Where it comes apart
Two points either side of a level-1 boundary are in different cells at every level, so they share no characters at all, however close they stand. The shortest possible crossing is one cell: 2.56 m north to south.
This is not a fault a better construction would remove. Any grid of fixed cells has boundaries, and any code built on one inherits them. What a format can do is put its seams on round lines, say where they are, and cross them correctly. All three are checkable, so here they are.
The seams
The equator and the 45th parallels north and south. The prime meridian and every 60th meridian east and west of it, which is 60°, 120° and the antimeridian. Nothing else.
| Seam | Across it | Shared | A quarter-degree away | Shared |
|---|---|---|---|---|
| The prime meridian | 2.8 m | 0 | 2.8 m | 9 |
| The equator | 4.4 m | 0 | 4.4 m | 7 |
| The 60° E meridian | 3.3 m | 0 | 3.3 m | 9 |
| The 45° N parallel | 4.4 m | 0 | 4.4 m | 7 |
The same two steps, taken on a seam and taken anywhere else. The distance is identical; the prefix is not. East to west crossings narrow toward the poles, which is why the figures differ between one meridian and another.
Two doors
At the Royal Observatory, where the line is drawn on the ground and people stand either side of it for photographs.
2.8 m apart, and not one character in common. A shared prefix proves proximity; proximity does not promise a shared prefix, and the specification says so in as many words.
What it costs
Rarely anything. Over 20,000 pairs measured at exactly 100 m apart, drawn uniformly over the sphere:
| Shared prefix | Share of pairs |
|---|---|
| 6 characters or more | 91.45 % |
| 5 characters or more | 98.31 % |
| 4 characters or more | 99.64 % |
| Fewer than 4 | 0.36 % |
| None at all | 0.000 % |
From section 10.2. The last row is the one worth reading twice: to share nothing you have to be within a few metres of a seam, and at 100 m apart that did not happen once.
Crossing them anyway
A cell's neighbours are the eight around it, and at a seam that means eight cells with a different first character. The operation crosses regardless: at Greenwich, the cell west of the line lists the cell east of it:
- West of the line
- R0NHX
- Its eight neighbours
- R0NJ0PX59XPX5C0PX5C9R0NHJR0NHKR0NHWR0NJ1
PX5C0 is the cell on the far side, and it is in the list. This page fails to build if that ever stops being true.