sea and from the edges of the picture and then flows where it can, so a pit that nothing connects to stays dry and the Randstad does not.

Follow me, top left on the map, is a switch and not a jump. On: the map stays under you and your height is re-read as you walk. Off: everything freezes so you can look properly. Dragging the map turns it off by itself. You are the black ring with the cross; the dotted circle round it is how well the telephone actually knows.

Because it knows badly. Horizontally it is out by a few metres, vertically by about twenty, which is useless in a country that lives between −6 and +3. So the height is never taken from the GPS: it is read out of the height map at your position, where the fix is good. Nudge it by hand and your choice stands until you press Follow me again. Tap the map to take that spot’s height instead — on a telephone that also moves you there, because a finger points where a mouse drags.

Scale runs from the pavement to the planet in seven steps, and the line is always at your height, never at the height of whatever sits in the middle of the picture — so at the wide settings you are looking at every coastline on earth that happens to be level with you.

Heights come from the open terrain tiles on AWS (registry.opendata.aws/terrain-tiles: SRTM, Copernicus, NED and a set of national models, about 30 m), and inside the Netherlands from AHN — half a metre, actually measured, in metres above NAP — with the tiles left to fill in what AHN does not know, which is mostly water. Water on the picture is a guess: everything below −10 m, which keeps the polders on the land side. Nothing you do here is sent anywhere.