How much of a bump a patch may miss before refining it stops being worth a worker's time, in meters.
Half a meter is the canonical floor the plan names for the elevation field itself, and the same number does for the mesh: a landing ship spans tens of meters, so ground that is right to within half a meter is ground.
It is CANONICAL_AMPLITUDE_FLOOR, not a second copy of it: the level past
which refinement stops buying detail has to be the level past which the field
stops having any, and two constants that must be equal are one constant.